Terra cotta is composed of practically the same material as bricks, and its characteristics, as far as the material is concerned, are the same. Terra cotta, however, requires for its successful production a much better quality of clay than is generally used for bricks, while the process of manufacture is entirely different.
The first consideration in the manufacture of terra cotta is the selection of the material. No one locality gives all the clay required for first-class material, and each shade and tint of terra cotta requires the mingling of certain clays from different localities to regulate the color.
A great variety of excellent clays are mined in Northern and Central New Jersey, large quantities being marketed annually for making terra cotta, as well as for fire bricks, pottery, tiles, etc. The color varies from light cream to a dark red.
A partial vitrification of the mass is also desirable in the production of terra cotta, as it enhances the durability of the body. To achieve this, different materials are added which tend to fuse the body to a harder consistency. The vitrifying ingredients usually added to the terra cotta clays are pure white sand, old pottery and fire bricks finely pulverized, and clay previously burned, termed "grog."
The clay after being mined must be properly seasoned before being delivered at the factory. After being received the clay is crushed and ground, or washed, then mixed with grit, "grog" and water. The clay is then piled in layers, each quality being in a separate layer or stratum. As many as ten or twelve strata or layers are piled together, and from this mass perpendicular cuts are taken, and the whole is again thoroughly tempered in a pug mill, or between rollers.
After passing through the machinery, which thoroughly mixes alt the ingredients, the plastic mass is moulded into small cakes for convenience in handling and sent up to the moulding rooms.
If several pieces of terra cotta of the same size and shape are required, a full size model of plaster and clay is first made, and from this a plaster mould is taken. In the making of these models and moulds the highest grade of skilled labor is required. When the moulds are dry they are sent to the pressing department; here the plastic clay is pressed into the moulds by hand, and when partially dry the work is turned out on the floor. The ware is then ready for the carver or modeler, if it is decorative work that requires the use of their tools, or for the clay finisher if it only requires undercutting or some special work to make it fit in with other work.
The work is then carefully dried on the drying floor, when it is ready to be put into the kilns, where it must remain seven days for burning and cooling before it is ready for use. The kilns commonly used for burning terra cotta are of the beehive, down-draft pattern. In burning terra cotta the alkaline salts contained in the clays yield an efflorescence, which, acting upon the silicates of the surface, vitrify to a greater degree the exterior of the terra cotta, and this harder face should remain intact and under no avoidable circumstances be allowed to be chipped, chiseled or broken, although the joints sometimes require chiseling or trimming to ensure a close fit.
If only a single piece of terra cotta is to be made, or where no repetition is intended, no moulds are used, the clay being modeled directly into the required shape. The finished product thus bears directly the impress of the modeling artist. It can be studied, improved or modified, and, when entirely satisfactory, burnt. On this account terra cotta possesses, for highly decorative work, an advantage over all other building materials.
Terra cotta is usually made in blocks about 18 inches long, 6 to 12 inches deep and of a height determined by the character of the work. To save material and prevent warping the blocks are formed of an outer shell, connected and braced by partitions about 1 inch thick. The partitions should be arranged so that the spaces shall not exceed 6 inches, and should have numerous holes in them to form a clinch for the mortar and brickwork used for filling.