For those who wish to make large, heavy pieces, grog clay is a safer medium in which to work. Grog clay is particularly adapted to large pieces of pottery and ceramic sculpture. It is not only of an interesting texture, but also has the advantage of a relatively small shrinkage. The pieces made of grog clay are almost indestructible to breakage in the building as well as in the firing.
Grog clay is a combination of pure clay flour and coarse clay flour which has been fired in a kiln. The grog may also be pulverized bisque ware, drainage tile, or bricks. It should be screened through a twenty-mesh screen—a fly screen will do—to secure the desired texture. The grog clay which has proved most successful is a mixture of half clay flour and half grog. The two are mixed dry and then moistened with water. It should be allowed to stand overnight before using. When it is too wet, it may be dried out on newspapers or a plaster slab.
The dilution of pure clay with grog may have derived its name from the term "grog" used by the English sailors in reference to the diluted rum served to them. "Old Grog," the nickname of Admiral Edward Vernon, was originally given him because of the coarse silk and mohair "Grogan" cloak he wore. Later he initiated a diluted rum to be served to his sailors which was later called
The grog clay will seem more firm to handle, quite crumbly, and less plastic. It should be used in much the same way that plain clay is used except that the coils should be thicker, shorter, and flattened, if handling is too difficult. Since the great weight will cause the walls to sag, an electric fan should be turned on the piece to harden and dry the clay. Turn the hand potter s wheel at intervals so that one side will not become too dry. In spinning the wheel, care should be taken to turn it slowly to prevent distortion. Slip is applied between the coils. The slip is made of the basic clay flour and water. If the grog clay seems to be drying and more tooling should be done, water may be applied with a spray gun or a sponge.
The finer the clay grog, the finer the detail of the finish.
The grog and the basic clay may be different. If there is fine bisque dust, it will affect the basic color of the clay. Particles of grog will rough up with a tool. A flat wooden tool or a rubber mallet tapped against the surface will drive the grog into the body of the piece and give a smooth surface without covering up the grog. Hammered marks, grooves, carving, relief decorations in grog clay all make interesting surfaces. Colored slip may be sprayed or sponged onto the clay. Different colors of grog clay may be used on the same piece, in ceramic sculpture.
The drying of grog clay does not require as much care as pure clay. For two days, leave a dry cloth over the grog piece. Remove the cloth and continue to dry it at room temperature. Always protect the open rim, handles, spout, or any other projections on the piece by covering them with a smaller cloth. These projections should be dried last. A vase or a bowl is usually turned upside down to retard the drying of the rim. The piece should be completely dry before firing. However, pieces that were rigid but not dry have been fired with no breakage. A piece may be forced dry by placing it in an ordinary kitchen-stove oven for several hours at a low temperature. The clay may warp, however.
Solid ceramic sculpture in grog should be hollowed out so that no section is more than 11/2 inch thick. It may be dug out with a spoon or a wire loop tool.
The shrinkage of a grog piece is small when compared to that of the pure clay. The kiln temperature may be higher. The rough edges should be rasped, ground, or sanded after the bisque firing. The clay is too fragile for this to be done earlier.
PLATE XXIV. The bottom of the bowl is shaped in a saucer-shaped mold. At left is the clay prepared for use by the famous Indian artisan.
Freshly molded bowls are in the foreground.
In glazing grog pieces, some care should be taken. The rough texture requires a great amount of glaze. There is less running and flowing of the glazes in the firing. Transparent and translucent glazes are most effective for this material. Slip and underglaze decorations are well adapted to the smoother textures of grog pieces. If the clay absorbs less than six per cent moisture, it may be considered durable for outdoor use.
PLATE XXV. The fire is started with cedar wood.