Process Ceramic

CERAMIC PROCESSING CERAMIC DEFECT SERIES 1-10 by JOHN T. JONES, PH.D.

All Rights Reserved copyright©2007 John Taylor Jones, Ph.D. John T. Jones, Ph.D. 213 3rd Avenue North Buhl, ID 83316 email: [email protected] 208-543-4053 (phone or fax) www.tjbooks.com www.ceramicdefects.com www.potterydefects.com www.chinadefects.com www.brokenceramic.com

ABOUT THE AUTHOR  John T. Jones served in the 17 th Infantry Regimental Combat Team during the Korean war: http://www.tjbooks.com/koreapics.htm After the war he entered the University of Utah, spent one summer at the Norton Company, and graduated in Ceramic Engineering. He then worked for Coors Porcelain Company in Golden, Colorado. After five years of service at Coors, he returned to the University of Utah in 1962 and received the Ph.D. degree in Ceramic Engineering and Metallurgy on 1965. He then joined Vesuvius Crucible Company in Pittsburgh followed by eight years teaching at Iowa State University. There he and Mike Berard wrote Ceramics: Industrial Processing and Testing: http://www.tjbooks.com/ceram.htm Leaving the university, he joined Interpace Corporation, then Pfaltzgraff Company, finishing his career after 17 years at Lenox China Company where he was Vice V ice President of R & D. Jones then became editor of  Ceramic Industry Magazine. Magazine. He is a fellow of the American Ceramic Society and past president of the New Jersey J ersey Ceramic Association. Association. He was Man of the year for that association, as sociation, the Philadelphia Section of the American Amer ican Ceramic Society, and for the University of Utah Ceramic Engineering Alumni Association. Jones served for many years on the Government Liaison Committee of  the American Ceramic Society and served as the committee’s chair.

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 How These Articles Came About  After I retired I wrote two detective novels and two western novels. I didn’t want to spend the rest of my life writing novels so I started to write shorter articles for  www.ezinearticles.com Those articles cover a multitude of topics but I let ceramics slip in there a few times  just from habit. I was writing several articles each day but a sore neck ended that  process. Now I write articles only occasionally. You can read the title and abstracts of my hundreds of articles at www.tjbooks.com After writing the ceramic articles I decided to write this book on ceramic defects. Most of my experience is in ceramic whitewares. My early experience in technical ceramics and refractories is probably obsolete so if you are looking for information in those areas, you may not find what you are looking for here. Well, I did add some information from my experience and from the literature. Much of the information on defects is in table form. For that reason I placed a Table Index right after the Table of Contents. You can use your search function to search the book. You can contact me if you like using usin g the information on the title page. Since I’m old and senile I would like you to point out errors and useful additions. I do not reject criticism. When we first published Ceramics: Industrial Processing  and Testing  our publisher was very angry about a book review published by the American Ceramic Society. The reason was that the book had been reviewed by several distinguished and very knowledgeable members of the ceramic community  before publication and the person reviewing review ing the book seemed to not have the proper  credentials. I simply contacted the writer of the review and asked if I could have his r eview notes. He provided me much more information than was in the review because he had studied the book with great interest. We made corrections where justified in the second printing. Many of of his suggestions were not made because because he preferred using Page 3 of 82

English rather than American terminology because he was English. He was technically wrong here and there. The first edition had over ten printings before the second edition was published. He made it a better book. The gentlemen who wrote the review and I became very good friends. He was very happy with my response and said that the book was very good except for the list of  corrections needed. He could have said that in the review! John John T. Jones, Ph.D. January 9, 2007

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The Ceramic and Pottery Defect Series 1-10 Abstract 1. Selected Definitions and Ceramic Definition References 2. Raw Materials and Batching of Bodies and Glazes 3. Forming 4. Drying Operations 5. Bisk Firing 6. Glost Firing and Glaze Formulation Notes 7. Decaling Operations 8. Enameling Operations 9. Gilding Operations 10. Process Control

TABLE INDEX References

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TABLE INDEX 2:1

Particle Size Defects

2:2

Particle Size and Surface Area Measurement Methods

2:3

Raw Material Contaminants

2:4

Glass and Glaze Constituents & Composition Range in Glazes

2:5

Typical Molecular Glaze Formula

2:6

Glaze Classifications

2:7

Properties Imparted to Glazes by Atomic Constituents

2:8

Ceramic Body Constituents

2:9

Ceramic Body Classifications

2:10 Materials for Advanced Applications 3:1

Forming Defects

3.2

Capacitor Defects (multiple thin film)

3.3

Glass Making Defects

3.4

Injection Molding Defects (See also Table 4.2)

3.5

Spray Drying Criteria

4:1

Drying Defects

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4.2

Injection Molding Drying Defects (See also Table 3.4)

5:1

Bisk Firing Defects

6:1

Glazing and Glost Firing Defects

6.2

Customer Complaints Glazed Ware

6:3

Thermal Shock and Autoclave Testing

6:4

Special Crazing Problems on Firing & Dunting

7:1

Decaling Operation Defects

8:1

Enameling Operation Defects

9:1

Gilding Operation Defects TOC

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Abstract

Defects in ceramics cost money; money that is often lost without compensation. Some defective ware can be reclaimed, but there is a cost to do this. Some ware can be sold as is which avoids a complete loss. In fact, selling defective ware can have almost the same profit margin as good ware except for the cost of  selling the ware. This Thi s occurs, for example, when the defective ware is retailed at the same discount that commercial customers receive. Defective ware should never be ignored. Noticing a defect and not taking action to eliminate it can result in major loss as the occurrence of the defect increases from day to day (or from hour to hour for that matter). The cause of a defect is not always easy to determine. A defect in the final stages of  decorating may have been caused by an impurity in a body raw material. For that reason, close contact with suppliers is always proper. To ignore the source of the material, such as a particular mining pit containing a known par ticular contaminant, can delay resolution. STP (Statistical Process Control) can help eliminate defects, For that reasons, references are given to implant STP for those not having already done so. I’ve tried to use charts as much as possible to help identify and eliminate defects. We start with the raw materials, then proceed through batching, forming, drying, biskfiring, glazing and glost-firing, decaling, enameling, and gilding.  Reference Text: The reference text for this e-book is Ceramics: Industrial Processing and Testing by John John Taylor Jones and M. F. Berard. The book has two editions. The  publisher has asked that a third edition be prepared but it is not available now.

There is a list of other references at the end of the book.

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CERAMIC AND POTTERY DEFECTS 1. DEFINITIONS Defects in ceramics are of of interest to potters and ceramic manufacturers manufacturers because they are a major cause of financial loss. They are of interest to collectors of ceramics  because they may (or may not) reduce the value of an item. They are of interest to users especially espe cially if they can cause damage or injury in use. Because the language of  ceramics is rich, I list here sources for ceramic definitions:  REFERENCES FOR CERAMIC DEFINITIONS:

International Porcelain Floor Facts (Tile) Astbury (Pottery) Turner Pottery (Pottery) Keramverband (Technical Ceramics) U.S. Census Ceramic Industry Magazine (definitions and suppliers, etc.)  DEFINITIONS FOR PROCESSES USED IN THIS E-BOOK 

For those of you who are not familiar with ceramic processing here is a listing of the ceramic processes we will discuss in this series of articles on ceramic defects: Formulating and Batching:

Selecting a composition for the ceramic and then choosing raw materials for the  batch. Forming or Mak Ma king:

Shaping the part by slip casting, pressing, hand forming, injection molding, roll forming, jiggering, or any other of a number of ways available.

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Drying: Controlled heating to dry the ware before firing or just leaving it on a shelf in Arizona. Bisque Firing (or Bisk Firing) and Single Firing:

A low-temperature firing followed by a high-temperature glost or gloss firing in the Porcelain Process. A high-temperature firing followed by a low temperature glost or gloss firing in the China Process. A single high-temperature fire in the (true)  Stoneware Process where the body and glaze mature together. The body porosity is less than 0.5 percent. A single lower-temperature firing in the Semi-vitreous Process resulting in a  porous body and mature glaze. The body porosity is usually between 4 and 8  percent. Glost Firing:

Firing of glazed ware (see also above).

Decal Firing: Firing a decal or decals on ware.

Enamel Firing: Firing an enamel usually on decaled ware.

Gild (Precious Metal) Firing: Firing gold or other precious metal decorations. (Now days, the decal may include enamel and gold so that a single firing can be used.)

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TOC

CERAMIC AND POTTERY DEFECTS 2: DEFECTS FROM RAW MATERIALS AND BATCHING ERRORS Formulating Formulating an d Batching Ceramic bodies are ar e formulated according to the requirements of the final product. p roduct. An important consideration is often the mineralogical composition of the final product. This often yields a chemical composition required in the initial batch. Materials are selected to satisfy the chemical and final mineralogical composition. Some bodies have a single component. In the case of glazes the composition is often strictly chemical and mineralogical considerations are not as important except where wher e a particular mineral may be added to the batch because of its particular composition. The final glaze is usually, but not always, completely vitreous. If you are not familiar with formulating and batching you might want to study the reference.  Reference Text: Ceramics: Industrial Processing and Testing by John Taylor Jones and M. F. Berard. CHAPTER 9  BATCH CALCULATIONS:  BATCH  CALCULATIONS: Oxide Formulas and

Formula Weights, Loss on Ignition and Moisture, Empirical Formula and Formula Weight, The Batch Recipe Many ceramic raw materials are already refined   by the supplier. If you can keep them clean  between the source and your factory, they will not be a source of defects. That is usually but  bu t  cour tesy Figure 5 not always the case. (The pic to the right is courtesy Page 11 of 82

of ECC International. The operator is controlling the blending of kaolin clay slur ries  by computer.)

