Fire clay
Fire clay is a range of refractory clays used in the manufacture of ceramics, 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."[1]
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 1,515 °C (2,759 °F).
Fire clay is resistant to high temperatures, having
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.[2] Chemical analyses from two 19th-century sources, shown in table below, are somewhat lower in alumina[3][4] although a more contemporary source quotes analyses that are closer.[5]
Thorpe[3] | King[4] | Shackelford[5] | |||||
---|---|---|---|---|---|---|---|
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 given in the text |
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.
References
- ^ "Calciners and Dryers in Mineral Industries" (Background Information for Proposed Standards). U.S. Environmental Protection Agency. 1985. pp. 3–48. EPA-450/3-85-025a.
- ^ a b Minerals Zone, World Mineral Exchange. Archived 2011-07-14 at the Wayback Machine Retrieved 2011-6-23.
- ^ 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.
- ^ a b Shackelford, James F. (2008). Ceramic and glass materials: structure, properties and processing. Springer. p. 121.