A trial should be made before any important work is undertaken, both to see that the color is right and to discover any discrepancy in shrinkage. If a clay shrinks too much, a little ground flint may be added. If it shrinks too little, a little ball clay will correct it. The tint produced by the color is apt to darken in the kiln but the general hue will be similar to that of the color used.
For some classes of work a native red clay gives admirable results. It may be lightened by the use of kaolin and flint and darkened by adding burnt umber. These colors are more satisfactory than greens and blues in clay because the brown and red tones are natural, the others are artificial.
If a good buff-burning clay be available, it forms the best possible foundation for color work. Burnt umber will darken it and red clay may be mixed with it, always having regard to the matter of shrinkage already mentioned.
Very pleasing effects may be produced by inlaying one clay with another. The pattern or design is first cut out as described under incising and then the second clay is pressed, morsel by morsel, into the excavation. The surface is cleaned off level with the body of the piece and the whole may be either polished or glazed.
A plastic clay can be polished when leather hard and the finish will remain after firing. Any tool of steel, boxwood or ivory will do the work but a good supply of patience is needed so that the whole surface may be uniformly treated.
For color decoration upon the pottery, ordinary underglaze colors are used, either upon the unburned clay or upon the burned ware commonly called biscuit. For use upon the clay, the colors should be mixed in water, using a little molasses, sugar, glycerine or gum arabic to make the color flow easily from the brush. Before burning, a little glaze should be sprayed over the work with an atomizer. Any ordinary fusible glaze will do. It is diluted with a good deal of water as only the very thinnest coat is necessary. The spray should not be held long in one place or the water will flow and smear the color. If the piece be turned slowly around the clay will absorb the water as it is applied. If this spraying be not done the colors will be apt to rub off after burning. Under-glaze colors are not fusible and hence they come from the fire as dry powders.