Clay Materials  Brick Clays and Shales Some folks use materials right out of the ground to make ceramics. A high-volume example of this is the brick manufacturer up the road. (If you don’t have clay or shale where you live, there is no brick manufacturer up the road.) Brick manufacturers and clay suppliers usually mine clay by the open pit method. That means that they usually don’t tunnel for the clay. They carefully remove the overburden (the dirt, weeds, trees, old cars, and what-have-you on top) leaving a clean clay or shale face. Then they mine the clay or shale and deliver it to the  processing plant or directly to the factory. (When the clay is depleted, the law usually requires that the terrain be restored to its original condition. However, a lake may replace the mining pit. Figure 6: Pit reclaimed courtesy of KT Clay. See an interesting article on clay blending for the sanitary ware industry at: http://www.k-tclay.com/FKBLend.a sp)

Figure 6

At the factory, it is crushed or ground as required, water is mixed in, and the clay is extruded by means of an auger into a very long rectangular cross section that is cut into brick as it moves along the conveyer belt.  Now days everything is automatic in most plants including the loading and unload ing of bricks into and out of the kiln. ( See Ceramics: Industrial Processing & Testing , John T. Jones & M. F. Berard, Iowa State University Press) Brick and sewer pipe manufacturers, etc., add barium carbonate or some other  chemical to tie up the sulfates in the clay or shale because barium forms insoluble Page 12 of 82

sulfates which can cause blisters and scumming, etc.. These materials are not  premixed into the clay. They are just sprinkled on the clay and the extruder auger  does sufficient mixing. Some other manufacturers may add barium carbonate such as the sanitary ware manufacturers that manufacture your bathroom fixtures. Some potters add it too. Better safe than sorry. Vanadium compounds cause scumming in otherwise pretty white bricks. This is a tough problem. How do you tie up vanadium. If you know how, you are in demand as a consultant. The defect is often called efflorescence. How to prevent stains and how to clean brick having such manganese and vanadium stains, etc., can be found at: http://www.bia.org/BIA/technotes/TN23.pdf  http://www.upchurchkimbrough.com/technotes/efflor1.html A useful source on materials found as impurities or used in ceramics and their  effects is: http://www.kickwheel.com/chemical.names.and.uses.html

 Primary Clays

Primary clays are in situ. That means they are mined from the spot where they were formed by hydrothermal activity. The composition of the clay depends on the composition of the parent rock, often granite. Granite is a coarse-grained igneous rock composed chiefly of orthoclase and albite feldspars and of quartz, usually with lesser amounts of one or more other minerals, such as mica, hornblende, or augite. There may be iron compounds such as hematite and magnetite, titanium compounds such as rutile or anatase, and manganese compounds such as manganese dioxide called pyrolusite. These minor compounds can cause trouble if they are not removed  by the supplier. Primary kaolins are often mined by using high-pressure water. I learned in England that I was not a very good washer and was causing unnecessary segregation in the slurry. Well, my grandfathers used dynamite, picks, and shovels to mine. Page 13 of 82

I will say this: If you get a chance to blast the clay from the pit in England, do it. It is great fun! Primary clays may be processed in slurry form or by air floating. Individual slurries from different pits can be characterized as to their chemical, mineralogical, and ceramic properties. Then several slurries can be blended to give a slurry with the  proper properties specified. (See Figure 5 above.) Even after wet grinding and screening and magnetic separation a mineral such as  pyrolusite may survive. It will decompose on firing. If the decomposition is complete during a preliminary bisk firing, there may not be problems. However, if preliminary firings do not finish the decomposition you can have bubbles form even in the last stages of decorating. For example, MnO2 does not decompose until 535 oC. If it does not completely decompose during bisk firing, it may show up as blisters in gloss and decorating fires as the oxygen is released. We had this problem. In one factory with a hightemperature bisk, there was no problem. The pyrolusite contaminant completely decomposed. In the other factory with a lower temperature bisk, we got blisters in glost and decorating using the same contaminated clay. This event never should have occurred as we were not to receive clay from the contaminated pit. The loses were high and the suppliers’s insurance company had to pay.  Secondary Clays

Secondary clays were washed down from primary clay deposits. This is a natural refining process in a way because the fines are removed leaving the coarser materials including heavy oxides behind. The material were collected in natural pools which contain other materials washed down with the clay. These materials may be organic such as tree limbs, etc., which has changed to lignite. Some kaolins and all ball clays were formed by sedimentation. Ball clays may contain excessive lignite and sulphate. If you walk around a ball clay  pit you may find an old palm tree.

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Ball clays may be air floated but they are generally ground and aged. The sulfates  break down as the clays age and sulphate control is required by the supplier. (You can spell sulfate as sulphate. It is okay. I generally just mix them up.) Fire Clays

Some clays form a natural fire brick refractory, often a single-component body. Diaspore in fire clay increases the alumina content giving a more refractory composition than kaolin alone. Fluxing Minerals

Feldspars are a product of hard-rock mining. The granitic rock is blasted from the quarry face, trucked to the processing plant, crushed, and ground. Screening and magnetic separations is important in these operations. Finally, the feldspar is floated off by chemical means, leaving the quartz behind. Rutile is a secondary float product. After the final grinding of the feldspar it is checked for color and fusibility. This is done by making small truncated cones and firing them to a specified schedule in the laboratory.  Note: Particle size distribution affects fusibility.  Silica

Silica for the ceramic and glass industry are from pure sandstone. Grinding and screening are the main operations. Impurities may be removed by magnetic separation as with all ceramic raw materials. The particle size distribution can be important in ceramic operations and must be controlled.  Refractory Oxides

Alumina is usually formed from bauxite by the Bayer process which involves crushing and grinding of high-alumina hydrates followed by crystallization and calcination. Tabular and fused aluminas are formed at high temperatures. Magnesia Page 15 of 82

can be formed from dead-burning magnesite from sea-water brine. Other oxides are lime from limestone and chrome-ore refractories from chromite, a complex oxide.  Miscellaneous Materials

There are many materials used in ceramics that do not fall into the above categories. They are formed by a variety of process. It is important to understand the processes used to create the materials you use in your operations.  Stay in Contact with your Raw Material Suppliers

It is also very important to stay in contact with your suppliers and to tell them about any problems that could be material related. When I was in the industry, I often asked suppliers what problems their other  customers were having. I didn’t always get a good answer but trust between the ceramic manufacturer and the material supplier is essential. When I suspected that something was not right in the material supplier’s processing and I was not getting decent answers to my questions, I would hop a plane and fly to the source. Then management would get on the band wagon and I would learn what was going on. Once we had a problem with the particle size of incoming nepheline syenite. My laboratory manager could not get the information he needed. I flew to the mine in Canada and asked, “What is going on?” I finally go the answer by talking to the grinding plant manager. He told me that he had increased production by changing the media distribution in the ball mills. That had changed the particle size distribution. When we complained, he changed the mills back. So we had a range of product that was always changing in particle size distribution. I told him that he would not have had to change the mills back if management has simply told me what was going on by telephone before the media distribution was changed. All we needed to know was the truth. We could have easily adjusted our  milling operations. So the company lost the advantage of the change and they faced a very embarrassing Page 16 of 82

situation. I had warned them on other trips that they were to inform me of any change in production procedures or product. You should tell your suppliers the same.

 Impurity Defects Defects caused by raw materials are usually related to impurities or particle size. The defect can turn up at any time in the process. Here are some more examples of defects caused by impurities:  Lignite

Lignite is common in many clays. It can be hard anthracite or softer bituminous coal or lignite which is softer than the first two. Screening can reduce lignite from slurries even after ball milling. (Ball mill and media pics courtesy of Paul O. Abby.) Soda ash can react with soft lignite and turn it into a useful Figure 7 colloid that will keep the slurry dispersed. In other words, some lignite can be   beneficial in slip casting operations and other operations that use slurry. But the hard anthracite and other coal varieties are not good. They can cause blisters and pits during bisk- and glost firing operations. Figure 8 Clay companies have some control over the size and amount of lignite in their clays (often by blending).

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You can run a screen analysis on the material and see how much lignite remains on the screens. If it is higher than the previous shipments, call the clay company and say these words: “What’s with all this lignite!” (Figure 9: Vibratory Screen image thanks to Sweco®.) That should get their  attention. Once I said, "What is all this plastic?" (The filter    bags had melted in the air  floatation equipment.) Make sure you blame them for all defects generated for  the next six months. (Hey, you think I’m kidding! Make that the next two months or  whenever that batch of clay is used up.) Figure 9

For a description of clays used in the ceramic industry go to: http://www.hamgil.com/html/claytype.html Grit 

We had a recurring blistering problem in fine china where I once worked and for  years nobody could figure out what was causing it. I was new with the company but not inexperienced in solving material problems. I called each of our suppliers and said that we had blisters and it was their fault. The representative from a kaolin company asked, “What’s the grit?” I wasn’t sure what he was talking about because I had not used that particular clay  before. Rather than tell him that I didn’t know what he was talking about, I said, “Hold on a minute.” Page 18 of 82

I asked the quality control manager, “What’s the grit in that stuff?” He had a questioning look on his face despite his 30 years of service. He pulled out the specification sheet for the last shipment, found that there was such a thing as "grit," and gave me a number. The number represented the amount of coarse material that showed up during a screen analysis of the clay. For this particular clay, the grit was in the form of mica. (Mica can cause blisters and discoloration. A close look at the blisters in our bisk ware revealed a slight discoloration. We knew we had found the mica.) Hearing the number, the representative said, “That is too high! I’ll call the mine!” We set a control standard and never had that problem again. I‘d paid my first years salary in five minutes a few days after joining the company. Learn how to evaluate clay materials at: http://www.ceramicindustry.com/CDA/Articles/Feature_Article/fcd8eaeeba25d01 0VgnVCM100000f932a8c0____   Iron and Manganese Compounds, Silicon Carbide, Soy Beans and Salts

Sometimes clay manufacturers ship clays to storage areas by rail, ship, or barge. Dockside raw material storage is always dangerous for contamination. The reason is that these facilities ship iron ores, ferro-silicon, silicon carbide and they are not very careful about cleaning out a shed of ferro-silicon or other contaminant before loading it with a shipload of clay. (We changed from bulk to Super Sack® shipments.) Some of these materials give off oxygen when they decompose. That occurs at some characteristic temperature. We used the same British china clay in two different plants. In one plant we had blisters in our decorated ware, the worst possible condition. The other factory using the same clay didn’t. Why? The bisk temperature was much Page 19 of 82

higher in the not affected plant. The contaminant, a manganese compound, decomposed before glazing and decoration. The affected plant had both lower bisk and glost temperatures. The material did not fully decompose at these temperatures and bubbles were still appearing at the decorating temperatures. The problem was complicated because we had to prove that the supplier caused a heavy loss that their insurance company should pay. I’ll not go into the details because they proved we were right and did pay. The reason for reimbursement was that I had requested them to never ship us material from a certain pit (mine). It had caused the company loses for years and I wanted no more of it. They shipped from that pit rather than shipping us the processed clay that I specified. I know I told you the above story before. I don’t want you to forget it.

