Fire clay is a range of refractory used in the manufacture of , especially fire brick. The United States Environmental Protection Agency defines fire clay very generally as a "mineral aggregate composed of hydrous silicates of aluminium (Al2O3·2SiO2·2H2O) with or without free silica."
Properties
High-grade fire clays can withstand temperatures of 1,775 °C (3,227 °F), but to be referred to as a "fire clay" the material must withstand a minimum temperature of .
[ Minerals Zone, World Mineral Exchange. Retrieved 2011-6-23.] Fire clays range from
to
plastic fire clays, but there are
semi-flint and
semi-plastic fire clays as well. Fire clays consist of natural
argillaceous materials, mostly
kaolinite group clays, along with fine-grained
and
quartz, and may also contain
organic compound matter and
sulphur compounds.
Fire clay is resistant to high temperatures, having higher than ; therefore it is suitable for lining furnaces, as fire brick, and for manufacture of utensils used in the metalworking industries, such as , , , and glassware. Its stability during firing in the kiln means that it can be used to make complex items of pottery such as pipes and sanitary ware.
Chemical composition
The chemical composition typical for fire clays are 23-34%
aluminium oxide, 50-60%
silicon dioxide, 6-27% ignition loss (water, organic matter), and various amounts of Fe
2O
3,
Calcium oxide,
Magnesium oxide,
Potassium oxide,
Sodium oxide, and
Titanium dioxide.
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 |
|
|
| 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 given in the text |
|
Extraction
Unlike conventional
brick-making clay, some fire clays (especially flint clays) are
mining at depth, found as a
seatearth, the underclay associated with
coal measures.
