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The Making Of Chinaware
If the reader wishes to follow up in detail all the intimate minutiae of porcelain manufacture, and to become acquainted with all the technical processes, the books noted in the bibliography at the end of the volume had best be consulted. As we are chiefly concerned here with the aspect of the finished product, it must suffice to point out a few of the salient facts connected with the transformation from the original elements to the ultimate stages of decoration.
After the materials are finely ground, washed and filtered, they are mixed in the desired proportions and the plastic clay is thoroughly worked and kneaded to ensure uniformity of texture. The clay may, perhaps, be "short" and inclined to crumble, or it may be highly plastic and "fat," as is apt to be the case when there is present in the mixture a high percentage of kaolin. The nature of the clay and its degree of plasticity necessarily somewhat determine the methods by which the articles to be made are shaped.
A lump of clay may be "thrown" on the potter's wheel and gradually shaped by the manipulation of the potter's thumbs and fingers as the wheel revolves. This method implies an object of circular form and a proper degree of plasticity on the part of the clay. Again, objects may be molded, the clay being pressed firmly into molds of the desired shape. Fluted articles, articles with raised patterns or perforations, and such members as the handles of cups, the handles and spouts of teapots, and the handles and knobs of tureens and vegetable dishes must all be molded as well as lids, the generality of plates, all plates and platters of form other than circular and, as a rule, most articles with the exception of such jars, bowls and vases of circular form as can be shaped on the potter's wheel. Statuettes and figures, of course, are molded, unless shaped by the casting process which, however, was unknown in the East. Handles, spouts, knobs and embellishments in high relief, such as figures and flowers, are molded separately and then attached in their proper places with "slip." "Slip" is a fluid mixture of the clay body, of a thick cream-like consistency. After the separately molded members are attached in their proper places, the articles are set away to dry until the time comes for them to be fired. Many objects of elaborate and complex shape have to be built up of a number of separately molded parts.
In casting porcelain a thick "slip" is poured into a plaster-of-Paris mold. The water is absorbed or drained off, and when the clay has dried and hardened to the right extent to hold its shape, the plaster mold is removed.
When the ware has been carefully dried, it is fired. In China it was customary, with a great deal of the porcelain, to apply the fluid glaze, directly to the surface of the air-dried object and then subject it to only one firing at intense heat. Much hard paste porcelain in Europe, however, had a preliminary firing at a dull to full red-heat. This "half baked ware" was afterwards glazed and subjected to a firing at the full temperature necessary, which served to make the glaze become virtually part and parcel of the body. In England the bone pastes were first fired at the most intense heat. Afterwards they were glazed and fired only to a temperature sufficient to melt the glaze and make it adhere inseparably to the body. The same method was followed for firing and glazing soft paste porcelains.
With the exception of underglaze colors, all colored decoration with enamel colors and all gilding are applied after the article has been through its second or glazing firing and these overglaze enamels and gilding must be fixed by an additional firing at little more than a clear red heat, a temperature much lower than is needed to melt the glaze.
The Glaze. The term "glaze" is really only another form of the word "glass," and there is not a great deal of difference in chemical composition between the glaze on the surface of china and the glass which we daily see and handle.
The glaze is a very important factor, not only so far as the aspect of porcelain is concerned, but also as one of the cardinal items of identification. There are many variations of glazing and some of these are peculiar to certain kinds of china; all of them add their quota of individuality and distinction to the ware of which they form the skin. The different glazes vary in their quality which the sense of touch distinguishes as well as in the appearance they reveal to the sight.
The glaze is transparent and ordinarily colorless, or almost altogether so, so that the white body or paste beneath is perfectly visible. In some of the early Chinese porcelain the decoration is effected by the use of one or more colored glazes, but such glazes are the exception rather than the rule. In still other cases the glaze, although perfectly transparent, is slightly tinged with a greenish, bluish or yellowish tint which, whether intentional or not, often enhances the beauty of the general effect. In some of the old Chinese blue and white porcelain the glaze has a slight bluish tinge which acts as a pleasant bond between the white of the body and the blue of the decoration.
In China the glaze was made of the pure petuntse or china-stone, sometimes softened with a little lime, and was applied in a thin fluid state to the air-dried but still unfired objects. They were then fired at an intense heat and finished at one firing, the glaze becoming thoroughly incorporated with the body. In Europe and Japan it was customary to fire the pieces first at a moderate heat of between 600 and 900 degrees Centigrade. Afterwards they were covered with the coating of glaze and subjected to a second firing at the full temperature of from 1350 to 1500 degrees Centigrade, the heat required by hard paste porcelain. Thus the porcelainisation of the body and the fusion of the glaze took place simultaneously.
