Fourth, glazing. It is well to have ready a small supply of a standard clear glaze. Each of the test pieces should be covered with this in a rather thin coat and then they should all be fired again, this time close together so that they will receive the same heat treatment. This will enable one to determine what degree of fire for the clay will best suit the glaze.
CHAPTER V
The Preparation of the Clay
A clay having been selected in accordance with the tests described, it becomes necessary to prepare it for use. A fairly large supply should be obtained and stored in a dry place. Most natural clays need some kind of cleansing for there are almost always foreign substances present. This cleansing is accomplished by reducing the clay to the fluid known as slip. The necessary appliances for making slip are as follows:
- A large sieve of quarter-inch mesh.
- A small wire sieve of about 14 meshes to the inch.
- A large barrel.
- Two galvanized pails.
The clay is, after drying, powdered and sifted through the large sieve. One of the pails is half filled with clean water and the clay, handful by handful, is sprinkled into it. The clay rapidly absorbs the water and sinks to the bottom. The addition of clay is continued until a small mound rises through the water, when the whole is left to soak for an hour. The bared arm is then plunged into the pail and the mass stirred vigorously. A stick or paddle will serve, of course, but the potter learns a great deal by the feel of the clay and therefore the hand is best. It is said that he is a poor sailor who will not dip his hands in the tar bucket and in like manner, he is a poor potter who fears the slip tub. This stirring will tell a good deal about the probable working of the clay. It may be stony or sandy or greasy. The large stones and roots will have been removed by the sieve but now, after thorough mixing, the slip is poured through the small sieve into the barrel. Both pails may be kept going at once, one being filled while the other is soaking and so on until the barrel is full or, at least, a good quantity of slip has been prepared.
If the clay prove very sandy it should be washed. The mixture in the pail having been well stirred is allowed to stand for a definite time, say one minute. The slip is then poured into the second pail and it will be found that a quantity of sand has settled. This is thrown away and the slip in the second pail is examined. If enough sand has been removed, the slip may be poured into the barrel, using the fine sieve as already described. If still sandy the process should be repeated, the settling being for two minutes. Experience is the best guide in this operation but all the sand should not be removed.
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]