Modelling a Decoration.—If the decoration is to be modelled it would seem wise to do it, in whole or in part, as the work is built up; but in this case great care will be needed to keep a firm hold on the relief and unity of the decoration. It will be easy to over-model the work.
How Under-glaze is Applied.—For decoration with under glaze the colours given under the head of glazing ([page 304]) are mixed in different proportions with dry powdered clay and water to form a colour paste. This is painted on the "green" or moist clay, forming a smooth and even surface. Experience will teach the proportions of colour to be mixed with the clay. These proportions vary greatly with different colours. A very strong colour like cobalt will give a deep blue if mixed in the ratio of one part by weight of cobalt to ten parts of clay. Colours like the oxides of iron and copper are of medium strength, and antimony is quite weak.
Before the work is left to dry it would be well to make sure that all corners and rough edges are smoothed off as they will show light and rough through the glaze. The use of sand-paper, however, is not advised, though it may occasionally be used in the emergency of an accidental roughness remaining after the piece is dry. The aim should be to have all clay work show something of the plastic nature of the material out of which it is made.
Firing.—Pottery must be "bone" dry before it is fired. A very satisfactory portable kiln for firing may be purchased for from $34 to $175, according to the size. The smallest size, which is illustrated in the next drawing, will be ample for the needs of one or two persons. If, however, the amateur does not care to go to the expense of purchasing a kiln, it is generally possible to find a pottery factory in the vicinity that will undertake the firing and perhaps the glazing.
A portable kiln
Temperature Required.—Pottery is fired at a temperature varying, according to the clay and the glaze used, from approximately 1800 to 2000 degrees Fahrenheit. Modelling clay fires at 1958 degrees Fahrenheit, or what is called "cone 04." This term comes from the fact that the heat is gauged by pyrometric cones, which can be seen through a spy hole attached to the kiln. These cones are graded compositions of various materials and possess different degrees of resistance to heat. They are usually set up in clay in groups of three or more. When one of them melts it bends over, as illustrated in the accompanying drawing, which shows cone 05—the most fusible one—completely "down," and 04 at a point indicating that it is time to shut off the heat from the kiln. This must be done gradually and the kiln must be allowed to cool completely before it is opened. This is especially necessary when firing the glazes described in the following section, which require the same temperature as modelling clay. Further details about firing and caring for the kiln may, of course, be learned from descriptions accompanying the apparatus. Pottery comes out of the first firing in the kiln a dull porous ware and in colour either cream, buff, or red, according to the amount of iron in the clay. In this state it is called "bisque" or "biscuit."
Pyrometric cones
Glazing.—It is necessary to bring the pottery up to the condition of bisque before it can be glazed; or, to speak more accurately, before it can be over-glazed; for it has already been shown how the so-called under-glaze is put on before the first firing. The subject is somewhat technical, and it will not be possible in a single chapter to take up the details extensively. Briefly speaking, the glazes most used for this class of work are the lead glazes—combinations of "white lead" or carbonate of lead as a flux with kaolin, flint, whiting, feldspar, and other ingredients. These are supplied in powdered form and are ground together in water by means of a mill or a large mortar and pestle; a mortar 8 inches in diameter will serve.