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.
Fig. 52. Method of glazing: use of the brush
The glaze was, as I have endeavored to show (cf. [pp. 37 ff.]), applied to the vase in leather-hard condition. At that stage the clay still contains enough water to prevent too rapid absorption, and the glaze, therefore, runs in an easy, flowing manner.[38] To carry out his work the painter could take his time, for, as we have seen (cf. [p. 16]), a vase can be kept leather hard for an indefinite period by the simple device of placing it in a damp box, that is, an air-tight case with a pan of water at the bottom.[39]
Fig. 53. Method of glazing: spraying
Cox, Pottery for Artists, Craftsmen and Teachers, p. 111, fig. 51
Fig. 54. Hydria showing brush marks
Met. Mus. Acc. No. 11.212.7
How did the Greek painter apply the glaze to his pottery? We know that on the broader surfaces the brush was used, for brush marks are clearly visible in many cases.[40] When possible the backgrounds and horizontal bands were painted while the piece was rotating on the wheel; where a panel had to be reserved, the irregularity of free-hand brush work can readily be observed (fig. [54]). For the “flat” glaze lines, in both the figured scenes and the ornamental motives, smaller brushes were employed. But what was the instrument which produced the famous “relief” lines, that is, those fine lines which stand out perceptibly from the surface and which were used for the contours and inner markings of the figures during the best red-figured period (fig. [55])? The quality of these lines has been justly admired by all who have studied them and it has been thought that the instrument used had much to do with the result.[41] After a number of experiments with fine brushes, single bristles, reeds, feathers, pens, etc., I have come to the conclusion that it is of little avail to discuss the instrument used before we can employ in our experiments a glaze identical with the Greek. For to get the wonderful flow of the Greek line, the glaze must run much more easily than any imitation black glaze which we have so far produced. The groove which runs down the middle of the relief lines hardly affords a clue; for slight pressure of any instrument seems to produce such a result.
Fig. 55. Detail of psykter showing relief line
Met. Mus. Acc. No. 10.210.18