Identifying Impurities Identification of impurities can be tricky without a Scanning Electron Microscope (SEM). Heavy liquids can be used, but that is a nuisance. Microscopes are good for those who know how to use them. You can look at the contaminants from the heavy liquid separation. Sometimes you must use an outside lab if you don’t have the correct equipment. To isolate a contaminant before you send samples out, elutriate clays and screen non clays. Wet screening is best. To elutriate a clay, take about ten pounds of clay and keep washing it down until only the contaminant is left. Put the contaminant in your body or glaze and reproduce the defect you are seeing in production. Some clays can be wet screened but not at 325 Page 20 of 82

mesh in most instances. Still, before you elutriate, wash the clay though a screen as fine as possible. Contaminants can be very fine. Common contaminates are quite dense and settle readily from slurry. Chloride can be detected from salts by washing the clay with de-ionized water and testing with a soluble silver nitrate solution. A milky precipitate is silver chloride. Salts cause blisters! We had soy beans once in one of the plants where I worked. The clay picked up the  beans from a hopper car. They make big holes in your ware but they are easy to screen out if you have screening in your operation, which we didn’t! There is a funny story that goes with the soy beans. We were putting up silos for clay storage in an automatic batching system. I wanted to set raw material specifications to avoid problems. I decided to call my cronies in the ceramic industry who were working with silo systems. I called my friend, Fred (not real name), out in Ohio and I said, “Fred, we are putting in a clay storage system. Have you had any problems with your clay from railroad cars?” “Nope!” “Fred, you never had problems with iron oxide, manganese, or ferro-silicon?” “Well, we had some iron oxide problems (from taconite, low-grade iron ore) but that was from the barge, not the railroad.” I said, “I see, you get some of your materials by barge. Was the iron contaminant a  problem?” “Hell, yes! It made ugly brown holes in our ware.” “So, Fred. What did you do with the clay?” “We had to take it out of the silo. We had to use a wheelbarrow.”

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“Who did that?” “Me and my technician.” “Fred, what did you do with the contaminated clay?” “Our pond was leaking so we put it in there.” I asked Fred, “What about soy beans?” “Well, the railroad brought them.” “Didn’t they cause you a problem, Fred?” “Hell, yes! They put big holes in the ware. We could see them coming down our clay conveyor belt.” “What did you do with that clay, Fred?” “Same thing! The pond didn’t really need it, but we dumped it in anyway.”  Particle Size

One of the most serious formulating errors is in not controlling particle size in the  batch recipe. For example, if you use too much of a certain clay having a very fine particle size, you will have problems. Let me say this, you must choose the correct particle size distribution for your process. Slip casting requires a coarser particle size distribution than a plastic forming process for the same formulation. What does all this nonsense mean? You must use some coarser grained clay(s) in your casting process. If you don't you will still be trying to cast a piece when the next millennium comes in. If you want to have control of your casting process, then try using a coarse-grained kaolin plus a fine-grained kaolin, and a coarse-grained ball clay plus a fine-grained Page 22 of 82

 ball clay. Changing the ratio of fine to coarse clays will give you control. You must maintain the total amount of kaolin and the total amount of ball clay to preserve color  of the product. Look at the particle size distribution of the clays you are using in your process. This information should be provided to you with each shipment. Look at the particle size portion that says <0.5 microns. (If that information is not there, the <2 micron information usually is. Use it but it is not as good for control.) The <0.5 microns is called the colloidal fraction. Keep the colloidal fraction the same in each casting batch. ( Send me an email if you can’t figure out how to do this. Hint: if you have 30% Clay A in your batch and the colloidal fraction of Clay A is 20%, the colloidal fraction added to the batch is 6%.) You control your shrinkage by controlling particle size (and water content). Too fine a formulation will cause excessive shrinkage and you will get distortion and cracking  problems.

Section 2 Tables

Table 2:1 Par ticle Size Defects

Wrong Distribution

Supplier may have Effects plasticity in changed  plastic forming. Effects  powder flow in pressing milling/grinding   procedures. Changed operations. Effects grinding media type, casting properties. size, or media size Effects shrinkage. distribution.

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Too Fine

Increases shrinkage. Slows casting rate. Changes plasticity.

Supplier may have changed material source. For clays, supplier may have moved to another part of the mine.

Too Course

Decreases shrinkage. Increases casting rate. Changes plasticity.

Supplier may have changed material source. (See above.) Supplier may have underground the material.

Table 2:2 Particle Size Measurement Methods (Chapter 11 in Ceramics: Ind. Proc. & Test.)

Sedimentation

Stokes Law

Light Scattering

Often LASER techniques

Screen Analysis

ASTM Series Screens

Microscopic Analysis

Optical or SEM techniques, etc.

Surface Area

Absorption Techniques

Table 2:3 Raw Material Contaminants

Silicon carbide, ferro-silicon, etc.

Source is storage areas such as  ports as these are man-made materials. Cause blisters in bisk  and/or glost firings.

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Manganese oxides, iron oxides, titanium oxides.

Source can be storage areas or  such oxides are sometimes found in mining deposits. Cause blisters and discoloration. Problems with these oxides may be related to  bisk firing temperatures. Manganese oxides may decompose at high bisk  temperatures and may not cause trouble in glost and decorating fires. For low bisk temperatures, the oxides may not completely decompose and bubbles and  blisters may appea r in glost and decorating firings.

Soluble salts like NaCl and KCl

Can be from sea water during shipping or from deicing operations at ports, etc. Detect chloride with silver nitrate.

Plastic, lint, paper, etc.

Can come from out-of-control drying operations (melted bag filters, etc.)

Sulfates

Inherent in ball clays, shales, etc. Can be controlled with barium carbonate which forms barium sulphate which is insolub le. High sulfates yield a very “mushy” cast in slip casting operations.

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Vanadium, titanium, iron, etc.

These compounds are tough to deal with in brick manufacturing  because they cause scumming. Vanadium scumming on fine white brick is particularly bad. See the text for references on how to clean such brick.

Lignite

Inherent in clays. Lignite may be  bituminous or anthracite coals or  soft lignins. Causes b listers, pits, and possible discoloration. Can be controlled by screening o r by material selection by supplier. Soda ash is helpful during grinding because it forms  beneficial compounds when it  breaks down the lignite and improves dispersion.

Mica

Mica is common in clays and comes as biotite or muscovite in most cases. It can cause pinholes and / or coloration. Often measured as “grit” by clay suppliers.

Organic Binders

Carbonization of organic binders can result in bubbles an d blisters and discoloration, especially in glazes. The kiln must be kept oxidizing with good air flow during the binder burnout period in bisk and glost firing. Page 26 of 82

Table 2:4 Glass and Glaze Ionic Constituents & Composition Range in Glazes. Compiled from reference 2. (LF = lead free)

Cation

Valence Radius

Ionic Potential (Valence / Radius)

Role in Glass

% Composition Range / Typical as Oxide in Glazes

Li

1

1.5

Modifier

0.0-1.0 / 0.0

Na

1

0.68 0.98

1.0

Modifier

0.0-8.0 / 3.0-5.0

K

1

1.33

0.75

Modifier

0.0-10.7 / 5.0-8.0

Be

2

0.31

6.5 Toxic: Do not use in any form.

Modifier

no information

Mg

2

0.71

2.8

Modifier

0.0-2.7 / matt 6.4

Ca

2

0.99

2.0

Modifier

0.0-33.6 / 5.0-8.0

Sr

2

1.13

1.8

Modifier

see Ba

Ba

2

1.35

1.4

Modifier

0.0-48.4 / 10.0 LF

Zn

2

0.74

2.7

Modifier

0.0-6.0 / 0.0

Pb

2

1.21

1.17 Toxic: Limit use.

Modifier

11.0-50.0 / 20-40

B

3

0.20

15.0

Glass Former

0.0-42 / 6.0-10.0

Al

3

0.50

6.0

Intermediate

0.0-25.3 / 8.0-15.0

Si

4

0.41

9.8

Glass Former

0.0-79.2 / 20-50

Ti

4

0.68

5.9

Intermediate

no information

Zr

4

0.80

5.0

Intermediate

4.0-6.0 opaque glazes

Sn

4

0.71

5.6

Intermediate

8-13 matt & opaque glazes

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Pb

4

0.84

4.8

Intermediate

0.0-61.4 / 20-30

P

5

0.34

14.7

Glass Former

no information

As

5

0.47

10.6

Glass Former   no information

Table 2:5 Typical Molecular Glaze Formula (Earthenware Type Glaze Reference: 2 For glazes but not bodies the basic oxides total is set to 1 mole and the other oxides adjusted accordingly.)*

Basic Oxides

Amphoteric Oxides

Acid Oxides

0.078 K 2O

0.261 Al2O 3

2.905 SiO 2

0.221 Na2O

0.370 B 2O 3

0.378 CaO 0.333 PbO (added as frit) *See Ref. 1 Chapter 9 to learn how to batch b odies and glazes. Table 2:6 Glaze Classifications (I call this the Jones Classification)

Single-Component Raw Glazes

Some materials form a natural one-component glaze. Albany slip clay forms such a glaze and can b e used to protect graphite refractories from oxidation. It can also be used on pottery, etc. Salt glaze is a single-component glaze formed when salt vapors react with the surface of the ceramic substrate.

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High-Lead Raw Glazes

White lead is a very good agent to have in a dipping tank. However, the material is toxic and sho uld be avoided.

High-Lead Fritted Glazes

Usually contain lead bisilicate frit. The density of the frit must be controlled by the su pplier. Frit  batches can be blended b y specific gravity or refractive index.

Low-Lead Fritted Glazes

The lead is combined in the frit with other glaze ingredients such as the alkalies and alkaline earths, Zn, Al, Si, etc. Ba may be add ed to improve brilliance.

Non-Lead Raw Glazes

Everything is in the batch but there is no frit or ground glass.  Not too common.

Opaque Glazes

Contain opacifier of course.

Crystallized Glazes

Crystal formation glazes not opaque.

All other glazes in the universe.

Excludes the above. Now where does the  snake skin glaze go?

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Table 2:7 Properties Imparted to Glazes by Atomic Constituents (Partly extracted from Reference 3 an d other references.)

Na

Increases expansion, decreases tensile strength and elasticity, increases fusibility, lowers viscosity. Should be used with K or  other alkalies to optimize properties such as electrical resistivity.

K

With Na increases cost, may increase gloss, lowers fluidity, increases scratch hardness, increases chemical resistance, decreases expansion.

B

Strong flux and solvent for other glaze constituents, stabilizes colors, lowers viscosity and surface tension, heals defects caused by air bubbles or liquid non-miscibility, reduces surface friction to improve knife marking / cutting resistance. Greatly improves chemical durability as glass former. Increases gloss /  brilliance, Excess can form pinho les and blisters.

Li

Reduces bulk needed for same number of equivalents of alkali  because of its low molecular weight. Reduces expan sion and greatly increases chemical and mech anical durability. Small amounts lowers softening temperature. Large amounts can cause crystallization. Improves mechanical properties.

Ca

Increases the mechanical and chemical durability of a glaze. Lowers expansion. Increases fusibility at higher temperatures.

