Fire clay
Fire clay is a term applied to a range of refractory clays used in the manufacture of ceramics, especially fire brick.
High grade fire clays can withstand temperatures of 1775°C (3227°F), but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1515°C (2759°F).[1] Fire clays range from flint clays 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 micas and quartz, and may also contain organic matter and sulphur compounds.
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[edit] Properties
It is resistant to high temperatures, having fusion points higher than 1,600°C, therefore it is suitable for lining furnaces, as fire brick, and manufacture of utensils used in the metalworking industries, such as crucibles, saggars, retorts and glassware. Because of its stability during firing in the kiln, it can be used to make complex items of pottery such as pipes and sanitary ware.
[edit] 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.[1][2] In the 21st century, Trinks [3] lists twelve modern firebrick compositions, all with an Al2O3 percentage of 43.9% or higher, and where SiO2 + Al2O3 represents at least 90% of the composition by weight in eleven out of twelve. Chemical analyses from two 19th century sources show somewhat lower alumina content: [4] [5]
| Fire Clay Compositions | ||||||
|---|---|---|---|---|---|---|
| Thorpe[4] | King[5] | |||||
| Stonebridge | Eisenberg I | Eisenberg II | Newcastle 1 | Newcastle 2 | Newcastle 3 | |
| SiO2 (%) | 65.10 | 89.8 | 64.7 | 51.1 | 47.6 | 48.6 |
| Al2O3 (%) | 22.2 | 5.40 | 24.0 | 31.4 | 29.5 | 30.2 |
| MgO (%) | 0.18 | 0.09 | 0.40 | 1.54 | 0.71 | 1.91 |
| Lime1 (%) | 0.14 | 0.20 | 0.37 | 1.46 | 1.34 | 1.66 |
| Iron Oxides (%) | 0.18 | 0.09 | 0.40 | 4.63 | 9.13 | 4.06 |
| Potash2 (%) | 0.18 | 0.61 | 2.40 | not provided | ||
[edit] Extraction
Unlike conventional brick-making clay, some fire clays (especially flint clays) are mined at depth, found as a seatearth, the underclay associated with coal measures.
[edit] References
- ^ a b "http://www.mineralszone.com/minerals/fire-clay.html" Minerals Zone, World Mineral Exchange. Retrieved 2011-6-23.
- ^ "http://digitalfire.com/4sight/material/index.html" Digitalfire Reference Database. Retrieved 2011-6-23.
- ^ Trinks, Willibald (2004). Industrial furnaces, Volume 1. John Wiley & Sons, Hoboken, New Jersey. pp. 399-400.
- ^ a b Thorpe, Sir Thomas Edward (1890). A Dictionary of Applied Chemistry Volume I. Longmans Green & Company, London.
- ^ a b King, William B. (1878). "King's Treatise on the Manufacture and Distribution of Coal Gas. self.
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