Fireclay is the name given to any clay which is capable of standing a high temperature without melting or becoming soft. Such clays are also called refractory.
Fireclay is required in buildings for setting stoves, ovens, backs of ranges, etc.
Where found. - Clay of this description abounds in the coal-measures, just beneath the several seams of coal.
1 Roorkee Treatise Of Civil Engineering
The following list gives some of the counties in which fireclay is found, together with the localities producing the best known descriptions: -
Kilmarnock, Dean, Hillhead, Perceton.
Brierly Hill, Wolverhampton.
Worcestershire .. .
Stourbridge, Dudley, Tipton, Hanford,
Wortley (near Leeds), Elland, Stannington, etc.
A refractory fireclay will contain nearly pure hydrated silicate of alumina.
The more alumina that there is in proportion to the silica, the more infusible will be the clay.
The composition of different fireclays varies, however, considerably.
From 59 to 96 per cent silica. „ 2 to 36 „ alumina. „ 2 to 5 „ oxide of iron.
A very small percentage of lime, magnesia, potash, soda.
The fire-resisting properties of the clay depend chiefly upon the relative proportions of these constituents.
It will not, however, be necessary to enter in detail upon the part played by each of the constituents that are found in fireclay. These constituents are the same as those found in brick earth (though their proportions are different), and the effect they produce upon the clay is the same in both cases (see p. 86).
The presence of an extremely small proportion of lime, potash, or soda, may, however, improve the clay, by soldering the particles firmly together.1
When a clay containing iron requires the addition of sand to prevent its cracking, it is a common practice to add burnt clay instead, so as to produce the same beneficial effect without risk of making the clay fusible.
1 Percy's Metallurgy.
The chemical analysis of a clay is not a very safe criterion of its qualities. The silica shown may be either soluble silica influencing the chemical constitution of the clay, or it may be sand which is chemically inert.
In the analysis there is no distinction made between the two.
"A good fireclay should have an uniform texture, a somewhat greasy feel, and be free from any of the alkaline earths." 1
Brierly Hill, Staffordshire, P
Dowlais, best, P
Glascote, near Tam-worth, P
Hedgerley, Bucks, G
Howth, near Dublin, P
Kilmarnock, Ayrshire, G
Plympton, Devon, G
Poole, Dorset, P
Stourbridge, "Worcestershire, P
Teignmouth, Devon, P
Wortley, Leeds, G
Titanic Acid •41
P, Percy's Metallurgy. G, Capt. Grover, R.E. Prof. Papers, vol. xix.
1 Page's Economic Geology.
It should be remarked that the infusibility of fireclays does not depend altogether upon their chemical composition, but also upon their degree of fineness. A fireclay with a coarse open grain will probably prove more refractory than one with a close even texture.
Firebricks are made from fireclay by processes very similar to those adopted in making ordinary bricks.
The clay is dug, weathered, tempered, ground under rollers, passed through riddles to remove lumps, pugged, moulded, burnt in cupolas or in Hoffmann's kilns at a heat slowly increasing until it attains a very high temperature, and then allowed gradually to cool.
There are several varieties of firebrick in general use, named usually after the locality providing the fireclay from which they are made.
Stourbridge Firebricks are made in a district about twenty miles southwest of Birmingham, which contains several varieties of fireclay.
The bricks produced are generally of a pale brownish colour, sometimes reddish or yellow-grey. They are frequently mottled with dark spots, which are stated by Dr. Percy to be due to the presence of particles of iron pyrites.
"With Stourbridge clay it is customary to mix burnt ordinary clay. For common firebricks the proportions of the latter to the former are often as much as two to one. This gives a brick capable of resisting the action of the heat caused by a house fire, though it would not be sufficiently refractory for resisting a furnace temperature. Fireclay being expensive, the inferior brick is naturally cheaper, and is much used." l
Kilmarnock and Newcastle furnish firebricks somewhat similar to those from Stourbridge.
Dinas Firebricks are made from a so-called fireclay found in Glamorganshire.
It will be seen from the table of analyses on the previous page that the so-called "clay" consists nearly entirely of silica. It is found in the state of sand. About 1 per cent of lime is added, and enough water to make it cohere. The bricks are then moulded by machinery under pressure, dried, and burnt in a close kiln.
The bricks made from this substance will bear a most intense heat, being the only description that will resist the temperature (4000° to 5000° Fahr.) of a regenerative furnace.2
They expand under heat, are porous, and will not stand rough usage.
The fractured surface of a Dinas firebrick "presents the appearance of coarse irregular white particles of quartz, surrounded by a small quantity of light brownish-yellow matter. The lime which is added exerts a fluxing action on the surface of the fragments of quartz, and so causes them to agglutinate. . . . From their siliceous nature it is obvious that they should not be exposed to the action of slags rich in metallic oxides."1
1 Abney's Notes on Chemistry of Building Materials. 2 Dr. Siemens, Chemical Society, 7th May 1868.
Guismuyda Firebricks, made near Swansea, and Narberth Firebricks, from Pembrokeshire, are of the same description as those from Dinas.