Thermal Decomposition
Thermal decomposition occurs when heat has imparted enough energy to a molecule that it becomes unstable, and breaks down into more stable components. The prime examples here are carbonates turning into oxides, and oxides turning into a reduced form.
Note that these temperatures are for a normal environment in air. Reactions giving off oxygen may occur at significantly lower temperatures in a reduction atmosphere, or in the presence of oxygen hungry substances such as carbon, aluminium or silicon carbide, which will react with the oxygen.
| Temperature ºC | Original | Reaction | Notes |
| 45 - 420 | Gerstley borate | x.Ca2B6O11·5H2O + y.NaO·2CaO·5B2O3·5H2O → x.Ca2B6O11 + 5(x + y).H2O + y.NaO·2CaO·5B2O3 | |
| 62 - 120 | Borax | Na2O.2B2O3.xH2O → Na2O.2B2O3 + x'H2O | x is variable. The decahydrate decomposes at 62°C and the pentahydrate at 120. |
| 80 - 160 | Yellow iron oxide | 2 Fe(OH)3 → Fe2O3 + 3H2O + O2(g) | |
| 140 | Cobalt carbonate hydrated | CoCO3.xH2O → CoCO3 + xH2O(g) | |
| 200 - 290 | Copper carbonate | Cu2CO3(OH)2 → 2.CuO + CO2(g) + H2O(g) | The amount of hydration of the carbonate can vary |
| 180 - 250 | Magnesium carbonate (hydrated) | MgCO3.x''H2O → MgCO3 + "x"H2O(g) | "x" may be 3 or 4. Upper temperature may be 450ºC |
| 230 | Nickel oxide - black | 2NiO2 → 2NiO + O2(g) | NiO is green nickel oxide |
| 300 - 600 | Colemanite | B6Ca2O11.5H2O → B6Ca2O11 + 5H2O(g) | The water is largely present as hydroxyl ions in the boron chain, and these change to water at 388°C, and then the water is given off at 400°C. Most water has been expelled by 450°C. The water is expelled very forcefully, which can blow glazes off the pots. |
| 335 | Cobalt carbonate anyydrous | 6CoCO3 + O2 → 2Co3O4 + 6CO2(g) | |
| 350 - 660 | Manganese carbonate | 2MnCO3+½O2(g)→Mn2O3+2CO2(g) | Some sources say decomposition starts at 200°C |
| 350 - 1974 | Zinc oxide | 2ZnO → 2Zn + O2(g) | Temperature highly dependant on the atmosphere: 350°C in carbon monoxide, about 600°C with carbon, or 1974°C in air |
| 500 | Potassium dichromate | 4 K2Cr2O7 → 4 K2CrO4 + 2 Cr2O3 + 3 O2(g) | |
| 510 - 550 | Magnesium carbonate (anhydrous) | MgCO3 → MgO + CO2(g) | May not be completed until 900ºC |
| 535 - 800 | Manganese dioxide | 2MnO2 → Mn2O3 + ½O2(g) | |
| 600 - 700 | Cobalt (II) oxide | 6CoO + O2(g) → 2Co3O4 | |
| 600 - 1050 | Talc | Mg3Si4O10(OH)2 → 3MgSiO3 + H2O + SiO2 | Formed over a number of steps, dependent on particle size and impurities |
| 656 | Antimony trioxide | 2Sb2O3 + O2(g)→ 2Sb2O4 | |
| 667 - 1497 | Strontium carbonate | SrCO3 → SrO + CO2 | Sources differ significantly on the temperature of decomposition |
| 685 - 915 | Dolomite (1st stage) | CaMg(CO3)2 → CaCO3MgO + CO2 | |
| 780 - 915 | Dolomite (2nd stage) | CaCO3MgO → CaO.MgO + CO2 | |
| > 800 | Manganese dioxide | 3MnO2 → Mn3O4 + O2(g) | |
| 800 - 1360 | Bone ash | Ca5(PO4)3(OH)→Ca10(PO4)6(OH)2-2xOxx + OH- | Exact temperature varies on conditions |
| 850 - 950 | Cobalt (II, III) oxide | 2Co3O4 → 6CoO + O2(g) | |
| 900 | Calcium carbonate | CaCO3 → CaO + CO2 | |
| > 900 | Tin oxide | 2SnO2 → 2SnO + O2 | Not all sources say that this happens |
| 900 - 1030 | Antimony tetraoxide | 2Sb2O4 → 2Sb2O3 + O2(g) | |
| 940 - 1090 | Manganese (III) oxide | 6Mn2O3 → 4Mn3O4 + O2(g) | |
| 1200 - 1250 | Red iron oxide | Fe2O3 → 2FeO + ½O2(g) | |
| > 1200 | Manganese (II, III) oxide | 2Mn3O4→6MnO + O2(g) | |
| 1350 | Barium carbonate | 2.BaCO3 → BaO.BaCO3 + CO2(g) | Decomposes much more readily in a reduction atmosphere |
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