Electric to Gas Kiln Conversion Undergraduate Research in the Arts Kyle A. Crowder May 16, 2014
Warren Wilson College Undergraduate Research in the Arts 2014
Introduction In my research I am looking at the feasibility of converting an electric kiln into a gas fired salt kiln. The feasibility will be based on the expense of labor and money, and also the product that is achieved through firing it. Depending on the functionality of the kiln, I will further my research with experimentation in surface treatment and glazing for this specific type of firing. I am very interested in the surface created by a salt glaze in terms of effects and aesthetics. I hope to see this inform the work that I create now and for my senior thesis project Background Many ceramic vessels we use are fired. This simply means that the clay is heated in an “oven” for a given amount of time until it is vitrified1. This is not the only method of firing to achieve this type of arrangement for functionality func tionality but the majority of functional ceramics use a glaze to create glassy surface that is smooth and clean for ease of use. One can think of a kiln like an oven because kilns burn natural gas, electricity, wood, or even oil. Each fuel has properties that change the effects one can create. For my research I am focusing on natural gas as my fuel. The benefits of this fuel are that it is reliable and consistent compared to wood. It is a Hydrocarbon2 so it allows for control over the redox 3 of the
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atmosphere. The costs of the fuel are dependence on a fuel company and gas line repair can be dangerous and should be done by a professional. The kiln that I have built is a 10 cubic foot updraft4 cone 10, natural gas burning atmospheric5 salt kiln. Cone 10 refer to the specific temperature to which one is trying to reach to vitrify the clay body, for the porcelains po rcelains and stonewares that I use this is about 2350 23 50 degrees Fahrenheit. For my project I am working only with salt, or sodium chloride as my flux. This salt added to the kiln when it is almost at peak temperature creates a glaze on bare clay. A glaze is composed of mainly three parts. There is a glass former which is silica or flint, a flux that lowers the melting point of silica and a stabilizer that bonds the surface of o f the clay to the glaze and keeps the glaze suspended on the pot. These three parts are present in all glazes in some shape or form. There are many man y other parts that are not essential like colorant, stains, secondary fluxes, and opacifiers. Glazes are typically applied to the pot by dipping or spraying a liquid mixture of the compounds. The purpose of the flux in the atmosphere allows one to leave parts of a pot unglazed when it goes in the kiln. The flux in the atmosphere replaces the flux that a glaze would contain; the other two parts of a glaze are in the clay, a stabilizer in the form of aluminum oxide and silica in the clay body. The flux acts on the surface of the pot and creates a glaze. Once one fires a kiln with these fluxes, it is forever going to have residue that will affect future firings. I am
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experimenting with only sodium chloride that is in the form of Morton’s Canning and Pickling Salt. This glaze is hard to describe in terms of color, it is typically a brown or orange o range color with stoneware clays do to the iron in the clay, and with porcelain p orcelain it is typically white because there is no iron present in the clay. c lay. For simplicities sake the glaze will take the color of the clay body because the clay body makes up two-thirds of the glaze. Salt glazed pots first appeared in Germany during the late 13th or early 14th centuries. (Rogers 2002) The technique is thought to have been a mistake by using old barrels which contained brine in a wood kiln. The salt from the brine would have deposited a strange texture most likely considered a mistake. Since then salt glazing has continued to spark interest in potters across the world. When building kilns it is important to know the difference between different bricks because they are made for different jobs. Hard brick or high fire brick is used as a face brick, meaning it is lining the inside of the kiln. This brick is more resistant to deterioration from the atmosphere. Soft brick is the insulating brick. It is less dense and holds heat better but does doe s not stand corrosion for very long. This is where the decision to use an electric kiln for an atmospheric kiln can become problematic. Electric kilns k ilns are only made out of soft brick so to get around this one can apply a kiln wash6 over the walls to protect them. Construction
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The construction design was based on William Baker’s design, Figure 1, a kiln builder and soda fired potter in Penland, North Carolina. It starts with a layer of cinderblock on a level surface. Then bricks are stacked to create a firebox7. The electric kiln is gutter of all elements e lements and electrical wiring and place on these bricks. b ricks. With all of this together you end up with Figure 1
about 11 cubic feet of space in the kiln. There
are two holes for the burners and on pull out brick that will act as a salt port. The lid from the electric kiln has a 9x9 inch square cut out for the chimney. A damper was constructed out of steel to allow a18x24 inch kiln shelf to slide over the hole to act as a damper to
Figure 2
the chimney to control the flame. These types of dampers work very similarly to ones one s for wood stoves or fireplaces. Here you can see the flame’s hypothetical path through the kiln. The shelf right before flue is there to help recirculate the flame. This creates a similar effect to what a down-draft kiln would have. It would usually have burners near the bottom of the kiln and the chimney port would also be at the floor level. The flame then, based on brick arrangement would have to go up Fig. 3
the wall, into the middle, through all a ll of the pots, and then out the bottom. This typically is more even in terms of temperature. With a C