Mg

Increases fluidity at higher temperatures, lowers expansion more than other bases, improves colors, excess causes opacity, improves chemical and mechanical properties.

Sr

Strong flux, slowly increases expansion, best added to frit, lowers solubility, improves mechanical and ch emical durability.

Be

TOXIC: DO NOT USE. Use Zn or Li instead for improved  properties. Page 30 of 82

Ba

Strong flux. Increases brilliance as it is a big atom with a heavy electron cloud. For that person, subs titute for Pb to lower Pb content of glaze. Incorporate in frit because of slight toxicity. Lowers elasticity. Does not reduce in kiln atmosp here like Pb or  Zn (as seen in kiln w recks).

Zn

Very imp ortant in color stability. A cure all  for many glaze defects but not quite as good as Pb in that regard. Improves melting and gloss. Excess causes opacity. Lowers expansion. Improves mechanical and chemical durability. Every glaze should have some in my op inion. Well, maybe not every glaze.

Pb

The best glaze cure all  but chemical leaching can be a problem. Should be used in low er amounts and always in frit form to  protect workers. Gives high brilliance, low expansion, increases elasticity, lowers viscosity, wide maturing range, lowers surface tension and improves smoothness.

Al

Improves melting characteristics. Improves chemical and mechanical durability. Can stop crazing. Important for some colors. Common glaze ingredient.

Si

Main glass former in glazes. Increases durability. Greatly lowers expansion in glazes (raises expan sion in crystalline bodies). Eliminates crazing but may raise required firing temperatures.

Sb, Ti

Opacifier and color former.

P

Not used much in glazes. Opacifier and increases brilliance.

As

TOXIC COMPOUNDS rarely used in glazes. Opacifier. Color  former.

Sn

Opacifier. Color former. Expensive.

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Zr

Opacifier and color former. Minute amounts can improve mechanical and chemical durability without causing exc essive opacity.

Bi

Like lead but not as good. May need an oxidizer like As to  prevent grayness. I was always disappointed with Bi glazes.

Cd

Like Zn but not nearly as good. Color former.

Cr

Color former  

Ce

Expensive opacifier required in large amounts. But could be useful in lead-free glazes in low amoun ts

W

Small amounts in frit can be helpful but expensive. See Mo.

Cu, Co, Au

Color formers.

Mn, Fe

Can be strong fluxes. Color fo rmers.

Mo

Lowers surface tension of glaze. Larger amounts add opacity. Best in frit.

 Ni, V

Color formers

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Table 2:8 Ceramic Bo dy C onstituents (See Reference 1 and Ta ble 2:9 Body Classifications)

Alumina, Beryllia, MgO

High-temperature bodies which have good chemical, electrical, and mechanical properties. May be transparent to radar. Used in many electrical, mechanical, and chemical processes. Can be bonded to metals.

Refractory Oxides, Silica, Graphite, SiC, Zirconia, Zircon, Chromite Ore, Magnesia

Resistant to slag and molten metal attack at high temperatures.

Complex Oxides, etc.

Used in many magnetic and electronic applications such as magnets and cap acitors.

Clays

Kaolins and ball clays are used in many ceramic  products including whitewares and structural clay  products. Kaolins fire white. Ball clays may fire off-white but add plasticity to ceramic bodiers. New Zealand h alloysite can be used for very white, high-strength, high translucency bodies with good plastic properties .

Fluxing Minerals

Talcs and feldspars used for tile and whitewares and other ceramics. Talc used to make ste atite.

Alumina Rich Minerals

Kyanite, sillimanite, and andalucite are used for  refractory materials.

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Table 2:9 Ceramic Body Classifications (See Reference 1)

 Near Pure Compounds

Alumina, beryllia, graphite, and o ther essentially one-component systems. Okay, I’ll allow up to 1% non-organic binding systems such as clay, magnesia, chromia, etc.

 Near-Pure Systems

Above with up to 10% clay, magnesia, calcia, chromia, etc., additions.

Mixed Oxide / Silicate Systems

Ferrites, capacitor compositions, optical or  electronic device compositions, etc.

Clay / Talc Based Systems

Brick, tile, sewer pipe, floor tile, etc. Also steatite having superior electrical properties.

Triaxial Systems

China, stoneware, porcelain, pottery, and such made form feldspar, clays, and silica.

Fused Silica

Low-expansion, chemically durable glass.

Borate Glasses

High chemical resistance.

Structural Glasses

Plate glass, automobile glass, glass blocks, etc.

Container Glass

You know, like Coke® bottles.

Specialty Glasses

Optical glass and other glasses with special  properties.

Glass Ceramics

Dinnerware and technical glass ceramics usually  based on Li compounds.

All other bodies in the universe.

Those bodies of which I don’t have a clue.

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Table 2:10 Materials for Advanced Applications (see ref. 1)

High Purity Aluminas

Purity beyond that of Bayer Process Alumina. Sodium vapo r lamps. TTA is zirconia transformation toughene d alumina. Titanium aluminate for automobile parts, etc.

Barium Titanate, etc.

Capacitors and transducers. PZT and PLZT also—incorporating lead, zirconia, and lanthanum.

BeO

Good electrical and nuclear properties. High thermal conductivity is advantage for heat dissipation from electronic devices such as klystron tubes. Powder is toxic and AEC standards should be used to prevent injury.

Boron and Titanium Carbides and Titanium Boride

Grinding tools, etc. Very hard and strong. Ceramic armor.

Boron Nitride

Good electrical properties. Go od for sp ecialty refractories, etc. Like graphite in some ways except it is an electrical insulator. See ref.

Rare Earth Oxides

Used in many diverse applications. See ref.

Iron and Other Oxides

Used in ceramic magnetic materials includes MgO, ZnO, BaO, SrO, etc.

Silicon Nitride

Refractories and automotive engine parts. Antifriction applications.

TOC

Page 35 of 82

CERAMIC AND POTTERY DEFECTS 3: DEFECTS FROM FORMING OPERATIONS Common defects from forming operations are discussed in this 3 rd article in the Ceramic Defect Series. Dry forming methods generally have defects related to particle size distribution, binders, and pressing parameters. Wet processing methods have defects generated by the state of flocculation of the slip and moisture control. Forming methods of ceramics are sometimes classified as wet or dry. Dry forming refers to pressing operations from dry or perhaps damp powders. Wet forming includes slip casting and plastic forming methods. For a review of industrial forming methods see Ceramics: Industrial Processing and Testing  by John T. Jones and M. F. Berard, Iowa State University Press. Dry pressing requires that a shape be dimensionally stable after firing. That will occur  if the pressing operations are in control and the firing is specified. If a pressed part is oversize after firing, it can be ground to size, but that is an extra operation usually not included in the costing of the part. If the part is undersize after firing, the part is scrap. Important factors in pressing are the die size, the particle size distribution of the  powder, the binder system, the pressing pressure and pressing cycle. Problems are  powder sticking to the die, powder not flowing into the die freely, and incorrect fired shrinkage. In isostatic pressing an additional problem can occur due to the incompressibility of  air. This can sometimes be relieved by flooding the tooling cavity with propane which is compressible and on release will evaporate into the air slowly rather than expand instantly like air expands which can crack the part. In isostatic pressing the part usually must be machined after forming. Laminations in pressed parts can be caused by the powder not flowing easily during die filling or during pressing. These laminations will not usually heal during firing and must be eliminated in the pressing operations. Experiments with binders such as Page 36 of 82

 paraffin, gums, and starches, etc., must be conducted to determine the correct binder  type and amount. Binders with lubricating properties can help . Sometimes a lubricant is needed to prevent sticking of parts to the die. See the reference for more information including spray drying of powders. Plastic forming by machine such as plate rollers and jigger machines usually require the clay to be in the flocculated state. Small amounts of plaster added to the body when in slurry form before filter pressing and extrusion will provide the desired  plastic properties. Cast jiggering is an exception but even then the clay slip is placed on a plaster mold providing what is needed to flocculate the slip as it builds up on the mold before jiggering. Clay for plastic forming must have the correct moisture content or the dimensions of  the dried product before firing will be incorrect. Firing will not improve this situation. Most other problems occurring during plastic forming can be resolved by keeping the clay flocculated.  Nepheline syenite and alumina tend to deflocculate plastic bodies bef ore forming so it may be essential not to delay forming in these cases only. Generally, aging of    plastic clay improves forming operations especially for bodies with low inherent  plasticity. Aging not only distributes moisture but also breaks down sulphates,etc. Information on clays used in plastic forming can be found at these sites: http://www.walkerceramics.com.au/practical%20information.htm http://cavemanchemistry.com/oldcave/projects/pottery/basic_clays.html http://web.clas.ufl.edu/users/sassaman/pages/classes/Ceramics/Clays.htm http://www.basic-stuff.com/hobbies/ceramics/ Slip casting requires that the clay slurry be dispersed. The slip specific gravity and viscosity must be controlled. The particle size distribution is critical and must be controlled by formulation as well as processing. See the referenced text for details of  testing and slip control. Whether from plastic forming or slip casting, leather hard ware must be handled carefully during finishing operations. Clay has a memory and distortions in green ware can reappear during firing.

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See the reference for other forming methods and problems.

Section 3 Tables Table 3.1 Forming Defects Shrinkage Problems

(see raw materials above)

Bubbles and Blisters

May be caused by air entrapment. The vacuum chamber of extrusion equipment must be maintained by auger replacement or repair, etc. Vacuum lines must be open. Sometimes too plastic clay will plug vacuum lines when it is already difficult to de-air. Pressing operations can also entrap air (see Laminations).

Laminations

In pressing operations caused by poor   material flow or air entrapment. Particle size distribution and particle fineness are to be suspected. Clay may not “mend” during plastic forming.

Cracks

(see also drying) Cracks are often related to the pressing cycle. In isostatic pressing, the expansion of  entrapped air can cause cracking. Propane is compressible and may be used to fill die cavities or isostatic  pressing fixtures to displace the air. Air is not compressible.

Cracks and Distortion During Firing

(see also firing) Cracks and distortions during firing may come from forming operations. Check pressing cycles,  particle size distributions, etc.

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Setter and Support Design Errors

(see also firing) Setters need to be designed to “pull” the piece in during firing and to support dimension controlling setters during firing. See text and illustration of setter design.

Pits and Blisters

Binder preparation may have left small lumps of organic material not destroyed during pressing. Check   binder preparation methods and  possible contaminants in material  preparation areas.

Table 3.2 Capacitor Defects (multiple thin film) See: http://www.ceramicindustry.com/CDA/Archives/cde08fabca9c7010VgnVCM100 000f932a8c0____  Swiss Cheese or Other Void Structures

Excessive binder. Solid particles not touching. Solid volume should be about 60%.

