When the barrel is full of slip it is allowed to stand over night when some inches of clear water will be found at the top. This is removed with a siphon which may be made of a piece of lead or rubber pipe. The removal of the water results in the thickening of the slip and the contents of the barrel should be thoroughly stirred with a long wooden paddle to insure a uniform consistency. If the slip is found to be still thin another settling and removal of the water will thicken it.
The slip thus prepared will keep indefinitely, provided that it is not allowed to become dry by evaporation. It improves greatly with age. This is the material which is used for casting as will be described later but for plastic work it must be still further thickened. A shallow box may be procured and made water-tight and the slip, when poured into it, will thicken much more rapidly than in the barrel, but it is better to have some shallow plaster dishes as the plaster itself absorbs the water and thickens the clay. Instructions for making these dishes appear in the chapter on plaster.
These directions will suffice for the preparation of a natural clay but it is sometimes desired to prepare a white body either of earthenware or porcelain. These bodies do not exist in nature and therefore a mixture must be made. The ingredients are kaolin or white porcelain clay, ball clay or plastic potters' clay, ground quartz or flint, and ground feldspar.[B]
[B] Georgia Kaolin and Tennessee Ball Clay may be procured from the John H. Sant and Sons Company, East Liverpool, Ohio, and flint and feldspar from the Golding Sons' Company, Trenton, N. J., or the Eureka Flint and Spar Company, Trenton, N. J., in quantities of not less than one barrel or sack.
Georgia Clay[C] 20 parts by weight Tennessee Ball Clay 30 " " " Flint 35 " " " Feldspar 15 " " " --- 100
Georgia Clay 45 parts by weight Flint 35 " " " Feldspar 20 " " " --- 100
[C] If English china clay can be procured it will make a whiter ware than Georgia clay.
The earthenware will be creamy in color and porous at an ordinary fire. The porcelain will need a hard fire and will be white and translucent. It is, however, non-plastic and hard to work. The preparation of these mixtures of course necessitates a pair of scales but otherwise the treatment of the mix is the same as that of natural clay. Washing is not necessary but the clay must be powdered, mixed with the flint and spar, and sprinkled into water as already described. In place of the wire sieve, however, a silk lawn of 120 meshes to the inch should be used.