On the other hand, the chemical and physical nature of the fireclays used in the manufacture of such crucibles requires careful attention in order to secure the best results.
In the basins of the Forth and Clyde the following subdivisions are well marked: (5) Upper Red Sandstone series (red and grey sandstones, fireclays, shales, marls); (4) Coal Measures (white and grey sandstones, dark shales, fireclays, coal seams, ironstones); (3) Millstone Grit (massive sandstones and grits, with fireclays, thin limestones and coal); (2) Carboniferous Limestone series - (c) sandstones and shales, with three or more seams of limestone; (b) sandstones, shales, coals and ironstones, but with no limestone bands; (a) sandstones, shales, fireclays, coals and iron XXIV.
They are in every form from the rare to the common-glass pot clay, ball clays, kaolins, flint fireclays, plastic fireclays, stone-ware clays, paving-brick shales, building-brick and gumbo clays.
Plastic fireclays, paving and brick clays are available in seemingly limitless quantities.
Some clays, however, such as fireclays, contain very little potash or soda, while they are rich in alumina; and it is a fair inference that hydrated aluminous silicates, such as kaolin, are well represented in these rocks.
The most essential point in good fireclays, or in the bricks or other objects made from them, is the power of resisting fusion at the highest heat to which they may be exposed.
The most highly valued fireclays are derived from the Coal Measures.
The principal French fireclays are derived from the Tertiary strata in the south, and more nearly resemble porcelain clays than those of the Coal Measures.