Glaze

“ 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. ”

“ Glass, or silicious glaze, is formed by fusing sand with an alkali—potash or soda. When to this is added the oxide of lead, transparent plumbiferous glaze is the result; and when to both of these oxide of tin is added, we have opaque stanniferous enamel. The glass and plumbeous glazes may be colored with a variety of other oxides, without losing their transparency.
When or where glaze was first applied to clay is not known. Like many other branches of knowledge and many nations, it has its roots in the East, but whether we are indebted for it to India, Egypt, or Assyria, cannot now be decided. ”

“ The glaze under the base is of pale buff tone and crackled, and a careful examination of the surface generally shows that a faint crackle extends over the whole piece. The glaze, moreover, is full of minute bubbles and consequently much pinholed, and the red colour has the appearance of lying on the body in a dust of minute particles which the glaze has dragged downward in its flow and spread out in a continuous mass, but where the colour and the glaze have run thick the particles reappear in the form of a distinct mottling or dappling. ”

“ But the principal decoration is to be found in the varied coloured glazes with which the wares, whether stoneware or porcelain, were covered. The glaze is never clear and white as at later times; it is generally uneven, imperfectly fused and presents all the marks of an imperfect technique. The nearest approach to white is found in an opalescent grey which shades off to greenish and bluish tints. ”

“ Early Assyrian faience, Della Robbia ware, the Maiolica of Spain and Italy, and the wares of Delft and Rouen are earthenwares coated with a tin enamel.
The silicious glaze here referred to is prepared by fusing silicious materials with soda or potash, and is known as Vitreous, or glass glaze. Plumbeous, or lead glaze, is produced by the addition of oxide of lead to the silicious glaze, rendering it more fusible, and still transparent. A white opaque enamel formed by using oxide of tin with the vitreous glaze, is termed Stanniferous, or tin enamel. ”

“ 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. ”

“ The glaze should not be too thick, for the fine, even quality of the paste is just as much an element as the glaze in giving the old lustre. The blue decoration under the glaze shows a perfect command of outline as well as colour. There is no soaking in of the colour, but the outline is applied by the brush with absolute equality.
The two specimen vases show "Lange-Lysen" or "Long Elizas" and flowers in fluted lotus-shaped medallions covering the whole surface, except under the lip and neck, where there are two bands of triangle-work diaper pattern. ”

“ Sometimes white lustre will retain its glaze over a lightly fired tint, but we suggest that the decoration be retouched after the colors are very dry, and powder it with ivory glaze. Pure glazes or fluxes mixed with oil should never be applied over the unglazed decoration—as it will invariably destroy the colors. ”

“ If the decoration has been executed under the glaze the surface will be uniformly covered with a brilliant glaze. Of the two, underglaze painting is probably the most difficult, as the colors are more liable to change under the action of the great heat to which the ware must be subjected, and the final results being, therefore, somewhat uncertain, can not be accurately counted upon, until experience has been gained from repeated failures. ”

“ The glaze was applied in several ways—by dipping in a tub of glazing liquid (i.e. glaze material finely levigated and mixed with water) , by painting the glaze on with a brush, or by blowing it on from a bamboo tube, the end of which was covered with a piece of tightly stretched gauze. One of the last operations was the finishing off of the foot, which was hollowed out and trimmed and the mark added (if it was to be in blue, as was usually the case) and covered with a spray of glaze. To the connoisseur the finish of the foot is full of meaning. ”

“ In the lower grades of ware a larger amount of lead is used for a flux, and this causes the glaze to fuse at the comparatively low temperature at which these wares are fired. A glaze made of materials that fuse at an unusually low temperature is called a “soft” glaze. Some glazes are so soft that, when fired, they can be easily scratched by the point of a steel instrument. The glaze of good and durable wares is, however, so hard, that the point of the sharpest knife will make no impression on its surface. ”

“ The Keen-lung class of blue and red under the glaze was derived from that discovered in the Yung-ching period. Its application was developed with surprising skill by the finest workmen, so that the application of copper red under the glaze, in combination with blue, gave splendid opportunities for these artists to display their pre-eminent skill for freehand drawing in applying designs upon the biscuit porcelain before the glazing took place. If, for example, you pass your hand over a piece decorated under the glaze, you will find a surface perfectly smooth, the colour has sunk into the paste. ”

“ It is a thick and rather opaque glaze, sufficiently flowing to have left the upper edges almost bare and formed thickly on the flatter and lower levels; the colour is broken by streaks and clouding, which mark the downward flow of the glaze; the surface has a barely perceptible crackle, which will no doubt become more marked with age, and a subdued lustre between the brilliancy of the old opalescent Chün types and the viscous, silken sheen of the Canton glazes [407] which also imitate them. ”

“ Reproductions of the Sung millet-coloured (mi sê) glaze: copied in form and colour from the fragmentary wares dug up at Hsiang Hu (q.v.) .
13. Sung pale green (fên ch’ing) glaze: copied from wares found at the same time as the last.
14. Reproduction of “oil green” (yu lü) glaze: “copied from an old transmutation (yao pien) ware like green jade (pi yü) , with brilliant colour broken by variegated passages and of antique elegance.”
15. The Chün glaze of the muffle stove (lu chün) . “The colour is between that of the Kuangtung wares and the Yi-hsing applied glaze [420] ”

