This double mill is driven by a single belt six inches wide and of good quality.
As the geared wheels revolve in opposite directions, the mo tion is very steady and free from the usual clanging of ordinary machinery.
One man and two boys can attend to this mill and insure good results. The man shovels the clay into the left-hand hopper, and as it passes out of the orifice at the bottom of the left-hand cylinder, one boy catches the lumps and throws them into the hopper of the right-hand cylinder, and as it is forced out at the bottom of the right-hand cylinder the second boy catches it and rolls it into blocks of about 5o pounds each ; these are now ready to pass to the presses, and any manufac turer who will use these methods will be sure of preparing good terra-cotta clay. The clay having been properly pre pared is next moulded or modeled into the required design, and if it be a panel or similar piece of work it is turned out of the mould on to a drying-board to be finished by retouching, undercutting, and drying.
These drying-boards are an important part of the furniture of a factory, and require to be carefully made. They should be strong enough not to bend in the slightest, when being lifted about by the workman, or else the piece of work upon them is liable to get broken by undue strain upon some weak point. They should be made of white pine, as that wood is the least liable to warp and split under the alternate wetting and extreme drying to which they are subjected while in use.
Drying-boards under two feet long may be made of one inch stock (smaller ones of even less thickness) ; those under three feet long should be one-and-one-quarter inches thick, those under four feet should be one-and-one-half inches thick, and all over this length should be two inches thick.
When the drying-boards are made of strips six inches wide secured to cleats of the same size, the strips having an open joint three-quarters of an inch, they are less liable to split and warp in use than when wider boards are used ; moreover, the open joint permits the air to circulate in the interior of the piece of work, and thus expel the moisture on all sides at once.
Figure 187 shows such a piece of work placed upon a dry ing-board such as has been described, and illustrates a practical method of drying terra-cotta work preparatory for the kilns.
A rough bracket, it will be seen, has been screwed on the lowed end of the drying-board to permit it being placed at a sharp incline without allowing the piece of work to slide off.
This incline is given in order to reduce the friction of the clay upon the drying-board, and thus avoid cracking. It is easy to understand this principle ; the clay contracts as it dries, and some clays are not tenacious enough to draw their weight over so much surface when laid level, but when placed upon an in cline, the material naturally falls into its place during the dry ing process. For large pieces it is advisable to have brackets at both ends and alternate the incline daily ; because the lower end has to sustain all the weight, and is apt to crack if not sometimes relieved ; also the top end dries quickest, so that the reversing of the ends tends to an equal drying of all the parts.
It is obvious to the most casual observer that a panel of clay-work such as is described above has four corners and four lines of exposed edges. The four corners will dry first because they are exposed to the air on three sides, the upper edges will dry next quickest because they are exposed on two sides, while the top surface will dry slowest of all the exposed parts, because it is only acted upon by the air on one side ; moreover, the centre of such a panel will dry more slowly because the moisture is constantly being attracted from the interior. If this panel is placed upon the drying-board and left to dry, it will surely crack unless the clay is very strong, and then it would be likely to warp badly.
The reason for this is very simple, and is in obedience to a natural law.