Pin Holes

Too little binder. Air is trapped.

Surface Craters

Too little binder. Air is trapped.

Table 3.3 Glass Making Defects Stones, Cords, Knots

Glass tank refractories

Bubbles

Improper melting and refining. Trapped gases.

Un-melted Material Grains

Improper melting and refining.

Page 39 of 82

Table 3.4 Injection Molding Defects (See also Table 4.2) Slumping and Warping

Improper cooling after injection. Binders too soft. Support not provided.

Air Pockets and Pinholes

Particle size distribution. Air   entrapped during extrusion. Use vacuum to evacuate chamber before melting.

Poor Microstructure

Microstructure depends on the close  packing of particles during forming. Slip casting gives a good structure  because of good particle packing. The structure is not as good in pressed  parts because often more and stiffer   binders are used. In injection molding, there is an excess of binder that must  be removed. Therefore the structure is sometimes inferior to other forming methods.

Table 3.5 Spray Drying Criteria Inlet and Outlet Temperature

This temperature difference combined with the airflow volume supplies the heat required to heat the slurry as it is fed into the chamber and to evaporate the water from the batch. More air is desirable over too-high an inlet temperature to control particle size distribution. Moisture control dictates air control parameters.

Page 40 of 82

Moisture Content and Angle of Repose A SiC bomb is a quick way to determine the moisture. Weighing and drying a small sample also works. Too much moisture will degrade the flow  properties. The angle of repose should  be determined at the same time the moisture content is being determined. An angle of repose of 32 degrees or  less is favored. Angle of repose above this mean that the powder may not flow properly in isostatic pressing and dry pressing operations.

Spray Nozzles Versus Spray Disc

Disc are usually desirable for  large chambers, nozzles for small chambers. Small dryers are oft en less desirable than larger d riers.

Binders

Paraffin can be emulsified and  blended with the s lurry. This is often more desirable for isostatic  pressing than using wax emulsions. A 0.1% addition of  gum arabic can reduce excessive fines. Dextrin and other binders and lubricants such as stearates (a salt or ester of stearic acid, containing the group C17H35COO), may improve dry  pressing operations but not always isostatic pressing. Alkaline  binders containing Na, K, etc., may degrade electrical properties.

Page 41 of 82

Particle Size Segregation

Large dryers cause segregation  but blending can be obtained in air- blending chambers.

TOC

Page 42 of 82

CERAMIC AND POTTERY DEFECTS 4: DEFECTS GENERATED DURING DRYING OPERATIONS In this 4th in the series on ceramic defects, drying of ceramic parts is discussed. At first, water is easily removed allowing rapid drying, but then diffusion takes over and the water must be removed slowly or the ware will crack or distort. Finally, the last tightly-held water must be removed. The process is moderated by temperature, humidity, and air-flow control. Drying operations relate to plastic forming operations and casting operations. Forced drying in controlled driers expedites production and guarantees continual controlled  production flow. Driers are usually built into automatic casting machines and roll forming machines. The drier is often designed to accommodate the different stages of drying. Air flow is adjustable throughout the drier. When a piece of ceramic ware is first formed, the particles are separated by a water  layer which can be easily and safely removed. For that reason, excessive heat can be supplied at this stage of drying. Once the particles touch, the process becomes diffusion controlled and the water  molecules must move slowly through the body matrix. Finally, the last tightly held water must be removed. These last two steps require careful heat and humidity control in the drier. See Ceramics: Industrial Processing and Testing by John T. Jones and M. F. Berard. For automatic plastic forming operations incorporating built-in driers, the slip must  be controlled as to particle size, specific gravity, and viscosity. The slip must be in the flocculated state during filter pressing and subsequent operations. If not controlled, the ware may not dry properly after forming and the operations can be drastically slowed down. Drier losses due to cracking or distortion can be dramatic. Page 43 of 82

Drying of cast ware is less critical except for automatic casting machines. Slip control is again the most important function to prevent loss. See the reference for more information on the drying process, slip control, and industrial driers. To obtain a psychrometric chart go to: http://www.engineeringtoolbox.com/psychrometric-chart-mollier-d_27.html

Section 4 Tables Table 4.1 Defects from Drying Operations

Cracks

Often caused by improper drying cycles. Plastic formed ware may be dried rapidly at high temperatures initially until the water layer between  particles is removed. Then things change when the particles touch each other. Now the removal of water is controlled by the diffusion rate of the water to the surface. This stage of  drying must be carefully controlled by higher humidity and lower  temperatures. The very last water is hard to remove and is often not removed until firing. See Chapter 5 in Ceramics: Industrial Processing and  Testing .

Distortion

Plastic formed ware such as dinner    plates may have to be supported during drying operations. Special setters are available.

Page 44 of 82

Laminations

Laminations may appear during drying. They may be related to forming.

Table 4.2 Injection Molding Drying Defects

Slumping and Warping in Injection Molding Dryers

Thermal plastic effect. Include a thermal setting resin in mix. Use range of thermoplastic binders  burning out over different temperature ranges. Control  binder burnout stages removing lowest-temperature binders before softening of other binders.

Voids, Air Pockets, Blisters, Etc.

Control binder particle size distribution. Remove air before injection by vacuum techniques.

TOC

Page 45 of 82

CERAMIC AND POTTERY DEFECTS 5: DEFECTS GENERATED DURING BISK FIRING OPERATIONS In this part of the series on Ceramic Defects we discuss the bisk (or bisque) firing of  fine china, porcelain, and industrial ceramics. Drag, shrinkage, and slump that occur  during firing must often be accounted for by using special setters and fixtures. The purpose of bisk firing is (1) to completely vitrify or densify a ceramic body in the case of bone china, fine china, and industrial ceramics or (2) to  partially vitrify a  body in the case of porcelain. Taking porcelain first, Case (2), the body is taken to a temperature to which enough strength is developed to be automatically handled in glazing operations (in modern factories) but porous enough to be easily glazed by dipping methods. Porcelain ware does not have to be supported during bisk firing and defects generated are usually chips or breakage from handling. Kiln dirt can cause defects here, but rarely. If they are formed, they often can be removed by grinding methods. Sometimes  sand , perhaps alumina, is placed between dinner plates so they can be stacked without any chance of sticking. This sand is easily removed by subsequent operations. If a speck of sand is left on the wear during glazing, it will cause a glostware defect as described in the nest article. In the first case, Case (1), the body is heated on a temperature / time cycle where full strength is developed (usually through sintering) in non-glazed industrial ceramics. For fine china and bone china, the body develops maximum translucency through vitrification. The ware often has to be supported during firing.

Page 46 of 82

Three major problems relate to the high-temperature firings of Case (1). The first  problem is shrinkage. The second problem is slump. The third problem is drag. These factors play a major part in the distortion of large parts during firing. For fine china and bone china, the dinner plates and similar items are fired in refractory saggers. The ware settles into the shape of the sagger as it softens during firing. Therefore, the ware is fully supported. The saggers are covered so there is usually little or no kiln contamination. Operations may be required to remove any  particles from the ware before glazing. Small parts are not a problem for industrial ceramics. Larger parts are a different matter. The shrinkage and slump and drag factors must be known. Shrinkage is caused as the particles move together during sintering or vitrification. Slump is due to the weight of the part on itself. Drag is due to the friction between the part and its setter or the kiln car deck. A shrinkage block of a body can be formed into a two-diameter cylinder about 3 inches high with a top half diameter of perhaps 1-3/4 inches and a bottom diameter of 2 inches. The drag shrinkage is determined by the diameter of the block touching the kiln car deck or setter, the slump is determined by measuring the total and shoulder height of the block, and the normal shrinkage is determined by Figure 10 Shrinkage Block  measuring diameters not affected by drag or slump. An example of how large cylindrical shapes are fired will illustrate w hat sometimes must be done. A setter is machined from the same material as the cylinder. This base setter is coated with a solution of PVA (polyvinyl alcohol) and sprinkled with sand (alumina). For  smaller cylinders this step can be omitted. Page 47 of 82

On top of this is placed a step setter. The step is about 3/8 inch. The ceramic cylinder has its ends coated with a solution of PVA (polyvinyl alcohol), dipped in setting sand (perhaps alumina), and is placed on the step setter. The upper step is the diameter of  the ceramic cylinder. (Well leave a few thousandths.) The step setter maintains the lower diameter of the part. As the base setter shrinks, the base setter (and step setter) will shrink, pulling the cylinder in with it. But we are not firing yet. Figure 11 Ceramic cylinder firing setup.

A hollow cylinder (perhaps 2 inches in diameter) made of the same material as the part is placed on the step setter  dead center. A top step setter, treated as above, is placed on top of the ceramic part, its minor  diameter fitting into the cylinder and resting on top of the inner support cylinder. The support cylinder will decrease slump caused by the weight of the top step setter. The top step setter maintains the diameter of the part. After firing, the cylinder will not be perfect, but if there is enough grind stock and the upper and lower diameter have  been maintained during firing, the part can be machined to the specified dimensions. I was not able to illustrate this in the original article. I hope Page 48 of 82

Figure 12 Ceramic Cylinder Firing Setup

with the figure you see it in your mind: A larger cylinder resting on a step setter with a step setter on top, the steps maintaining diameter as they fit into the cylinder. An inner cylinder supports the top step setter. Got it! For some shapes and very large cylinders, a refractory solid support cylinder is used. In this case, the support maybe placed right on the kiln deck. A hole is cut in the base and bottom step setters to accommodate the refractory shape. To protect shapes from shrinkage, drag, and slump, you must design the firing fixtures for every affected part. Sometimes bubble alumina comes in handy. Aluminum silicate wool such as Fiberfrax® can be useful for support at fine china  bisk temperatures and lower. For a discussion of ceramic kilns go to Ceramics: Industrial Processing and Testing , John T. Jones and M. F. Berard.

Section 5 Tables Table 5.1 Defects from Bisk Firing Operations Distortion

Plastic formed ware may have to be supported during low or high bisk  firings. Refractory setters serve this  purpose.

Drag

Foot distortion caused by a part dragging on kiln furniture or  refractories. Bubble alumina, alumina sand, etc., can reduce drag but it may require a setter between the ware and the kiln deck.

Slump

Usually caused when the firing is near   the maturing temperature for the body especially with vitreous bodies. Support setters are required (see text).

Page 49 of 82

Flame Impingement

Flame impingement can cause cracks,  blisters, and distortion. Sometimes a simple burner adjustment will alleviate the problem. More air to the burner can cool the burner temperature.

Scumming

Caused by impurities such as vanadium. See raw materials table above.

Bloating

In late stages of firing after the body is sealed, clay or other materials are still decomposing forming internal bubbles. Keep the firing oxidizing and control time / temperature stages. Over-firing also can cause bloating.

