TREATMENT OF CLAYS.
The peculiarities of the clays necessary for the manufacture of architectural terra-cotta have been enlarged upon in Chap ter II.
Having found the clays which he considers suited to his pur pose, the maker next proceeds to further reduce the natural shrinkage by mechanical means, in order to place the contrac tion as far as possible under his own control ; upon his skill in this detail will very largely depend his success in the produc tion of sound, straight, and well-formed terra-cotta.
There are various methods of doing this, such as by the admixture of ashes, refractory sands, crushed and ground shales, or of crushed and ground burned clay, with the crude or natural clay. The best results are obtained by the use of a proportion of the same clay burned and ground to powder, technically known as grit (some persons call it cement, but it is hard to tell why they do so, because its use is the exact oppo site of cementing). Having burned and ground to powder a portion of the crude clay, the next step is to prepare a mixture of such proper relative proportions as to make a homogeneous and workable body, or stiff mud. The exact proportions will always depend upon the quality of the clay and the knowledge of the workmen.
A safe and reliable rule will be found in aiming to make a mixture which, under sufficient burning, will contract only one sixteenth in bulk. An easy method of finding the proportion suited to the crude clay is to measure out three separate parcels of mixture as follows : A. 2 parts crude clay, i part grit.
B. 3 parts crude clay, i part grit.
C. 4 parts crude clay, 1 part grit.
Mix each parcel separately, adding water enough to make a mud capable of being pressed into a mould. It is important to knead thoroughly, and have all three samples of the same de gree of stiffness, then press into a mould, making each kind so as to know them after they are burned ; dry slowly at the same time, and in the same temperature, and burn in the same part of the kiln.
If these trials are made carefully, they will indicate clearly what is the correct proportion of clay and grit to be used in order to produce good terra-cotta.
In judging of these trials it will be important to notice first the form, second the hardness, third the shrinkage, fourth the color.
The form determines the marketable quality, the hardness indicates the durability of the form, the shrinkage demonstrates its working limits, while the color is capable of adjustment and is therefore of least consequence.
When the exact proportions of admixture known as body, which will reproduce good forms, in a semi-vitreous material of approximately correct size and of a reliable color, has been as certained, the next step to consider is how to prepare it in the most economical manner.
There can be no question of the value of the slipping or washing of all clays for terra-cotta work. This is the method adopted by Ernest March at Charlottenburg, and where the area of the works and the available capital will permit, it should always be done, indeed it must be done for high class work.
For ordinary work many of the advantges of the washing process can be obtained by the use of a cylinder-crusher and stone-separator for plastic clays, and the use of a pulverizer for clays that are of a shalt' quality, such as Indiana clays, ob tained from the block coal regions, and many other clays that are mined in the neighborhood of coal-fields. The kind of machinery to be used must be determined by every manufac turer for himself. The nature of his clay will indicate what work he ought to do in order to properly prepare and temper it. Every machine has some especially good feature, while no one is absolutely perfect, or suited to every description of clay or work. Therefore, in selecting machines, it is well to get only those which can readily be understood and operated by the purchaser. Clay-working is like farming, as shown in the maxim "The man is of more consequence than the land," so in our case the man is of more consequence than the machine.