When this is the case great care must be taken in fixing the blocks, otherwise the long lines of a building, such as those of the string-courses or cornices, which are intended to be straight, are apt to be uneven, and the faces ol blocks are often "in winding."
Twisted and warped blocks are sometimes set right by chiselling, but this should be avoided, for if the vitrified skin on the surface be removed, the material will not be able to withstand the attacks of the atmosphere, etc.
Another drawback is the uncertainty of getting terra cotta delivered as required, whereas a stone may be taken and fixed at once, the carving being left, if necessary, to be completed afterwards.
Terra cotta is made in several colours, depending chiefly upon the amount of heat it has gone through.
White, pale grey, pale yellow, or straw colour indicate a want of firing.
Kich yellow, pink, and buff varieties are generally well burnt.
A green hue is a sign of absorption of moisture, and is a sign of bad material.
A glazed surface can be given to terra cotta if required.
Porous Terra Cotta is made in America "of a mixture of clay and some combustible material - such as sawdust, charcoal, cut straw, tan bark, etc. When baked the combustible material is consumed, leaving the terra cotta full of small holes. It is fireproof, of little weight, great tenacity, strong, can be cut with edge tools, will hold nails driven in, and gives a good surface for plastering."1
Inferior Terra Cotta is "sometimes made by overlaying a coarsely-prepared common body with a thin coating of a finer and more expensive clay."
"Unless these two bodies have been most carefully tested and assimilated in their contraction and expansion, they are sure in course of time to destroy one another; that is, the inequality in their shrinkage will cause hair cracks in the fine outer skin, which will inevitably retain moisture, and cause the surface layer to drop off in scales after the winter frosts."
"Another very reprehensible custom is that of coating over the clay, just before it goes into the kiln, with a thin wash of some ochreish paint, mixed with finely ground clay, which produces a sort of artificial bloom, very pretty looking for the first year or two after the work is executed, but sure to wear off before long."2
Where used. - Terra cotta has been extensively used in Dulwich College, in Messrs. Doulton's warehouses, Lambeth, in the Albert Hall, in the new Constitutional Club, in the Natural History Museum, and in several other of the new buildings near the South Kensington Museum.
1 Proceedings R.LB. A. 1886, p. 129. Reports on Exhibition, 1876, p. 14.
Stoneware is the name given to articles made from the plastic clays of the Lias formation, obtained chiefly in the south of England.
The best comes from Poole, in Dorsetshire, or from the vicinity of Teignmouth, in Devonshire. It contains about 76 parts silica and 24 of alumina, with a very small proportion of other ingredients.
This clay contains very little infusible matter. It is generally mixed with a certain proportion of powdered stoneware, ground and calcined flints, ground decomposed Cornish granite (see p. 16), or sand, to prevent excessive shrinkage.
They are burnt in domed kilns like fireclay goods, but at a very much higher temperature.
A fractured surface shows that this ware is thoroughly vitrified throughout. It is intensely hard, dense in texture, close in grain, and rings well when struck.
This material is admirably adapted for all purposes where strength and resistance to atmospheric, chemical, or other destroying influences are required.
Stoneware articles are often salt glazed (see p. 129), but the material is in itself non-absorbent, and will resist the effect of moisture even when unglazed.