It is impossible to say which is absolutely the best machine for any of these purposes, there are so many, and so many good ones ; therefore in this chapter on clay-working machinery only those machines will be mentioned which long and practical ex perience has led Mr. James Taylor to adopt as best suited for the purpose of preparing the soft or plastic clays for the use of the terra-cotta maker.
They are simple, require but a small amount of power, have few wearing parts, and can be operated by any practical clay worker.
The clod-crusher and stone-separator is very necessary to crush all lumps and to remove any stones or foreign matter which may by accident or carelessness have got into the clay.
The Brewer machine has proven in practice to be a very satisfactory one for this purpose.
The next machine in order is one for preparing the grit for mixing with the clay which has been passed through the crusher.
For this work there are very many excellent machines in the market, and nearly every terra-cotta maker chooses a different one.
Some use a simple wheel of iron, very heavy, revolving in an iron track or in a pan. Others use the Blake crusher or some similar form of alligator jaws, while some use very powerful combined crushers, elevators, and grinders. Each person sees in the machine he selects and purchases some special quality that commends it to his judgment, hence his selection.
Mr. Taylor for several years used at the Chicago works a double set of rollers for this purpose, and obtained good results from their use as crushers, but not as grinders ; still, as they required too much power to work them, their use was not profitable enough to pay for repairing them, so when they be came worn they were discontinued. The stone breakers or alligator jaws are open to the same charge ; they cost too much for power to run them, are expensive at first, and costly for repairs, so that they are not profitable for the terra-cotta maker's use.
Few machines are profitable unlets they can be fully em ployed. Powerful machines are costly, demand great power to operate them, and require much fuel to furnish that power. If they can be fully employed in production, doubtless they yield great results and are economical ; but if they have to stand idle, it is not good policy to purchase them.
For a terra-cotta manufactory requiring not more than ten tons of grit per day, the most economical and satisfactory ma chine to employ is a six stamp mill, with a sixteen-inch drop, an illustration of which is shown in Fig. 186.
The next machine to consider is the clay-mixer. For this purpose there are many, and all of them have some good qual ities. The simplest one is the old-fashioned traversing wheel and pit. This is a good mixer, and for soft mud is perhaps the best all-around machine. But for the preparation of terra-cotta clay, it is important that while the clay is being thoroughly mixed, it should at the same time be closely packed or com pressed, so as to expel as much air as possible from the clay or body. This the wheel does not do, therefore it is best to use a powerful pug-mill.
With the clay-mixer the list of preparatory machines is corn plete. From this machine the clay ought to pass into storage cellars or pits, to remain a sufficient time to allow of complete soaking or ripening ; the longer this period is, the better for the material.
When the clay is required for use, it is found necessary to temper the stiff-mud to a proper consistency, so as to render it easy to be handled and pressed into the moulds which are to give it its destined form, so that it may take a good impress.
In doing this it is essential to have the clay thoroughly pugged, so that all stiff and all sloppy portions shall be abso lutely intermixed until the whole is kneaded into a uniform condition of stiff mud. Should one part be soft while another part was stiffer, the work would surely crack and warp in the drying, and would scarcely reach the burning stage. If the work should successfully pass through the kiln, it would be so unequal in size that it would entail much trouble and cost, and would never be a good specimen of the clay-maker's skill. In order to guard against the carelessness or neglect of unskilful workmen it is a good plan to insist upon a double pugging of all terra-cotta clays ; and to insure this it is well to use a double pug-mill, passing the mixture through one, and then through the other immediately afterwards, working both at the same time.