Temperature Events in Ceramic

TEMPERATURA

TIPO

DESCRIPCION

NOTES

50°C-250°C

Dec o om m po positi on on

Hygroscopic w wa ater re rem ov oved in in cl cl ay ay bodies

80°C - 250°C

Decomposition

Calcium Sulphate decomposition

Occurs during heat-up of bodies on first fire Water vapor is expelled. Many clays contain small amounts of calcium sulphate.

120C

Decomposition

Borax

It loses crystallization water.

150C

Decomposition

Epsom sa salts de decompose to to lo lose wa water

180C

Decomposition

Boric Acid expels water

185C

Decomposition

Copper hy hydroxide de decomposes to to Cu CuO

200C

Dec o om m po positi on on

Manganese Carbonate dec o om mposed to MnO

Loses about 18% weight as it decomposes to the heat stable CuO Release of carbon dioxide to give manganese(II) oxide: MnCO3 -> MnO + CO2 Organics are removed during first fire of ceramic bodies

250C - 370C

Decomposition

Organic burnout

250-200C

State Change

Cristobalite inversion (alpha/beta)

Above 180C it loses crystallization water and the oxide B2O3 melts at 300C.

Occurs in cooling clay bodies at 225C (an on heatup for vitrous ware being refired). It is accompanied by a sudden volume change. Cristobalite, a less stable form of crystalline quartz, can be present in the matrix of stoneware clays where sufficient fine quartz, time and temperature (above 1100C) are available during firing. The range of temperatures is given here because when there are variations in temperature in the cross section of ware changes in volume occur as waves across a piece and can crack it. Thus if one side of a piece is 250C and the other 200C, the inversion and associated volume change are happening in the center of the piece. Less cristobalite is formed in faster firings and in more vitreous clay bodies where the feldspar takes the small quartz grains into solution. 260C

Decomposition

Bismuth Subnitrate decomposes

300C - 330C

State Change

Copper carbonate basic decomposes

Loses 28% weight as it decomposes to to the heat stable CuO At 180C Boric acid loses crystallization water and the remaining B2O3 melts at 300C. Thus this is the first material to enter the liquid phase in the fritting process.

300C

Melting Point/Range

Boron from Boric Acid melts

370-650C

State Change

Carbon / iron oxidation

400C

Decomposition

Colemanite reacts to water loss

It has a strong reaction to the state change associated with water elimination at 400C.

425C - 650C

Decomposition

Sulfur ev e volution

This happens on first fire of many bodies (sulfur is an impurity in many ball clays and related stoneware and earthenware raw clays).

480C - 600C

Decomposition

Dehydroxylation in clay bodies

Chemical water is removed

500C - 600C

Decomposition

Magnesite decomposition

The process of removing the CO2 from magnesite to product light magnesium carbonate is called Magnesite Decarbonization.

512C

Decomposition

Hydrated lilime de decomposes (2 (25% H2 H2O)

535C

Decomposition

Manganese di dioxide de decomposes to to Mn MnO

TEMPERATURA

TIPO

DESCRIPCION

NOTES

550C - 600C

State Change

Quartz inversion (alpha-beta)

650C - 900C

Decomposition

Dolomite decomposition

This term refers to an abrupt 2% change in volume that occurs in quartz crystals when they are heated from the room temperature stable alpha phase to the beta crystal phase that exists about 573C. It is referred to as an inversion because the process is reversed when the temperature falls back below 573C. Since the change occurs suddenly ware will crack if there are significant temperature differences across its cross section. For example, if one side of a piece is at 573 and the other at 600 at point A on a cooling curve, then as the piece cools the volume change will move horizontally across it and obviously start a crack at the first weakness it finds. In a way, quartz inversion can be beneficial because it can put the glaze under compression and thus prevent crazing. Dolomite reacts similar to calcium carbonate but its decomposition starts earlier.

Calcium carbonate decomposition

Converts to Quicklime. If burned higher or too long it converts to Dead burned lime. The decomposition process of this compound is quite complex and the subject of much discussion and research. Nilo Tozzi, an expert in tile manufacturing, says that 'calcium carbonate decomposes at about 880C when alone but when mixed into a ceramic body its decomposition starts at about 700C'. Clearly the decomposition can occur over a wide range and its rate is related to many factors (particle surface area, atmosphere, pressure, interactions with surrounding particles, density and thickness of the containing matrix). This makes this material highly suspect in glaze flaws related to the generation of gases from constituent materials in the glaze recipe. It may be advisable to source CaO from a frit or wollastonite if you have a problem. See the book Thermal Decomposition of Ionic Solids by Andrew K. Galwey, Michael E. Brown.

750C - 1000C

Decomposition

800C - 1100C

State Change

Strontium carbonate decomposition

Some data sheets (and Wikipedia) indicate that decomposition occurs at 1100C, the same as the melting temperature. Technical references are more specific, saying that decomposition can occur earlier in a purely oxidizing atmosphere.

850C+

State Change

Sintering and densification

Occurs in a clay body during first fire

850 - 950

Eutectic

Zinc oxide boils and volatilizes

850C

Decomposition

Sodium Carbonate decomposes

990C

Decomposition

Chrome oxide decomposes

1025 - 1325

Decomposition

Copper Oxide breakdown

Decomposition occurs at melting point. CO2 and NaO are emitted. Copper becomes increasingly volatile and its crystalline structure breaks down

TEMPERATURA

TIPO

DESCRIPCION

NOTES The mechanisms of decomposition for BaCO3, CaCO3 and SrCO3 are similar. Decomposition of this material can be a concern (relating to glaze defects, for example) if the powder is used in ceramic clay bodies and contains granular +200 mesh material. This temperature is from Thermal Decomposition of Ionic Solids by Andrew K. Galwey, Michael E. Brown

1025C+

Eutectic

Decomposition of Barium Carbonate

1026C

Decomposition

Copper Carbonate decomposes to CuO

1050C

Decomposition

Copper carbonate basic breakdown

CuCO3 -> CuO + CO2 7% weight loss involving partial loss of oxygen to form a mix of cuprous and cupric oxides

Spodumene converts to beta phase

Undergoes an irreversible phase transition to yield beta-spodumene. This is accompanied by an expansion of ~30% and a decrease in specific gravity from 3.2 to 2.4. After conversion to its beta form at spodumene enters a stage of great thermal stability.

1082C

State Change

1100C

Melting Point/Range

Strontium carbonate melts

1100C+

Eutectic

Antimony volatilizes

1300C

Decomposition

Li2O Decomposes

1325C

Melting Point/Range

Copper oxide melts

1330C

Melting Point/Range

Fluorspar melts

1360C

Melting Point/Range

Barium carbonate melts

1380C

Zircon melts, slowly dissolves

1418C - 1428C

Eutectic Melting Point/Range

Spodumene melts

1420C

Melting Point/Range

Talc melts

1550C

Melting Point/Range

Iron oxide red melts

1785C

Melting Point/Range

Manganese oxide me

1990C

State Change

Chrome oxide melts

2300C

Decomposition

Praseodymium oxide decomposes

2320C

State Change

Neodymium oxide melts

2750F

Decom position

Kyanite decom pos es to Mullite and Si lica

Above 1025C copper becomes increasingly volatile and its crystalline structure breaks down, then it finally melts.

It forms a liquid phase and zirconium oxide, but its solubility in the liquid phase is poor, during slow cooling it will recrystallize.

Associated specific gravity change from 3.6 to about 3.0.