Chemical Composition
The chemical composition typical for fire clays are 23-34% Al2O3, 50-60% SiO2 and 6-27% loss on ignition together with various amounts of Fe2O3, CaO, MgO, K2O, Na2O and TiO2. Chemical analyses from two 19th century sources, shown in table below, are somewhat lower in alumina although a more contemporary source quotes analyses that are closer.
| Fire Clay Compositions | |||||||
|---|---|---|---|---|---|---|---|
| Thorpe | King | Shackelford | |||||
| Stonebridge | Eisenberg I | Eisenberg II | Newcastle 1 | Newcastle 2 | Newcastle 3 | N/A | |
| SiO2 (%) | 65.10 | 89.8 | 64.7 | 51.1 | 47.6 | 48.6 | 58.1 |
| Al2O3 (%) | 22.2 | 5.40 | 24.0 | 31.4 | 29.5 | 30.2 | 23.1 |
| MgO (%) | 0.18 | 0.09 | 0.40 | 1.54 | 0.71 | 1.91 | 1.00 |
| CaO(%) | 0.14 | 0.20 | 0.37 | 1.46 | 1.34 | 1.66 | 0.08 |
| Iron Oxides (%) | 0.18 | 0.09 | 0.40 | 4.63 | 9.13 | 4.06 | 2.40 |
| K2O (%) | 0.18 | 0.61 | 2.40 | not quoted | |||
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