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.
139. The material in terra cotta is practically the same as that in brick, but a much better quality of clay must be used, and the method of manufacture is also different from that of brick. The proper selection of the materials used requires considerable skill and knowledge of effects to secure desired results. The clays used are obtained from different places, and must be mixed in certain proportions in order to obtain certain results. Much artistic skill is necessary to produce the elaborately decorated work now so common, and consequently terra-cotta manufacture ranks much higher than brickmaking.
140. After the clay is mined, it must be seasoned by exposure to the air for some time. It is then ground and mixed with water and materials known as "grog," which, in burning, produce a partial vitrification, thereby increasing the durability of the terra cotta. Grog usually consists of very fine white sand, pulverized firebrick, partly burned clay, and fragments of pottery. The mixed clay is then piled in layers, each quality being kept separate. Ten or twelve strata are laid, and the mass is cut up into sections and again thoroughly mixed by being run between rollers, or through a pug mill, the object being to secure a complete and uniform mixture of the ingredients. The plastic clay is then formed into cakes of convenient size for handling, and is ready for the molder.
If only a single piece is to be used, the clay is modeled directly into the required shape, no molds being used. When, however, numerous pieces of the same size and shape are to be molded, a full-sized model of plaster and clay is made, from which a cast of plaster is taken; this is thoroughly dried before use. The tempered clay is compacted into the mold and allowed to become partially dry; it is then taken out and sent to the carver or. modeler, if it requires decoration, or to the clay finisher, if it merely needs " touching up," or trimming. The unburned terra cotta is next removed to the drying floor, which is kept at a temperature of between 70° and 80° F., and is dried thoroughly, and is then ready for the kiln, in which it remains about 7 days for burning and cooling. In the burning an efflorescence is formed, which, on cooling, becomes hard and vitrified, rendering the material more durable; this glaze should not be broken unless it is necessary to do so.
141. Terra cotta is usually made in blocks from 18 inches to 2 feet long, and from 6 to 12 inches deep, the height depending upon the position and character of the work. To economize material and prevent the blocks becoming distorted and out of line, they usually consist of an outer shell, braced by partitions about 1 inch thick; these should not be more than 6 inches apart, and should be perforated, so that the mortar may form a good bond between the piece and the brick filling.
Owing to the improvements in methods of manufacture, terra cotta may now be had in almost any shade, from nearly pure white to a deep red; but previous to about 15 years ago, most of the terra cotta produced had a red color. The shades most commonly used at present are tints of gray, white, bronze, red, etc. Any color may be produced by chemical means, but a better quality of material is likely to result if those colors are used which are natural to the clay, and which do not require overburning or underburning of the clay to produce the desired effect.