When the clay-reducing plant has been selected, the next step in order is to prepare the grit. For cheap and coarse work, sharp and semi-vitreous sands will be sufficient, but for making a uniform grade of body, of good color and density, there is no better material for grit than some of the crude clay burned, crushed, and ground to a powder so as to readily pass through a i6-mesh sieve.
This can be readily and cheaply obtained by putting lumps of clay in all available vacancies among the goods in the kiln. The burning thus costs nothing, because the lumps hold the heat better than air-spaces, and the saving of fuel will fully repay the labor of putting in and taking out.
Having the crude clay and ground grit in condition, we are now ready to think of the best way to prepare the batch or mixture. Where the space, water, and power will permit, it is best to run through a washer a stated quantity of clay reduced to a thick semi-fluid condition. This by being allowed to flow through a long trough having a broad flat bottom, and some trap provided, will leave all stones and refuse behind in its pro gress from the washer to the pit or pan. After this given quantity of clay has been sent to the pit, allow it to stand for awhile until the settlement of the clay has allowed the water to gather on the top. A simple contrivance will form a well in one corner of the pit into which almost all the water will run ; once there, the water can easily be drawn off by a syphon of rubber tube. When the water has been taken away, the grit can then be evenly scattered over the whole surface, and any other material which the workman deems necessary to a good terra-cotta body. This alternate deposit of washed clay and scattered grit, etc., is repeated until the pit is full, and the whole allowed to rest until it is stiff enough for the pugging process.
Another method, when space is limited, is to prepare a level platform about 25 feet square, as shown in Fig. 183, which will give a little over 600 feet of surface, and is capable of holding 50 tons of mixed clay.
As soon as the platform is ready, spread over it evenly forty five wheelbarrow loads of clay that has been passed through the rollers or pulverizers, as the case may be, each load strike measure of about 30o pounds ; use a standard size of wheelbarrow for all material, and insist upon the workmen using a striker to insure equal quantities in each wheelbarrow. This
will save much unnecessary expense in weighing the different materials used in the mixtures, and insure uniformity.
When the crude clay is leveled off to a regular thickness, scatter on the top of it the predetermined proportion of sand or grit. While the grit is being spread over, have a trustworthy man with a hose and coarse sprinkler pour on a steady stream of water sufficient to wet the mass ; this will wash the grit down into the interstices of the clay lumps, and aid greatly in the mixing afterwards. It will, be found useful to mix at least five batches, one over the other in each mixing, as this will give about 45 tons, and it will help to avoid differences or errors in mixing, as it will be certain that the 45 tons will be all alike if care be taken in moving it from the platform to the mill, and from the mill to the pit. After spreading the five batches, go around the edge of the mass and trim up all the loose material, throwing it upon the top of the pile, taking care to keep the top level. Then begin on one side cutting square from A at the top to B at the bottom, and pass the mixture through a pug-mill regularly ; then spread it evenly over the surface of a storage pit as level as it was before on the platform. By this process the vertical section of the platform strata will become the horizontal section of the pit strata, and it will be scarcely possible for two shovelsful to come together in the pit in the same relative positions which they occupied on the platform. Fig. 184 represents five batches of clay and grit laid up in alternate layers, the whole being about 45 tons of dry material. A and B indicate the directions of the vertical slice to be cut off in taking the clay mass to the pit or cellar by way of the pug. mill. The figures in cubes are seen to lie in uniform planes, but when the vertical section becomes the horizontal section by lying in the pit as described above, it will be found that the cubes will be mixed as indicated in the next cut, Fig. 185, where A and B form the bottom layer instead of the end, as in Fig. 184. Fig. 185 represents the change of position of each cube after being placed in the pit. This system prevents either the clay or grit from predominating in any part of the mass. The clay, after mixing, should remain at least one month in the pit before tempering it in the pug-mill.