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kiln this size the most important part is arranging the shelves to allow an even distribution. I have found that staggering the shelves dramatically increases the e venness of the firing. The next step was the natural gas and burners. I contacted Mark Ward of Ward Burner Systems in Tennessee and laid out the right burners I needed to fire this kiln in about 12 hours. I built an adjustable stand for the two MR-750 Venturi burners8. These are controlled manually with ball valves and a Safety Shut-off valve9. This is the most expensive part of the whole set up. Natural gas can be a dangerous if the right precautions are not taken to ensure that everything is done correct and safely. The last step was the chimney and damper set up that I designed to be done very W$47&/ O
cheap and quick. We had been given a quote of about $2000 to install a stainless steel insulated chimney through the roof. I could not see that much money go to something that was relatively temporary. It is import to think of the scale of a project and make sure that one considers the longevity of a kiln when they are building it. I don’t expect this kiln to last more than 20 or so firings because of the damage the salt atmosphere can do to the soft brick. The chimney I built was made of zinc coated steel piping made for heating and cooling ducts in homes. It’s cheap, only about $30 for all the pieces. The reason I even
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included the chimney was because of the heat buildup and the smoke from the salting under the roof. This is not a great solution, after 6 firings it has mostly fallen apart. The damper is a control of the airflow leaving the kiln. kiln. This in turn controls the draft or pull of the kiln. By not allowing much air to escape through the chimney, having the damper mostly closed, creates a reduction atmosphere in an an updraft kiln because there is fuel that is not n ot being burned due to a lack of oxygen being pulled in the kiln. Open the damper and oxygen is effectively pulled through the burner port and a mostly complete combustion of fuel is achieved in the kiln. It is important to control this because they affect the color of glazes. Oxidation will give you a pale celadon glaze. Reduction can make a copper green glaze, into a copper red glaze. These colors based on the
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different isotopes10 of the colorants based on more or less complete combustion of fuel changing the levels of carbon and oxygen interacting with the colorants in the glaze. Firing the Kiln The first firing happened Monday the 31st it was a failure… It was a hard night/early morning watching the kiln stall at about 1900 degrees F. I struggled to get up the next day and think about trying to do it again. a gain. I stated by investigating the burners. Turned out the orifice was !\
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the wrong size. Because we had found these in the department, this was not a fault of Mark Ward, it was the orifice of the burners I had ha d found that were drilled at about abo ut 3/16 of an inch, built for high pressure propane. I had assumed that because they were used on a natural gas system before that they would work. After realizing this I changed out the orifice to the proper size of 1/8 of an inch. I tried it again on the following Wednesday. I could not even believe it, but it made it all the way up to cone 10 within 13 hours. Price Breakdown The 3-D Department had old hard and soft bricks that I used so this cut my cost down. Hard bricks run at about $3 dollars for a 9x4 ! x3 brick and soft bricks are about a dollar more per same size brick. I used scrap kiln shelves and cinderblocks. The electric kiln was donated by the department but it is not rare to find someone looking to get rid of an electric kiln for free. I also have a background in fabrication so small pieces like the burner stand and damper slide were built for free. •
$370.00 for Safety-Eye automatic gas shut-off valve
•
$500.00 for Natural Gas piping and instillation
•
$38.00 for chimney
•
$100.00 for 3 ft. x 100ft. roll of Kaowool
Total cost to build: $1008 Firing Breakdown After one failed attempt and five successful ones I am very pleased with what has ha s been achieved in a kiln of this caliber. I have honed in on an ideal firing schedule that hopefully will
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grant consistent results in the future. I start the kiln in the morning, candling11 for three to four hours. During this time salt port and peeps are removed and the damper is at about three inches. In the first bump I close the peeps and salt port. I bump the gas to about a quarter of total pressure and push the damper in to two inches. This must be slow because this is a very fragile state for clay going up and cooling. I hold this until cone 012 is totally flat top and bottom. At this point the gas is bumped again aga in to almost full pressure and primary air at a half h alf inch. The damper is adjusted within half an inch usually increasing with the temperature. This lasts three to four hours until cone 6 falls top and bottom. The gas and primary air can be totally opened at this point. Salt is added after cone 9 is half over every 20 minutes or so. However much salt you are adding should be evenly spaced within the last hour or two of the firing. Once cone 10 is half over I shut the kiln off and close it up tightly and wait till it is all the way cool to open it. This is not perfect but it’s a place to start. It should all happen within 12 hours hou rs but it varies on many things. Conclusion Overall through the whole process, ending up with the result I did, I believe be lieve that this is a worthwhile experiment. For only about $1000 I was able to achieve passable salt fired pots. I don’t know how many more firings I could really do in it based on the state of the bricks after 6 firings. I hope that some of the resources and techniques I have laid out help and encourage others to attempt this. It would be perfect for a beginner potter or a school, like Warren Wilson College, without the kiln facilities large schools have.
