The clay, when dry, has completed its shrinkage so far as it can do until fired. Therefore as soon as the corners are dry they cease to contract ; then the exposed edges still continue to contract, moving most in the direction of the centre, because they dry from the corner toward the centre, the result being that the edges are all curved inward when dry, from the efforts of the mass to contract toward the common centre of the panel.
The edges having completely dried, in turn like the corners cease to recede, but the entire central surface has considerable shrinkage to perform yet, and there is no avoiding it ; until dry it must shrink, and as the corners and edges, having done their part, have become rigid, the centre can only shrink by breaking loose somewhere, hence the cracks so often found in large pieces of work during the process of drying.
The remedy for this is to guide the elemental movement. It is possible to direct, even if we cannot oppose, natural forces. By covering the edges and corners of such a panel with a light rubber cloth (which can be bought for twenty-five cents per yard), and leaving about one-third of the upper surface of the centre only exposed, the centre can be made to dry first ; then the edges will contract upon the centre, drying from the centre, and the result will be a sound and perfect form. The raising of this drying-board to an incline not only reduces the friction, but it also allows the air to pass freely through the open joints of the drying-board into the cells and back side of the panel, and thus aids in an equable drying of the whole mass. This principle, with modifications to suit the particular shape, weight, and bulk of the pieces being made, will insure success in the making of any piece of clay work that it is practicable to make. Successful drying demands constant watchfulness.
In any factory that employs steam as a motive power for its machinery, steam piping will furnish the best and safest heating for the purpose of warming the shops and drying the work in process of manufacture.
The steam pipes should be hung over head, about one foot below the ceiling or lower edge of the next story floor-beams, and should consist of numerous single lines of one-inch pipe connected with a four-inch header at both inlet and outlet, care being taken to allow ample escape for the exhaust-steam, so as to avoid any back pressure upon the engine.
The pipes should be placed in the second section of beams from the wall, and the floor over the pipes should be laid with open joints of half an inch. This enables the entire section of the floor next the windows to be used for working near the light, and, of course, there the floor is tight-jointed, and should be double with paper between to prevent dust and water leak ing through.
The finished work is set on the open-jointed portion of the floor, and is thus in a constant but gentle circulation of air, which enables it to dry equally, and without cracking or warping.
After the work is dry enough, it is moved toward the central sections of floor, which also is tight-jointed and double, to await its removal to the kilns.
There is no necessity for either steam-pipe heat or open floor in the kiln-end of the building, as it will be found that the kilns themselves, when in constant operation, will ventilate and heat the workshops, and dry the work made in that portion of the factory.
In addition to the above, there should be a circuit of steam piping for live steam, to be used in extreme cold weather only. This should be placed at about seven feet six inches from the floor and three feet from the outer walls, and encircle all the space not warmed by the action of the kiln heat. The posi tion, of course, can be varied to suit any other conditions, but in a building such as is described this would be the method to adopt, and the circuit would be a direct one ; the top story only requiring three lines of pipe, the next lower one line of pipe more, and so on until the first floor is reached ; this would thus require six lines of pipe, which would give ample heat at a low pressure of steam.
The question of drying the clay forms, after they are taken from the moulds, and preparing them for the kiln, is also one that must be governed by some general rules, but these rules can only be observed subject to the special case of the several forms of clay work produced. It is possible to state some method that will apply generally to the drying of simple forms, such as brick and pottery, which are of universal shape and bulk ; but in case of architectural terra-cotta work, which varies so exceedingly in its size, shape, and bulk of material, no spe cific method can be stated so as to be of universal application.
The rules to be observed are simple and few. First, avoid too violent a draught of air. Second, reduce the friction of drying clay upon the drying board as much as possible, and third, dry uniformly.
Extreme draughts of air cause the external surfaces to dry too rapidly, and the result is warping and cracking. The weight of clay in a piece of terra-cotta work is often greater than the tenacity of the clay is able to move in proportion to the shrink age, hence cracks in the weak parts of the form which is being dried.
Unequal drying of the parts also produces warping and cracking.
Suppose a piece of work has to be made three feet long, by two feet wide, and six inches thick : this should be moulded of a uniform thickness of one-and-a-half inches, with cross parti tions about six inches apart each way in its interior or back side, such partitions being only one inch in thickness, and care being taken to make a circular hole in each section of the par titions to allow of a free circulation of air among the cells thus formed in the body of the piece of work.