1 Percy's Metallurgy, p. 238. 2 Mr. Baldwin Latham, Sanitary Engineering.
Many localities furnish clay from which terra cotta may be made, as, for example, Tamworth, in Staffordshire; Watcombe, in Devonshire; Poole, in Dorsetshire; Everton, in Surrey, and other places in Northamptonshire and Cornwall.
The great difficulty to be overcome in making terra cotta is the uncertain shrinkage of the clay.
This mixture is ground into fine powder, thrown into water, finely strained, pugged, kneaded, forced into plaster moulds smeared with soft soap, very carefully dried, gradually baked in a pottery kiln, and slowly cooled.
The drying process requires to be conducted with extreme care. If the blocks are subjected to draughts of cold air, if they are of unequal thickness, or if the operation is conducted too quickly, they will warp, twist, and become useless.
This fusibility is aggravated by the presence of lime, magnesia, and other impurities, and the resulting terra cotta is not so hard and durable as that from the more refractory white clays.
In some cases the white clay is used with an admixture of oxide of iron just sufficient to make it burn to a good red colour.
Terra cotta made from fireclays, when properly burnt, is excellent in texture, colour, and surface, but appears ragged and porous directly the outer skin is removed. It manifestly suffers for want of a small proportion of some flux, such as that afforded by the lime and alkalies in the mixed clays.
The mixed clays used for terra cotta contract from 1/10 to 1/12 of their linear dimensions in drying and baking.
The red clays shrink only about 1/20 lineally, while fireclays shrink as much as 1/8 More than half of this shrinkage is in drying, and the remainder in burning.
The blocks used for building purposes should average from about 1 to 3 cubic feet in bulk, and no block should contairi more than 4 cubic feet.
Such blocks are generally made hollow, the thickness of the shell of terra cotta being from 1 inch to 2 inches.
Large blocks should have a diaphragm, or partition of terra cotta across them, to prevent their warping.
The blocks are usually made from 12 inches to 18 inches long, 6 inches to 15 inches high, and from 41/2 to 9 inches thick on the bed. These dimensions are suitable for bonding into brick backing.
The advantages of terra cotta are as follows: -
If properly burnt, it is unaffected by the atmosphere, or by acid fumes of any description.
If solid it weighs 122 lbs. per foot cube; but if hollow, as generally used, it weighs only from 60 to 70 lbs. per foot cube, or half the weight of the lightest building stones.
Its resistance, when solid to compression, is nearly 1/3 greater than that of Portland stone.
Mr. Page found by experiment that it lost 1/16 inch in thickness, while York stone lost 1/4 inch with the same amount of friction. It is, therefore, well adapted for floors.