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Recipes The Kiln Wash is sprayed or thinly painted on the walls of the kiln. This recipe comes from Gay Smith who got it from Tony Clennell whom used it for a soft brick soda kiln in Washington. Tony’s Kiln Wash 2 parts kyanite (sillmanite) 2 parts zircopax (ulltrox) 2 parts alumina hydrate 1 part vee-gum For an atmospheric kiln one needs to wad their pots because the flux in the atmosphere will melt everything together. This will keep your pots from sticking to the shelves. Wading Recipe 1 part alumina hydrate 1 part EPK (Kaolin) Flashing slips can add a great color variation to your work. This can be applied on bisque or leather hard depending on the amount of clay in the slip. Testing is a good idea for these because when I was given the recipes, they said they worked on bisque clay but I found they bubbled and cracked when I did. Coffman Yellow Nepheline Syenite Syenit e 60 EPK 40 Soda Ash 1 Titanium Dioxide 5 Matt Long’s Red Grolleg 70 Nepheline Syenite Syenit e 30 Yellow Iron Oxide 0.5 Titanium Dioxide 0.5 Bower’s Orange EPK 42 OM4 Ball Clay
42
Zircopax
10.5
Borax
5.5
One has to be careful with glazes touching the flashing slips, sometimes they bubble. These glazes I have included work very well and some have been specifically formulated for salt or I have found look really nice. Randy’s Green Flint 16 OM4 Ball Clay
10
Whiting
9
Zircopax
7
Barium Carbonate
13 !\
Dolomite
6
Custer feldspar
21
F4 Feldspar
18
Tin oxide
4
Copper Carbonate
6
Bentonite
2
White/Yellow Salt *(For Yellow Salt add 2% Red Iron Oxide) Nephlyne Synite Synit e
60.7
Dolomite
20
Zircopax
15.2
OM4 Ball Clay
4.1
Bentonite
4
Shaner’s Oribe Custer Feldspar
31
Whiting
22.1
Silica
25.3
EPK
12.6
Talc
7.9
Bone Ash
1.1
Copper Carbonate
5.2
Snowfall Rutile Custer Feldspar
30
Whiting
19
Silica
24
EPK
16.5
Talc
10.5
Dark Rutile
6.2
Copper Carbonate
1.25
Bentonite
2 !!
Resources Olsen, Fredrick L. The Kiln Book Materials, Specifications & Construction. Radnor, Pennsylvania: Chilton Book Company, 1973. Rice, Prudence M. Pottery Analysis; A Sourcebook. Chicago and London: The University of Chicago Press, 1987. Rogers, Phil. Salt Glazing. Philadelphia: University Of Pennsylvania Press, 2002. Wentworth, Becker. General Chemistry. Boston: Houghton Mifflin Company, 1973. Baker, William Converting and Electric Kiln to Soda. Wordpress. http://design.williambakerpottery.com/2009/07/23/converting-an-electric-kiln-to-soda/ Web. Bowers, Bruce. Converting an Electric Kiln for Wood or Gas Firing . Ceramics Arts Handbook. Web.
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