An accurate mental picture explanatory of this process is most happily conveyed by Burton, in his Porcelain: Its Nature, Art and Manufacture. He says:--
"It will be readily conceived that under such circumstances the melting glaze, containing fusible ingredients similar to those used in the body, will also attack and partly dissolve the outer layer of the body substance, and we may picture to ourselves a piece of glazed porcelain of the first class as consisting of many layers of different silicates, some of them of excessive thinness and none of them sharply defined, ranging from the outer skin of the glaze, which in perfect pieces is always the clearest, down to the body itself, which is a felted mass of minute crystalline rods imbedded in a more glassy substance. Only by forming some such mental picture, which is in harmony with the knowledge obtained by a microscopical examination of thin slices of the material, can we understand where the distinctive beauty of porcelain resides. When light falls upon a piece of true porcelain it penetrates these successive layers, which, so to speak, filter, soften and subdue it, so that the lowest depths shimmer and glisten with the light they reflect to the observer's eye through the successive envelopes of more translucent substance."
The process of glazing was different for the soft paste or glassy porcelains and for bone porcelain. The unglazed articles were fired up to a temperature of 1100 to 1150 degrees Centigrade. They were then coated with a glaze that was virtually a glass, rich in lead oxide or borax, and subjected to a second firing, at about 1000 degrees Centigrade, a temperature sufficient to melt and fuse the glaze. Of such glazes Burton observes:
"Glazes made in this way are always thinner, more transparent and brilliant, more `glassy,' in a word, than those in the first class for hard paste porcelain, and from their nature and method of formation they lack the subtle depth and unctuous richness of the latter, because they affect the light less as it passes through them."
THE DECORATION. China may be decorated in two ways. First, the decoration may be contained in the body of the piece itself and consist of engraving, embossing, perforations or fret work, or of applied reliefs. All of these devices are perfected before glazing. This may be called decoration in the white. Second, the decoration may be accomplished by means of colors or gilding.
These methods may be employed singly or in combination.
DECORATION IN THE WHITE. Decoration in the white is effected by engraving or incising patterns in the body of an article before it is fired, or the patterns may be impressed by the mold in which the article is first shaped. The engraving or incising process is exemplified in such pieces as the Oriental ware decorated with the rice-grain motifs. When patterns are embossed in low relief, they are made by the molds or, when higher relief is desired, patterns are produced by painting with thick "slip" (a thick fluid form of the clay) upon the surface of the previously air-dried article. When the slip painting, in turn, has dried, the article is ready for glazing and firing. Sometimes the piece of porcelain is first covered with colored glaze and fired, and then painted with white slip, thus necessitating a second glazing and firing. In certain cases these slip painted reliefs are as delicate as lacework. Examples of the low embossed or raised patterns are to be found in the basket work now and again found on the rims of plates. Perforations or fretwork, in such instances as plate rims, fruit baskets and the old Chinese porcelain lanterns, often supply a distinctive decoration. Separately molded reliefs, such as rosettes for the intersections of fretwork, sprigs, flowers and the various sorts of figures employed as knobs and handles are attached with slip to the body of the piece before glazing. In much of the old Chinese Fuchien porcelain these molded decorations in the white are very beautiful and not infrequently of an elaborate nature.
DECORATION IN COLOR AND GOLD. The colors used in decorating china are of two sorts, the underglaze colors applied before glazing and firing, and the enamel colors and gold applied after glazing. Enamel colors and gold thus applied require a second firing to make them fuse with the glaze and become permanent.
The most substantial and reliable underglaze color is blue, made from cobalt. In the old blue and white ware of China the decorations were painted with this blue on the air-dried bodies of the articles which were subsequently glazed and fired. At a comparatively early date the Chinese developed also an underglaze red. This was difficult to produce, however, and the secret of it was afterwards lost. Another underglaze red followed later but was never nearly so much used as the blue. Underglaze blue was likewise commonly employed by the European makers of porcelain. An underglaze rose was invented in England in the eighteenth century. Foreign potters frequently speak of it as "English pink," and it has been very extensively used. Somewhat later a few other underglaze colors were developed on the Continent and, about the beginning of the nineteenth century, the range of these colors was increased by a good underglaze green and some other colors that were not so pleasing. After all is said and done, blue was the one reliable and serviceable underglaze color that could always be depended upon and was almost exclusively used, although in many cases overglaze blues likewise occurred.
The overglaze enamels were easy to manipulate and remarkably varied effects were produced with a comparatively limited palette, although by the eighteenth century the Chinese had devised an adequate chromatic range much ampler than the color resources at their command in earlier times. In Europe, too, especially in France, during the eighteenth century, the colour possibilities were greatly enriched. Enamel colors on the glaze of hard paste porcelain often stand up perceptibly from the surface of the glaze, for the glaze is so hard that the enamel colors, which require a comparatively low temperature for their firing, do not thoroughly fuse with it. On the other hand, enamel colors applied over the glaze of soft paste porcelain very often melt into and become thoroughly incorporated with it so that their presence above the surface of the glaze is neither visible to the eye nor palpable to the touch and you feel nothing but the soft glossy coating of glaze. This absorption of the enamel colors by the glaze both protects them and adds to their lustre.
The third way of applying color decoration to china is by the use of colored glazes, previously mentioned. The glaze may be of one color over the whole of a piece, as in the case of the old Chinese Celadon ware, or glazes of several different colors may be applied to different parts of the same article. This method was used in adorning some of the old Chinese wares. In some cases a piece covered with a single-colored glaze was further embellished with designs painted upon its surface in enamel colors.