GLAZING
Besides shaping and firing a vase, a potter must understand the art of glazing, to many the most alluring part of the craft; this comprises both the preparation of the glaze and its application to the pot. A glaze is a chemical compound, known as a silicate, which upon firing becomes a glassy substance. It has three chief ingredients: (1) an oxide of lead, of lime, of alkali, etc., which forms the foundation of the glaze; (2) alumina and boron oxide, which regulate the behavior of the glaze in the fire; and (3) silica, which controls the fitting of the glaze to the body. Modern glazes are divided into two chief categories: lead glazes and leadless glazes. Of these the former are by far the more numerous; but alkaline glazes, tin enamels, etc., are also used, especially in studio potteries.
In the preparation of a glaze the ingredients have first to be weighed out carefully in the required proportions; the mixture has then to be ground with water and sieved; and when the right thickness is attained (usually about the consistency of cream), the glaze is applied to the vase, which should first be soaked in water to saturation to prevent too rapid absorption. Gum tragacanth is used as a binder to make the glaze stick.
The glaze can be applied in various ways—by dipping (fig. [50]), by pouring (fig. [51]), by the use of the brush (fig. [52]), or by spraying (fig. [53]), the method depending chiefly on the individual choice of the potter. To apply a glaze properly to the clay body is a great art, and upon it, as much as on the preparation of the glaze, final success depends. The glaze mixtures when applied are whitish or grayish; it is only after firing that they assume the wonderful range of colors which delight us so much today.
Fig. 50. Method of glazing: dipping
Fig. 51. Method of glazing: pouring
The Athenian potter had no ambition to produce brilliant color effects in glazing. He was content with the one variety which he had brought to perfection—a thin, luminous glaze of a deep, velvety black color and of astonishing durability. After generations of experiments we cannot yet say that we can successfully imitate it. Its composition is, however, no longer a secret. It has been shown by analysis and synthesis that the chief component parts of the glaze are an alkali (potash or soda), a clay (which would contain some silica such as flint naturally), and ferrous oxide.[34] The exact proportion of these parts, and above all the manipulation of the glaze are still unknown. At least, nothing completely corresponding to the Greek glaze has as yet been produced, though Mr. Tonks[35] and especially Mr. Binns[36] have come very near it. Whether the secret lies in the proportion, in the medium used for binder,[37] or in some undiscovered element one cannot tell. It is only certain that when the discovery is made, as doubtless it must be, it will be of great import both to archaeologists and to modern potters.