Black Coring

Oxidize carbon compounds for   removal before the final stages of  firing. Can be exasperated by iron compounds causing early melting. Provide good air circulation around  parts. Thick sections take longer firing times if carbon is present.

Lime Pop

Too large limestone particles can cause  blow out. A white particle is often seen in the “moon-shaped” crater. Screen out large particles and tell supplier of  your problem.

Delayed Crazing (See Next Section)

Hard to overcome by just glaze adjustment. The body must be sealed from moisture absorption by reducing  porosity and closing porosity with glassy phase. Moisture expands the  body placing the glaze in tension. Time delay reaction. May take years.

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CERAMIC AND POTTERY DEFECTS 6: DEFECTS GENERATED DURING GLAZING AND GLOST FIRING OPERATIONS In this segment of Ceramic Defects we discuss glazing operations and how to prevent certain glaze defects by formulation before they can happen. We also describe the common glazing operation defects and what can be done to remove or eliminate them. A glaze is a specially formulated glass applied to ceramics. In the case of electrical insulators the glaze must maintain specific electrical properties even in inclement weather. For chemical porcelain it must have high chemical durability. For fine china, porcelain, and semi-vitreous china, it must have high gloss and be resistant to dish washing chemicals. In all cases it must be resistant to thermal shock. For my notes on glaze formulation see below: Notes on Glaze Formulation and Firing. The properties of a glaze are determined by its chemical composition. Potassium feldspar is a single-component high-temperature glaze for chemical porcelain. Lead is a component of fine china glazes because it “fixes” many of the application and flow problems of glazes and adds high gloss. Lead is not used as it once was, in the white lead form, which was desired in dipping glazes. All most all lead is now contained in frits. A frit is a special glass used in compounding glazes. It ties up toxic and soluble materials and sometimes coloring oxides. Clays and insoluble oxides and or  carbonates may be added to the frit to form the glaze composition. To learn how to formulate a glaze, frit, or ceramic body, see Ceramics: Industrial Processing and Testing, John T. Jones and M. F. Berard, Iowa State University Press. While not all ceramic bodies are ground, all glazes are ground to a specific particle size. Over grinding can cause crawling of a glaze. The crawling of the glaze from the ceramic substrate usually does not show up until firing. Firing does not always fix this particular application problem. Sometimes binder additions li ke PVC or dextrin can reduce crawling. Read on for more information on crawling. The glaze is sprayed on to the ware in most cases. Many shapes are dipped. Have you Page 51 of 82

ever wondered how the innards of a toilet bowl can be glazed? A Nerf Ball® is soaked in glaze and sucked through the trap. In the   porcelain process the body is porous after a low-temperature bisk fire. Therefore it is easy to dip. This process is automated for dinner plates and such in modern factories. In the china process the ware is vitrified and it must be heated during the glazing  process. This is usually done by burners in the first section of the glazing tunnel. Glazes can also be applied in powder form by electrostatic spraying. This is a good way to put enamel on household appliances. I have little experience in this area but I have seen such glazing machines. I might mention that in the zillion tile factories I have toured in Italy (and one in Columbia, S.A.) a water fall works well for glazing. The tile industry has many clever  ways of glazing and decorating the tile as it moves down the line. I’ve learned that  people are generally not required in modern tile factories. Everything including lunch  break is automated. Typically most glazes have a small amount of montmorillonite. This can be a refined synthetic white material in glazes for fine china or Wyoming bentonite. Methocellulose, dextrin, and gum Arabic are also used as binders. Crawling  is a  problem that can be prevented by using a proper binder system and avoiding overground glazes and overly-thick glaze application. Crawling usually appears during the firing process but it can occur on drying of the glaze. Some glazes tend to craze after firing. This can occur during decorating operations or sooner or much later with time. One form is delayed crazing that occurs when a  porous body (semi-vitreous ware) absorbs water which can expand the body, stretch the glaze, and crack the glaze. Sometimes adding nepheline syenite or talc to form more glass in the body, adding silica to raise the body expansion, or other body adjustments can prevent this. Crazing during decorating operations indicates that the glaze needs reformulating. This may be done by simply adding a small portion of silica or low-expansion frit to the batch. Changing the cooling cycle during the decorating process can help. I’ve Page 52 of 82

seen large shapes crack on cooling during glost firing, sometimes crazing the glaze. This is called dunting .The solution was always in controlling the cooling cycle especially though the silica conversions. ( See the reference.) It is interesting that silica can raise the thermal expansion of a body and lower the thermal expansion of the glaze. Can you figure out why? [Answer: the silica is in crystalline form in the body and in the vitreous form (or very low thermal expansion form) in the glaze.] For all practical purposes the thermal expansion coefficient of the glaze must me lower than that of the body. This keeps the glaze in compression after firing. Glazes always fail in tension. Keeping the compressive forces higher than the tensile forces is the key. Silica and boron compounds can lower the expansion coefficient of the glaze. Replacing alkalis with alkaline earths can help. There is a thing I call multiplicity in glaze formulation: The more different materials used in compounding a glaze, the  better. It minimizes the negative effects of a particular material while retaining the good effects. (Anyway, that is my theory and I’m sticking to it.) Kiln contamination can be a problem in some operations. Pits, pin holes, and pocks caused by impurities dropping on the glaze during firing can be removed by grinding. The ware is resprayed with a thin coat of glaze and then refired. The thickness of a glaze before firing is always a concern so you don’t want to over do it. Keep your kilns clean. This should be a routine for the kiln loaders. Pinholes can be caused by body contaminants. If body impurities are still releasing gases on refiring, your problem may not be resolved until you fix the body problem. There is a discussion of glaze defects at: http://www.ceramicstoday.com/articles/122000.htm . One of the defects discussed there is shivering caused by too low a thermal expansion coefficient for a glaze. In all my days I’ve seldom seen this except in the laboratory. The problem is almost always the opposite (crazing). Anyway, what can be easier  than raising the thermal expansion coefficient of a glaze? Page 53 of 82

Notes on Glaze Formu lation and Firing

I hesitate to write an article on glaze formulation when th ere is so much on the Internet. Still, there are a few generalities that might be useful. Some years ago I categorized many glaze formulations accor ding to firing temperature and surface finish. I did this for lead and non-lead glazes both glossy and matt. I was amazed how easy it was to correlate composition with firing temperature (cone or B uller ring). You might run the same type of exercise if you are look ing at many glaze compositions trying to come up with just one. You can start your  formulation activities using the averages of the ranges of comp osition for  each oxide addition for the temperature range you are shooting for. Also my associates formulated hundreds of glazes over the years for  earthenware and fine china. Much of this work was done in a joint effort with frit suppliers as existing frits don’t always do the trick in large  production operations. Here are some factors that I found to be important: Glaze Composition

Lead is magic in glazes. It must be used in fritted form and in t he minimum amount possible to maintain the glaze flow in the molten state. The glazed ware must pass all FDA and other restrictive testing. If lead is allowed, your problems are minimized. Your workers must be monitored for lead in the blood if lead air levels exceed OS HA standards. Bor ate frits can help reduce lead conte nt but may increase solubility.

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Complexity of formula is important. Every element you add to a glaze impacts the final-glaze properties. For example, too much alkali content will increase the solubility of a glaze, raise its thermal expansion, and in general raise havoc. A minimum amount of alkali content will give the glaze fusibility. Also, individual alkalis act differently in exten t. Therefore one would use more t han one alkali and the minimum amount of each that yields the best balance in properties. The same is true of the alkaline earths, the glaze modifiers, and the glass formers. You should consider the entire periodic table of the elements when formulating a glaze. Remember that a very small addition of a particular element might give you a property you want, but increasing the amount only slightly can ruin your   progress. One other thing: You should use the minimum amount of binder. Sometimes it’s best to use several binders in small amoun ts rather than just one binder. Binders can be purified clays or organic compositions like gums or resins. Like I say, a mixture is usually best. None would be better. To learn how to formulate gazes from scratch or using frits you can find it in my book  Ceramics: Industrial Processing and Testing . Use http://tinyurl.com/gbxbz to obtain a new or used copy at Amazon.com. You don’t need the second edition if you are only interested in glaze formulation. Firing Conditions

The firing curve naturally has heat up and cool down periods. Page 55 of 82

In between these slopes is a flat or modified soak ing time/temperature. During heat up, the b inders are removed from the glaze. They must be completely removed and they must not be reduced to carbon during heat up (preheat). This does not mean that you can’t approach reducing conditions. Some compounds like MnO and FeO can greatly improve melting although they are often in the glaze only in tiny amounts as impurities. These compo unds do not form under oxidizing conditions. Anyway, do not reduce the binders to carbon. It’s near impossible to remove for many glazes during the rest of the firing. There are a number of instruments to determine the nature of the burnout for a particular binder. Thermal gravimetric analysis (TGA) and Differential Thermal Analysis (DTA) come to this old head . The preheat or heat up is usually a production standard for tunnel kilns. Therefore if you are not getting what you are looking for, you must adjus t the burners or heating input in this zone. You don’t want bubbles from   binder forming all during the soak period. You have enough bubble  problems w ithout that. During the soak phase and at all higher temperatures the glaze is changing composition. That is, the glaze is losing volatile elements because of the high temperatures. This can be complex as when one component of a glaze is removed it may increase the volatility of other comp onents. For you chemical engineers, this would be somew hat like steam distillation. When you lose volatile elements you also increase glaze viscosity in the Page 56 of 82

molten state. For this reason, and others I suppose, fas t firing is often better  than a long firing cycle. In fact, I’ve seen glazes that required fast firing. Cooling is important to both the glaze and ceramic ware. You can cool quickly to just above the silica phase transition and then cool slowly through the transitions. This may not be g ood for the glaze wh ich is trying to get rid of the bubbles created after the glaze melted. This can put you  between a rock and a hard place. You have to be able to con trol the whole cooling zone of a tunnel kiln from the minute the ware come s out of the hot zone.

Section 6 Tables Table 6.1 Defects from Glazing and Glost Firing Operations

Rough Foot-unglazed

If the glazed ware is fired on refractory setters glaze may cause sticking during firing. The solution is to clean the foot carefully after glazing, often using a sponge belt. The foot can be waxed prior to glazing. This can  be done by melting paraffin in a flat electric frying pan with a sponge bo ttom. Waxing belts are also available. After firing, the foot should be checked for  roughness and smoothed using a  burnishing stone or a foot burnishing machine.

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Rough Foot-glazed

If the foot is glazed, pin-firing is required. Small refractory spheres are often used instead of ceramic or metal pins. After firing, the foot should be checked for roughness and smoothed using a burnishing stone or a foot-burnishing machine.

