This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
Terra-cotta is a superior kind of brickwork, but possessing in the main the same general characteristics. It is burnt from carefully selected and prepared clay, and may be had of several colours and shades - buff, brown, pink, red, and blue - the red being most generally preferred. The face of the blocks is carefully smoothed with a table-knife or other instrument before the clay is thoroughly dry. This gives a close finish to the surface.
On account of the development of cracks and twists in solid blocks during the processes of drying and burning, terra-cotta blocks, unless of small dimensions, are made hollow, the clay being either forced through a die, forming blocks of a square-channel shape, or filled to a thickness 1½ or 2 inches into moulds of the desired shape, with clay struts where required to give rigidity to the blocks. After delivery at the building-site, the blocks are "loaded" with concrete, which is sometimes composed of only one part of Portland cement to ten parts of coke breeze; this is a very weak concrete, too weak where much weight has to be borne, but in stronger concretes great care must be taken that the cement has been properly air-slaked by being exposed for two or three weeks in a dry building, and occasionally turned, as otherwise the Blight expansion of the cement may injure the block.
Of the details of manufacture nothing need be said, but a few hints on the design and use of the material may prove of service . The design should not necessitate the use of large blocks; the smaller the blocks the less is the amount of warping in each, and, consequently, the less is the difficulty in fixing the blocks to make neat work. Terra-cotta should always be subjected to compressive stress, not to transverse; for example, it ought not to be used in the form of lintels, but of arches, flat or otherwise. In moulded work, the profiles of the moulds should be such as will allow the walls of the block to be of uniform thickness throughout, otherwise warping and cracks may be developed in drying and burning. "Undercut" moulds especially should be avoided.
Frequently, where inferior clay is used, the dried blocks, immediately before being placed in the kiln, are dipped in a thin fine clay, in order that the finished blocks may have a smooth surface and uniform colour. Under certain little-understood conditions of clay and burning, such surface coats are apt to peel off. If dipping has been practised, it can usually be detected by striking the face of the block with a sharp chisel, when some of the thin coat will probably off, or at least be revealed, or, better, by gently tapping the end or bed of the block with some hard substance close to the face which has been dipped.
Sometimes a scum is formed on the face of terra-cotta during the process of drying. . As this would detract from the appearance of the finished work, usually removed with brush and water before the terra-cotta is placed in the kiln, but 18 the formation of scum on the face can be prevented by preventing evaporation taking place through that face, such a damaging process as brushing ought not now to be adopted.
The durability of good terra-cotta is beyond question, but, as in all other materials, the quality is by no means uniform, and much terra-cotta is far from satisfactory. Ruabon terra-cotta has been found to absorb water weighing 51 per cent of its dry weight in I second, the same quantity in 1 minute, 1.80 per cent in 30 minutes, and 5.67 per cent in 1 day. No increase took place with a week's immersion. This shows that Ruabon bricks and terra-cotta are of equal quality as regards absorption, and much better than many facing-bricks.
Table I., page 95.