“ The varieties in colour are innumerable and clearly due to the opalescence of the thick, bubbly glaze, combined with the ever–changing effects of copper oxide on a highly fired ware. Lavender grey, dove grey, brown, and grey green are conspicuous, but as the thickness of the glaze varies with its downward flow, so the colour changes in tone and intensity from a thin, almost colourless skin on the upper edges to deep pools of mingled tints where the glaze has collected in thick masses. ”

“ Colors for painting under the glaze are specially prepared for the purpose from various metallic oxyds. As mentioned in the previous chapter, the colors used under the glaze have less flux or fusible matter in their composition than those used for painting overglaze: this is necessary, because in the former the colors are expected to bear a much greater degree of heat in firing than in the latter. They are so prepared as not to fuse except at the temperature required in the manufacture of the pottery to which they are applied. ”

“ Putting aside salt glaze we find a countless number of both lead and leadless glazes. They range from the thin silicious coating of the ancients up to the rich alkaline glazes of the Persians and Chinese; from the raw galena of peasant pottery to the rich Majolicas and fine hard glazes of modern commerce. Salt glaze is obtained by the vaporizing of salt inside the kiln at a great heat. The sodium oxide so formed combines with the silicates in the clay to form a very thin coat of refractory glass, intimately connected with the body. ”

“ Effects of colour of great depth and brilliancy may thus be obtained, and after the glaze has been floated over the surface a touch of the thumb, where the under painting is loaded and lights are required, will so far thin it as to let the underlying colour show through and blend with the deeper tint of the glaze in the shadows. ”

“ After this decorating, the ware is generally passed to a man who stands before a tub of glaze, and dips in each article, though sometimes he stands before the pieces of ware and sprays them with an air brush. Many different kinds of glaze are used, made of ground flint, feldspar, white clay, and other substances. Common sea salt works exceedingly well, not in liquid form, but thrown directly into the fire. The chief thing to look out for in making a glaze is to see that the materials in it are so nearly like those in the ware that they will not contract unevenly and make little cracks. ”

“ Where it is not possible for the amateur, for whom these hints are written, to obtain such wares for decoration, very pretty work can be produced, as has been mentioned before, by the use of a body of naturally colored clay upon which white or a light-colored clay can be applied.
Relief-work can be finished with a full glaze, a very slight glaze, or can be left unglazed. As a general thing it may be said that the modeling in a design in high relief will look best with but a slight glaze, or none at all. ”

“ It is too fluid, add a little clay and flint.
5. A matt glaze burns to a bright surface. Matt glazes must be used in a very thick coat. If too thin they will inevitably brighten. The fire may be too high. The fire may be "reducing," that is, with insufficient air.
6. The glaze crawls or rolls up in lumps. Notice whether the glaze is cracked before burning. If so [167] it will surely crawl. Too fine grinding is usually the cause of this trouble. Too much clay in the glaze may cause it, or a too porous body. A body which is underfired will almost certainly cause the glaze to crawl. ”

“ Certain glazes, as the turquoise and the purple of the demi grand feu, are always finely crackled. In other cases the crackling, which is caused, as we have already said ( (p. page 32) , by the glaze after solidification contracting more than the subjacent paste, may be produced or modified at the will of the potter by adding various substances to the glaze. A rock that has been identified with steatite has been often mentioned in this connection, and the increase in the shrinkage of the glaze attributed to the magnesia contained in it. ”

“ When the piece decorated is thoroughly dry, it should be fired at a temperature sufficient to make the body durable, and at the same time to perfectly fix the colors of the painting, so that there may be no danger that the glaze will cause them to run in the final firing. ”

“ The first firing completed, the ware is said to be in the “biscuit,” and is then ready to be glazed. This matter of glazing is a very important one, and the success of the whole very largely depends upon the manner in which the operation is performed. To insure a successful result there must be the nicest adaptation of the materials composing the glaze to the body of the ware. The glaze for each kind of ware must be suited to its especial characteristics, and it can therefore be imagined that the number of glazes in use is very large. Each pottery has its own glaze and the variety is infinite. ”

“ Majolica consists generally of terra-cotta covered with a glaze. A glaze is a fusible substance, we may say a kind of glass, mixed with colouring matter, which is at the same time a protection and an ornament. Enamel is glass in fine powder melted, used generally on metal or by itself. The base of the paint is a substance fusible by heat which is mixed with colours also fusible. ”

“ The application of glaze has already been described. A pot possessing a noble form and glaze is obviously in no need of decoration; no artist would attempt it. The Chinese and Japanese are safe guides here. Their rich glazes and fine forms are set off in the simplest and most effective manner. This is potting, pure and simple.
Under relief, we group all modelling,—raised or sunk, embossments, flutings, mouldings, feet, handles, or applied figures. At one extreme come the earliest attempts at decoration in slip or clay, highly developed in the Gallo- and Egypto-Roman and Romano-British wares. ”

“ The ware will be clean and white, and the glaze [276] pure, limpid, and lustrous, but with that faint suspicion of green which is rarely absent from Chinese porcelain. The general effect of the body and glaze combined is a solid white like well set curds. The base, to which the connoisseur looks for guidance, is deeply cut and washed in the centre with glaze which reaches about half-way down the sides of the foot rim. This patch of glaze is usually pinholed, as though the nemesis of absolute perfection had to be placated by a few flaws in this inconspicuous part. ”