Pinholes

Pinholes can be caused by overfiring, impurities in the glaze, or contaminants in the body. Closed setters can be used to  protect the ware from flame impingement during firing. (See  blisters.)

Spit Out

Occurs in decorating operations. It is often due to moisture trapped in earthenware or in fine china if the  bisk wa re was und er-fired. Pits left from spit out can c ollect dirt. Some refire earthenware before decorating or a small hole is drilled in the back of the ware to allow moisture to escape. Aged earthenware is more prone to this default and sho uld be re-fired. Decorators are advised to  purchase vitrified china rather  than earthenware blanks for  decorating or refire may be required. Page 58 of 82

Cut Glaze (also see crawling)

Often from dipping operations. The glaze is very thin or  nonex istent on some areas of the ware because the w are failed to take up the glaze. It can be caused  by dirt, grease, non uniform vitrification due to soluble salts, oil, perspiring fingers, or water. Over-grinding of the glaze or  mechanical handling of the ware causing knock-off  may result in cut glaze. Binder additions can help reduce cut glaze.

Starved Glaze

Loss of shine due to overfiring or   too thin glaze. Can be caused by loss of glaze ingredients during firing.

Sucked Ware

Dullness of glaze near cranks or   setters. The porous refractory is sucking up the glaze constituents. Use vitrified cranks or coat them with a suitable surface sealing wash.

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Droppers (Kiln Drip)

Glaze drip from the roof of tunnel kilns. (We had a similar problem using a kiln that had previously  been used to calcine fluorite or  calcium floride.) Kiln ve ntilation may help. Refractory coatings to increase the viscosity of the glaze so it does not drop may help. Protecting the ware with setters or  cranks is a sure but inefficient way to protect the ware.

Sulfuring, Staring, Feathering

Surface patterns cause by combustion pro ducts at the end of  firing forming calcium and or lead sulfates that do not dissolve in the glaze but crystallize on cooling into needle-like shapes. Avoid high sulfur fuels and keep good circulation in the kiln. Aired ware is similar but caused by the formation of calcium or zinc silicates. It occurs during the crystallization period of the glaze typically 700-850oC. Almost always caused by accidental kiln firing cycle disruption. I’ve never  seen it.

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Stuck Ware

 I hate it when this happens. The glazed ware sticks to the kiln setters or kiln furniture. The cause is glaze on the foot or glaze flowing to the foot area. This can  be caused by overfiring, too-thick  glaze application, or too fluid glaze. Setter washes can reduce the trauma. Ugh!

Specking

Another irritating fault. Cobalt dust floating in the air can put tiny  blue spots on glazed (or bisk) ware. Color contaminants can occur in decorating operations. Possible contaminants mus t be controlled and confined. Chromium oxide is another  contaminant to watch for. A little dust goes a long way. Reference 2 says that even birds in the beams can drop dust (and you know what) onto the ware. A blast of air  on the ware before glazing or  decorating can remove unwanted dust. Plant as well as k iln cleanliness is absolutely essential in ceramic operations. Vacuuming should be done with care and absolute filters should be used where warranted. And don’t for  get to maintain the filters on your  air lines. Page 61 of 82

Delayed Crazing (Moisture Expansion of Body) See Crazing below.

Hard to overcome by just glaze adjustment. The body must be sealed from moisture absorption  by reducing porosity and closing  porosity with glassy phase. Moisture expands the body  placing the glaze in ten sion. Time delay reaction. May take years to occur.

Blis ters (se e also Eg g S hell)

To o fa st firing or in com plete firing. Non oxidation of carbon compounds from organic binders due to too little heat in the preheat zone and or non-oxidizing conditions. Impurities such as limestone or SiC from grinding or  clay storage operations. Hardening of the glaze may help.

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Bubbles (in general)

Egg Shell (see also Blisters)

Bubbles in glazes degrade the  properties of the glaze and cause many glaze defects. Knife marking and kn ife cutting  problems are enhanced by  bubbles. Electrical, mechanical, and chemical properties suffer. Modification o f the body (including the removal of  impurities) and / or the glaze may help. The grinding and application of the glaze are important. Do you have soluble salts migrating to the surface? Add barium carbonate or  chloride to the slip. You might also want to check your molds in slip casting operations. Are they too dry? Is there oil on the surface of your ware or in your glaze. Clean air filters and add oil traps. What about fermentation? Does your glaze tank stink? Is something decomposing? Do you see bubbles forming in the slip? Failure of blisters to heal due to underfiring or too-hard glaze.

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Crazing

Cause by a glaze / body mismatch where the coefficient of expansion of the glaze is higher than that of  the body. Addition of silica or a low-expansion frit usually corrects the problem. An addition to the bod y of silica (at the expense of plastic clays and or  feldspar) of 5-10% will also help correct the problem. (Silica has low expansion in glaze becau se if  is fused. It has a higher expansion in the body because it is crystalline.) Increase ball clay at the expense of china clay. Underfiring of earthenware or  underground flint reducing the formation of crystobalite can be a cause of crazing.

Shivering or Peeling

Caused by the opposite of the above. The glaze is in so much compression that it shivers off. Curved or sharp surfaces enhance the effect. This is not a common default and is the e asiest fault to resolve simply by increasing the thermal expansion coef ficient of  the glaze. Simply lowering the flint content (or grinding the flint coarser) is the common cure. A more fluid glaze also can reduce  peeling. Page 64 of 82

Pinholes, Blisters, and Crowfooting in Engobes

May appear before firing. Check  the composition and properties of  the slip, slip applications, and firing procedures. SCROLL DOWN

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Crawling

Crawling of the glaze from the substrate can occur before or  during firing. During firing viscous glaze can pool and overcome surface tension forces  binding the glaze to the ware. Over-grinding can increase the tendency to crawl due to the formation of colloidal factions that tend to shrink more crack ing the glaze on drying. The glaze formula may have to be changed to reduce or increase the f ired viscosity of the glaze. Too-fluid glaze may extract silica and alumina from the b ody d uring firing changing the glaze composition. Fine clays can cause excessive drying shrinkage. Opacifiers and some underglaze colors can cause crawling. Ware should be clean before glazing. The glaze thickness must be controlled to about 1 0 thousands of an inch or so. Too-thick glaze crawls. Binder additions can help crawling. About one-tenth percent of dextrin often does the trick. PVA can also be used. Methocellulose in small amounts may also help but it dries slowly and may cause trouble in Page 66 of 82

Kiln Dirt, Spits, etc.

Dirt from refractories and setters can be removed by us ing small hand grinding tools. The ware can  be touched-up and re-glazed.

Table 6.2 Customer Complaints on Glazed Dinnerw are Rough Foot (see above)

Suggest the use of doilies between  plates during storage. Use a  polishing stone to remove roughness. Replace w are if  required to keep customer happy. Send a brochure on h andling ware. Rubbing one plate over  another can scratch the glaze. See also  Knife Cutting .

Knife Marking

Rough Glaze marks easily, especially those with ZrO 2 opacifier. Sometimes the markings can be removed with Soft Scrub® from the Dial® Corporation. Improving glaze flow during firing can smooth the surface.

Knife Cutting

Steak knives are very hard metal compositions and can easily cut into soft glazes. Hardening the glaze usually means higher  temperature glazes. Fine china manufacturers should provide a  brochure on the care and handl ing of fine china.

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Gild Rub Off

The ware should be regilded if    possible and returned to the customer.

Fussy Complaints

Some customers are more picky than others. Inform your customer  as to your production control  procedures and control limits. What size speck to you allow? What size pinhole do you allow, etc? If the ware is out of your  control ranges, repair or replace the ware.

Table 6:3 Thermal Shock and Autoclave Testing

Thermal Shock  Testing

Determine the ambient temperature of the water placed in a tub. Maintain this ambient temperature throughout the test. Place the items to be tested (usually 6) in an ove n and heat them long enough to be u niform in temperature. The first temperature shou ld be about 50 degrees F above the water  temperature. Using the same procedure every time, slide each piece into the water. Check for  craze lines. Check for cracks in the p iece. Use ink or die penetrant if required to find the cracks. If no cracks, raise the ov en temperature 50 degrees F. Repeat until failure by crazing or  cracking. If your ware can pass a test of 300oF to 80oF water, you may be okay in the field. 400oF would be better. Page 68 of 82

Autoclave Testing

Dinnerware should pass a 150 psi controlledtest autoclave pressure. Tile usually passes 200  psi. The heating and cooling to and from  pressure should be gradual. If your ware does not pass the test, you may have a delayed crazing problem due to moisture expansion. Sometimes just soaking ware in ho t water will cause crazing due to moisture expansion o f the  body. Earthenware bo dies are particularly  prone. Earthenware will sometimes craze during decorating operations. In that case, chang ing the firing cycle doesn’t always help.

Hot Water Test

Sometimes ware will craze when soaked in hot or even cold water for 24 hou rs. In this case, the body p orosity needs to b e reduced and the glaze expansion may have to be lowered. Sometimes a small talc addition to the body will help seal off the porosity.

Table 6:4 Special Crazing Problems on Firing & Dunting Crazing During Glost & Decorating Fires (See  Dunting  below)

This sometimes occurs, especially with earthenware and the body may crack too. The silica transition points sho uld be put in suspicion. Quartz conv erts abruptly at 573oC with a density change of 2 .65 " to 2.30 $. Kiln control may help but not always from my experience.

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Dunting

Cracking of ware caused by stress due to silica  phase transitions during the firing and c ooling  process at slightly over 400 F and ag ain at slightly over 1000 F. Glaze often crazes during this process. Dunting is not always evident immediately upon removal from the kiln. It sometimes occurs as much as a m onth or more later but it should not be confused wi th cracks caused by "thermal shock". Glaze fit can be a factor.

TO C

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CERAMIC AND POTTERY DEFECTS 7: DEFECTS GENERATED DURING DECALING OPERATIONS In this section of Ceramic Defects we discuss decaling. Two internet sources describe the current processes. The loss due to decaling is not high in the ceramic industry however the decal can often not be repaired after firing. Some decals incorporate enamels and precious metals eliminating further firings. Ceramic decals have been used for decades to enhance the value of ceramic products. Hand painting using colored glazes (enamels) is more expensive and the production rates are much slower. Decals can be produced by silk screening or by lithography, the former being more expensive. The process to make a decal is to add colored oxides to drying solvents and binders to form an ink. Low-melting frits may be incorporated in the inks to assure that the decal will sink into the glaze on firing. The colors are printed or silk screened each color individually onto a special decal paper. A cover coat is applied to the printed decal sheet before the final drying of the decal sheets. Go to http://www.beldecal.com/create.cfm to learn the actual details of the process. (It’s a pretty stinky process. Take a sniff while you are reading.) Several color separations are used. For fine china, seven passes through the silk  screening process are often required to obtain the color detail desired. Three-color  separations are okay for many other products.  Now I am going to send you away again. Richard Wasowski, a long-time friend of  mine, (don’t call him “Dick,” please) wrote an article for  Ceramic Industry  Magazine (I was once the editor of this magazine) on the digital process for  decaling. Just click on the Ceramic Industry link. You will need to register with the site for free.

