Fireclay is the variety of clay which is used for the manufacture of firebricks, gas-retorts, crucibles, glass pots, chimney-pipes, and other articles, most of which require to resist the action of high and long-continued heat. Ordinary fireclay is chiefly found in beds not usually much exceeding two feet in thickness, in the coal-measures, interstratified with seams of coal and other rocks. In the British Islands it is most largely worked about Glasgow, Newcastle-on-Tyne, and Stourbridge in Worcestershire, at which last place it is said to have been discovered about 1555 by some wandering glassmakers from Lorraine. But it occurs, more or less, in most places where true coal is found. It is mined in Germany, Belgium, France, the United States, and other countries. Stourbridge fireclay, owing to its excellent quality, is largely exported to foreign countries, as well as bricks and other objects made of it. Refractory clays are found, though more rarely, in other formations besides the coal-measures. For example, some of Tertiary age found in Dorsetshire and Devonshire are made into firebricks. The following table shows the principal constituents of fireclay:
| No. 1. | No. 2. | No. 3. | No. 4. | No. 5. | |
|---|---|---|---|---|---|
| Silica ..... | 65.10 | 51.10 | 59.49 | 53.52 | 54.20 |
| Alumina ..... | 22.22 | 31.35 | 28.95 | 33.68 | 33.80 |
| Potash ..... | .18 | trace | trace | ||
| Lime ..... | .14 | 1.46 | trace | .76 | trace |
| Magnesia ..... | .18 | 1.54 | .14 | .02 | |
| Oxide of Iron ..... | 1.92 | 4.63 | 1.05 | .52 | .01 |
| Water ..... | 9.28 | 11.05 | 11.34 | 10.86 | |
| Organic Matter ..... | .58 | 10.47 | .. | .. | .15 |
No. 1, Stourbridge; No. 2, Newcastle-on-Tyne; No. 3, Gartsherrie, Scotland; No. 4, Poole, Dorsetshire; No. 5, Morgantown, West Virginia, United States. See CLAY.
Fireclays from the same locality often differ considerably in their composition and quality. Some of the Newcastle clays, for example, contain from 70 to 80 per cent. of silica with from 9 to 18 per cent. of alumina. A high percentage of silica and alumina together (their relative proportions being comparatively immaterial) and small quantities of alkaline substances and oxide of iron constitute a refractory fireclay. If not small in amount, alkaline bodies in the clay tend to make it easily fusible, so that bricks formed of it are apt to soften and yield in a furnace. Fireclays are generally yellow in colour after being fired in the kiln. No very sharp line of distinction can be drawn between hard infusible clays and softer marly clays used for terra-cotta, garden vases, and some kinds of house bricks. In districts where fireclay is abundant composition bricks for ordinary building purposes are partly made of it. For furnace-building materials which resist the action of a very intense heat, such as dinas and bauxite bricks, see BRICK-MAKING. Powdered flint, as well as chrome iron ore, is likewise used for furnace bricks.