The demand for these goods is quite large, and the prices paid for them make them extremely profitable, unless the profits are too much cut down by losses in the course of manufacture. The same general laws apply to this branch of glazed work as to enameled brick, but the methods employed are quite different. The ware is molded in plaster molds. This requires men skilled in this work. The clay required may be different, in some of its qualities, from clay for enameled brick, as it is not subjected to heavy pressure in a repress, and as the method of manufacture tends to eliminate all tendency to laminations, the non-laminating quality of the clay is not of so much importance. The most important point of all is the clean burning qualities of clay. The slightest tendency to iron specks, lime flaws or coal flaws is extremely objectionable. A single flaw is just as capable of spoiling a bathtub worth many dollars as it is of spoiling a brick worth fewer pennies. The clay must dry without either warping or checking, and must also burn without either of these defects. The cracking of heavy pieces is occasioned by different conditions. Sometimes it is from the clay being too rapid a dryer and drying at edges and corners, while the center remains wet
If this is the case, drying in air-tight rooms may overcome the difficulty. With other clays cracking occurs only in the part of the piece upon which it rests, and is due to the clay not being strong enough to overcome the resistance of friction produced by its own contraction. In this case, drying upon heavy blocks of clay that will shrink with the piece may overcome the difficulty. This throws the strain upon the block instead of on the piece. Bathtubs are sometimes dried in a sort of hammock or sling made of four-inch webbing, or they may be dried by tilting them at an angle of 45 degrees, bringing the weight upon two sides, and consequently distributed over a larger surface. It is difficult in nearly all clays to dry a bathtub and keep it absolutely straight, and much care must be taken. The sides of the large-size tubs are about six feet by two to two and one-half feet, and about one and one-half to two inches thick, and only held at bottoms and ends. The top edge will easily warp, and, once warped, it is hard to do anything with it Heavy clay strips, made of the same clay as the tub and notched to fit over the top edge, will form a tie that will prevent either edge from warping independently. These ties will need constant attention; in fact, the whole business is one requiring the constant personal supervision of an expert
I have been connected with one case in which the cracking of large pieces was finally traced entirely to the water with which the clay was mixed. This water was drawn from a pond, and it was finally noticed that when the water became muddy cracks were sure to follow. By taking the water direct from a spring the cracking was entirely overcome. I merely cite this to show to what apparently insignificant and remote causes grave troubles may sometimes be traced.
Our English writers tell us to pick the clay carefully by hand so as to get rid of bad clay. This is unsafe, as by the slightest carelessness - in fact, even when great care is exercised - some bad clay will get through and spoil work. My advice is to use a pure clay. It can be obtained. Then, if you desire, you can hand-pick that, keeping the very best for bathtubs and the other for the smaller pieces; but do not use a clay which places you at the risk of carelessness or lack of judgment on the part of a workman, especially when it may completely ruin goods.
The preparation of grit for these goods will vary with the character of clay used. Coarse grit, say up to one-eighth-inch mesh, and plenty of it, tends to reduce cracking and warping, while fine grit tends to make much smoother surfaces, but, as usually prepared, does not prevent cracking. In grinding fine grit much of it is reduced to powder and mixes with and shortens the clay. If double sieving is resorted to, by which all the powder and extremely fine grit is eliminated and nothing but clean grit, free from dust, is used, it will, of course, increase the expense somewhat, but a very fine faced goods can be made without much tendency to crack. The elimination of dust is probably easiest done by washing grit. With some clays it will be a question of steering between two evils - rough, coarse-looking goods on one side and cracks on the other. The judgment of the manager then comes in. Some clays require weathering; others work best without it This point will have to be determined by experiment. As a rule, a mixture of clays is better than a single clay. If you have a clay that works well, and can be relied upon to run uniform in quality, nothing can be better; but most clays vary, and by mixing two or more kinds a variation in one of them, only affects the mixture to the extent to which that clay is used. All of the clays are not likely to vary in the same direction at the same time, consequently the mixture, as a whole, is apt to remain more uniform than when a single clay is used.
The mixing and pugging of clay must be very carefully attended to. There is probably no doubt that the wet pan is the best pugging device for this class of work. If hard clays are used (I do not mean flint clays), it is often better not to pug too long, and when a limit is once fixed it should be adhered to so as to obtain uniformity. If a plastic or natural clay is used, the more thoroughly it is pugged the better, as a rule. There may be a few exceptions, but they are rare.
The molding, I have already said, is done in plaster molds, and must be done by an expert The forming of large pieces, like bathtubs, so as to have perfectly even thickness and true, smooth finish, is a matter of considerable skill. If the molding is poorly done no amount of care can make a perfect piece of it
The enameling is done with slip and glaze, the same as with brick. The small pieces, that are easily handled, can be lined with slip by pouring it in and allowing it to run evenly, and then pouring it out again, but the usual method is to put the slip on with a flat brush when the ware is partially dry. Some prefer to put the slip on after the goods are thoroughly dry. In this event, a short slip, with some liquid glue added to it, is the proper thing to use. The slip is put on in several coats with a brush. Each coat must be allowed to dry thoroughly before the next is applied. Only a small amount of slip must have the glue added to it at one time, for it will start to ferment and spoil very quickly.