A suitable mixture for earthenware is—
| Georgia Clay[C] | 20 parts by weight |
| Tennessee Ball Clay | 30 " " " |
| Flint | 35 " " " |
| Feldspar | 15 " " " |
| —— | |
| 100 |
and for porcelain—
| Georgia Clay | 45 parts by weight |
| Flint | 35 " " " |
| Feldspar | 20 " " " |
| —— | |
| 100 |
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.
The lawn is simply a fine sieve and is named because of the material (also called bolting cloth), with which it is covered. Have a carpenter make a box without a bottom. Cypress or oak should be used and this should be a full half inch thick. Four strips of the same thickness are also to be provided. The box may be of any convenient size; eight inches square and four inches deep is about right. The sides should be fastened together with brass screws to avoid rust and a piece of lawn is strained tightly across the bottom and secured with copper or brass tacks. A strip of coarse muslin folded and laid along the edges will help to prevent the lawn from tearing, the tacks being, of course, driven through both muslin and lawn. Then the four wooden strips are set upon the muslin and secured with brass screws. The completed lawn is then a tray of which the bottom is formed of lawn. The strips of wood beneath serve to protect the lawn when placed on a table as well as to assist in holding it firmly.[D]
For storing clay in the plastic state there is nothing better than stoneware jars. These may be had of any size and a tinman should make close-fitting covers. Earthenware covers do not fit tight and are always getting broken. A little water is poured into each jar and a support provided for the clay so that it does not rest in contact with the water. Under any conditions clay will slowly harden so that not too large a stock should be kept. Slip, on the other hand, keeps well so long as some water is always on the top and it is not a long process to stiffen it into clay.
CHAPTER VI
Mold-making and Plaster
Plaster is almost a necessity to the potter and therefore something should be learned about it. Even if one does not use molds there are numberless purposes for which plaster is convenient. For stiffening slip into clay, and for absorbing water from glazes, shallow dishes of plaster are used, and for holding work either in making or drying, plaster bats or round slabs are always in demand.