This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Size Of Pieces. In designing any terra-cotta work, care should be used in limiting the size of the pieces to the most practical and economical dimensions; these may be said to be under 3 ft. X 4 ft. X 18 in. Columns have been made 14 1/2 feet in length, the shaft being in one piece 12 feet long, but such sizes are very costly, as great skill and care is necessary in their manufacture to prevent warping.
Window openings of more than ordinary width, should not be spanned by single pieces of terra cotta. Sills are usually formed of blocks, not over 2 feet long; the height of the pieces composing the jambs should not be more than 12 inches. In fact, all the work should be divided into as many pieces as possible, care being taken to insure proper bonding. Short lengths are more easily handled, and less liable to break than long ones. When brick structures are trimmed with terra cotta, it is essential that the pieces be of the same height as the courses in the brickwork, in order that they may bond well into it.
143. As considerable time and extra expense are required for manufacturing special shapes of terra cotta, use should be made of such standard forms and sizes as are readily obtainable, when these can be used; but when the pieces must be made to order, the drawings should be sent to the manufacturer at least two months before the terra cotta is needed, to allow ample time for manufacture and delivery. While small pieces may often be obtained in less time, it is not advisable to force the work, as it increases the cost greatly, besides preventing thorough drying of the blocks.
Weight And Strength. Solid blocks of terra cotta will weigh about 120 pounds to the cubic foot. Hollow pieces, with walls 1 1/2 inches thick, will weigh from 65 to 85 pounds per cubic foot, small pieces being heavier per cubic foot than large ones. An average weight is 70 pounds per cubic foot for blocks 12 in. x 18 in. or larger on the face.
Two-inch cubes of terra cotta crush under a weight varying from 5,000 to 7,000 pounds to the square inch. Authorities give the safe working strength of terra-cotta blocks in the wall at 5 tons per square foot, when unfilled, and 10 tons per square foot when filled solid with concrete.
Setting And Pointing. Before use, each piece of terra cotta should be carefully inspected. It is important that abutting surfaces shall match perfectly and that each piece shall fit exactly in its proper place. Terra cotta should give out a clear metallic sound when tapped with a hammer; and a fracture should show a close and homogeneous texture and uniform color. The surface should be hard enough to resist a knife scratch. Broken or twisted pieces, or any having the glazing chipped off, should not be used.
The mortar used in setting terra cotta should be composed of good cement and sand, mixed in about the proportion of 1 to 2. The method of laying the blocks is similar to that of stone setting, and is generally done by the bricklayer, the terra-cotta work being carried up simultaneously with the brickwork. The blocks should be solidly built into or anchored firmly to the walls, and all voids should be filled with brick and mortar, to make as strong work as possible. Immediately after the pieces are set, the face joints should be cleaned out at least 3/4 inch in depth, so as to prevent spalling the edges of the blocks, and to make ready for pointing. The mortar in all horizontal joints exposed to the weather, should be raked out to a depth of 2 inches, and the joint should be calked with oakum, about 1 inch deep, the remaining space being pointed in the usual manner. The pointing mortar should be made of about 1 part each of cement and sand, and colored to correspond with the terra cotta.
146. Fig. 71 represents a good example of terra-cotta work, showing the accuracy and neatness with which the pieces may be manufactured and laid. The Figure represents a doorway, a being the jambs, and b the lintel, which is formed of several pieces, joined at c.
147. When window sills are made in sections, they should have lap joints, as represented in Fig. 72, in which a shows the pieces of terra cotta; b, the joint, protected by the half-round roll e; c, the wood sill of the window; and d, the joint between the terra cotta and the wood. This is a very-good method of construction, as the insertion of the terra cotta under the wooden sill prevents water penetrating the wall during driving rains.