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This newer process will probably someday completely replace the older methods. (You can also see the speed and utility of the digital process at: http://www.designpoint.com/custom_digital_ceramic_decals.htm ) Oh, you came back again! Decals are typically printed on large sheets. A worker cuts the sheets into individual decals and places them into water (usually with a wetting agent) where they soak until another worker slides the decal off the paper and places it on the ceramic ware. He or she then squeegees out the water making sure that the decal is placed correctly on the ware and that there are no bubbles trapped under the decal. The decal is dried, usually in open air, and then fired on a 1-3 hour cycle. I’ve seen decals fired in 15 minutes or less in England. Decaling is a low-loss process. If there is a flaw in the decal after firing the ware usually must be scrapped. It may not be profitable to continue processing a piece of  ware that you know will be sold as seconds.  Now days, enamel and or precious metal are often incorporated in decals. This saves two extra firings if both enamel and precious metal are added. If precious metal is added separately, it still saves one firing. This can be very cost effective. For one thing, you are not wasting gold or enamel nor do you have to worry so much about environmental considerations. There are usually problems getting these decals working in your factory but close liaison with the decal manufacturer should resolve  problems with test decals before making a production run of decals. SCROLL DOWN TO TABLE

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Section 7 Tables Table 7.1 Defects from Decaling Operations

Improper Decal Application

Train ed w orkers mu st apply decals to ceramic ware. The must  be supplied with clean water  containing a wetting agent and squeegees to remove the air   pockets from under the de cal.

Excessive Decal Sink during firing on overglaze deca ls.

The problem can be the glaze or  the decal. Soft glaze, too-fluid at the decaling kiln temperatures, will allow the decals to sink too much. Too much frit in the decal can cause this problem. Glaze control is very important and the viscosity of the glass must be controlled. The decal supplier can solve the other problem. This should be don e with test decals  before production starts.

Too Little Decal Sink

Opposite problem. The decal is not down into the glaze and the situation is easily recognized by moving your fingers over the rough decal. See above for the solution. You can soften the glaze or add more flux to the decal.

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Color Problems

Some times you will see a change in color as you go through a batch of decals. Call your supplier. Remember that firing conditions can effect the color.

Deco Kiln Atmosphere

An addition of about 4% moisture to the deco kiln atmosphere can improve decal sink and reduc e the emission of lead and cadmium from the decal and glaze.

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CERAMIC AND POTTERY DEFECTS 8: DEFECTS GENERATED DURING ENAMELING OPERATIONS In this segment on Ceramic Defects we discuss decorating processes using stains, enamels, or colored glazes. Some processes have been automated which means that the control standards for enamels must be maintained. Color is what sales ceramics. Okay, I lied. Shape is more important. But color is very important and is used under-glaze, over-glaze and in-glaze. If the color sinks deep into the glaze on firing, the process is called in-glaze. Many manufacturers use pad printing or silk screening to apply colors and enamels to bisk or glazed ware. Stamping, bulbing, and hand painting are also used. Go to: http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP__Feature s__Item/0,2710,8117,00.html for more information on specific decorating processes. Some stains are more reactive than others. Iron and manganese compounds tend to  be so. Sometimes a frit is added to the stain to increase the reaction with the body or  glaze. Stains may be encapsulated. This is tr ue of cadmium colors, for example, that are tied up in zirconium silicates. This prevents leaching of the cadmium from food ceramics. Grinding can destroy the encapsulation. If the stain is made into a paint or paint, as in the normal method, it is called enamel. Stains can also be applied in powder form. With tile, a binder pattern is silk screened onto the ceramic. Then stain or colored frit in powder form is dusted onto the tile and shaken off. When the ceramic is fired, the binder pattern is reproduced in color. Hand painting is another method for applying color. It can be automated. Remember  that bond signers on Wall Street in New York sign fifty or more bonds at a time. The movement of a painters hand can be recorded and then transmitted repeatedly to servo motors that drive the brushes that do the actual painting. They can even wipe the Page 75 of 82

 brush. Bulbing can also be easily automated. (Well, maybe not that easy. It took some engineering for us to do it in two of the companies I worked.) The Inkjet process can be used to apply colors but there are restrictions as to the amount of color that can be deposit. At least, that is what I last heard. The British Ceramic Research Association has put a lot of effort into this area. The many variations in color formulations, and also in the application processes, can lead to defect generation. That’s what manufacturers don’t want. However, once a  problem is resolved in a process, it usually doesn’t come up again. For example, say that a formulation error in the color causes crawling, faint or off  color, running, etc. Correcting the formula eliminates the problem forever (usually). Close control parameters must be adhered to by both the color supplier and the customer. Color meters are not always the complete answer. I preferred to use standard color standards. After instituting this, the design department could just give us the color number which we could match without argument. Most people have eyes that are more color discriminatory than any color meter. That’s the reason why some scientists believe in God. Colors can be purchased in enamel form so that no preparation is needed at the factory. Often, the pigments are purchased and then frits and binders are added as needed at the factory. The particle size of the prepared enamel must be controlled as must the specific gravity and the viscosity. A hand painter can adjust for variations in these properties but a machine can not. Drying is the usual process before firing. However, some enamel can be cured much faster by using ultraviolet light. You must purchase the u.v. light-sensitive resins. Infrared lights can dry regular enamel. More heat might be required in automated enameling and glazing lines. Enameling on decaled ware requires skilled workers or precise machinery. Errors are easily corrected before firing. The enamel can be removed and you can start over. After firing, forget it! Page 76 of 82

Enamels are usually fired on decal or glazed ware in a 1-3 hour cycle. I didn’t mention that stains can be applied to green ware by stamping, bulbing, or  hand painting. Well, I forgot.

Section 8 Tables Table 8.1 Defects from Enameling Operations Enamel Batch Related Problems

Colors must be ball-milled, roll-milled, screened, and the proper amount of binder  added according to the enameling operations. Banding machines, bulbing machines, handdecorating operations, pad printing machines, etc., may each need a particular viscosity and specific gravity control.

Color Variations

The use of ceramic stains rather than raw oxides can help this problem. Firing conditions are also factors.

Complex Decals

Colors are often printed on decals along with  precious metals. This eliminates one or two firings. Testing of such decals is absolutely essential before purchasing for production.

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CERAMIC AND POTTERY DEFECTS 9: DEFECTS GENERATED DURING GILDING OPERATIONS In this last segment of the Ceramic Decorating Defects series we touch on the  problems of gilding. The fusibility of the glaze is critical. Too soft glaze means hard gold. Too hard glaze means soft gold. Soft gold rubs off. Hard gold will not burnish  properly. The secret is glaze control because the gold emulsion supplier is not going to let you know how he formulates his gold emulsions. However, he will work with you to help resolve problems. You are technically helpless in some ways when it comes to gelding. You are at the mercy of your gold supplier. Preparations of gold emulsions used in ceramic decorating are prepared in secret. Any technical problems relating to the emulsions used must be resolved by the gold vender. Also, the emulsion suppliers often adjust emulsions to resolve problems not caused by regularly used emulsion formulations. Gold and other precious metal emulsions are used in the ceramic industry to add value to a product. This is usually in the form of banding or hand painting. Banding can be by hand or by machine. On firing, the gold must sink to the proper extent. It is called soft if it easily abrades off with a pencil eraser. If it sinks too far into the glaze and looses its ability to be buffed to a fine polish, it is called hard. Hard gold can be caused by soft glaze. That is the glaze is more fusible than usual. The decorating supervisor tells the lab that the glaze is soft, lowers his decorating kiln temperature, and hopes the next lot of glazed ware will not cause him more problems. Soft gold from hard glaze causes him to raise his decorating kiln temperature. This may cause blistering of the gold or other problems. The gold emulsion supplier can make the emulsion more suitable for higher firing. So the problem is glaze control. Why do you have variation in the hardness of the glaze? Remember that we are talking about fusibility here. Most fine china glazes are lead based. Lead bisilicate is a lead frit commonly used. It is make in large batches and the specific gravity of each batch is different. Therefore the laboratory at the factory must calculate the lead content of a glaze made with a particular frit batch. If  the lead content is too high, then the lead bisilicate must be cut back. If the calculated lead content is too low, it must be increased. Page 78 of 82

The best control for the glaze itself is the FIRED specific gravity. Fire a cup of glaze,  break out a chunk, and measure the specific gravity. Use this as a control procedure. To maintain better control, blend frit batches before glaze formulation to even out the composition. Work with your suppliers to standardize their materials. Blend glaze  batches to get the right glaze hardness.

Section 9 Tables Table 9.1 Defects from Gilding Operations

Too Much Sink

The control of the glaze is essential in gilding operations. If  the glaze is too soft, the gilding will be sink too much. You can see this but check it out with a rubber eraser. The gold, or o ther   precious metal, will be very hard. The precious metal solutions can  be adjusted by the su pplier or inhouse by using a harder frit in the metal solution or by reducing the amount of the softer frit.

Too Little Sink

Hard glaze can be the problem. The precious metal will easily come off with an eraser. Soften the glaze or harden the precious metal solution.

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Complex Decals

Complex decals containing color   and precious metal should be tested and problems resolved  before purchase for produ ction operation.

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CERAMIC AND POTTERY DEFECTS 10: STATISTICAL PROCESS CONTROL (STP) The object of   Statistical Process Control is to refine and re-engineer    processes to prevent losses and to keep processes in control within specified statistical limits. The cost of SPC must be weighed against   benefits. I have made a list of books describing the benefits and management of SPC program. Click on the link below to see the list available at Amazon.com. SPC Book List STP or Statistical Process Control is now common t hroughou t the ceramic industry. Workers and management contribute to this effort. If you are not familiar with STP yo u can take a course o nline at: www.flexstudy.com: http://www.flexstudy.com/catalog/index.cfm?location=des&coursenum= 9561a The course ou tline states that you will: Learn How To: ! ! ! ! ! !

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Apply statistical thinking to quality improvement Use statistical tools to analyze a wide variety of daily work problems Select appropriate control charts for different applications Interpret variability in work processes Construct a histogram of set data Develop and analyze run charts and c ontrol charts for variables and attributes data Quantify process cap ability through data ana lysis and control charts Compute and interpret process capability indexes Page 81 of 82