POTTERY DECORATION UNDER THE GLAZE.
POTTERY DECORATION UNDER THE GLAZE.
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Wewill consider in this chapter some facts in regard to pottery, which, although of a very elementary character, are not so generally understood as may be desirable. These facts relate to the substances of which pottery is composed, the processes of its manufacture, and the methods by which it is or can be decorated.
Two chemical substances, viz., silica and alumina form the basis of all pottery clays. These substances are themselves infusible (except under the compound blow-pipe), but by admixturewith other materials more susceptible to the action of heat, they are made useful to the potter’s art, and, when subjected to a high temperature, fuse and form the hard, insoluable combination with which we are all familiar.
Porcelain, and the finer kinds of earthen-ware, are made from clays artificially combined of various natural elements, in such a manner as to produce the qualities desired, while the coarser wares are generally made from clays in a natural or unmixed state. These clays agree in their essential characteristics, but contain other elements which cause certain differences of color, susceptibility to heat, plasticity, etc. The differences, therefore, which exist between the many kinds of pottery manufactured, may be said to be due, not so much to a diversity of the materials used, as to the changes produced by the combination of the same materials in varying proportions.
It is unnecessary that we should here enter into the details of the mixture of different pottery clays, for which each manufactory has itsown formula. A few words, indicating, in a general way, the causes of certain easily-recognizable qualities, will suffice.
The peculiar beauty and translucency of fine porcelain, is due to an excess of silica, or the vitreous element of pottery, in its composition. This is attained by the use of kaolin, a fine, white clay, produced through the agency of natural causes in the decomposition of feldspar, which is itself a silicate of alumina, and one of the principal ingredients of granitic rocks. In porcelain, it may be said, that the proportion of silica to alumina is about three to one, and the other ingredients, such as iron, potash, chalk, and soda, exist only in such quantities as are necessary to cause the fusion of the two first-mentioned elements. It is fired but slightly the first time. This leaves the body of the ware very soft and porous, and upon the application of the glaze the latter is absorbed into the body, and by its action upon the materials composing it, produces a translucent effect.
We refer, here, to French porcelain. In themanufacture of English china, the process is radically different. The last-mentioned ware is sometime spoken of as “bone china.” This term has reference to the use of bones from which the lime required in its manufacture is obtained. This element produces a translucent quality in the body of the ware independently of the action of the glaze, which in this case forms merely a coating upon the surface of the previously hard baked body.
Earthenware possesses less of the vitreous element, and the first firing is continued until the ware becomes so dense that it will not absorb the glaze, and the body remains opaque.
The term pottery, is more strictly applicable to the kind of ware last mentioned. It is to this that especial reference will be made in the following pages, in describing methods of manufacture and decoration.
Earthenware is of various colors, ranging from pure white, through the cream white and cream-colored wares to decided yellow, and lastly red. The clays can also be artificially colored, and changes in color are also producedby different degrees of temperature in firing. Ordinarily the process of firing produces a material change in the color of clay. Thus, a grey-tinted clay will assume a bright, yellow color, and a dull brown, green, or blue will be changed to a bright brick-red, under the action of the fire.
The heat used in firing should produce partial vitrification. If a piece of ware has not been fired sufficiently to produce that effect, it is said to be “soft-baked.”
White earthenware requires the greatest degree of heat in firing. In regard to the wares made from clays retaining the colors produced by the elements which they contain, in a natural state, it may be said that the amount of heat necessary for proper firing, decreases with the depth of color. This rule, although it may accurately indicate the temperature at which any given clay should be fired, is simply artificial, except in so far as the depth of color shows the presence of an amount of foreign matter, which by its action increases the fusibility of the clay.
Red clay, being very susceptible to heat, is commonly used as a test in the firing of white ware. The same clay will change in the process from light red to dark brown, and so enable the experienced fireman to determine the degree of heat existing in the kiln.
Clays should be prepared for the making of pottery by careful sifting, dissolving in water, and, as a final precaution, straining through sieves made of fine, silk lawn. In this way all foreign particles are removed, and the clay, having attained the consistency of dough, is ready to be molded into the shapes desired. This is done in molds made of plaster of paris, or it is “thrown” by the hands of the potter on a turning-wheel, or, again, by a combination of these processes.
Before firing, the ware is said to be in the “green state,” in which condition it passes from “wet” to “hard green,” and finally to “white,” when it is ready to be fired. If fired before it has become sufficiently dry, it will be liable to crack, or scale, from the sudden expansionof the moisture confined in the clay. When dry, it is placed in “seggars,” or boxes, made of fire-clay, which are piled upon each other in a kiln, constructed for the purpose of firing the ware.
During the process of drying and firing, the clay loses a certain portion of its bulk. The amount of this shrinkage varies in different clays, but ordinarily may be said to be about one-eighth. A piece of ware, therefore, made from clay which exhibits this degree of shrinkage, would be one-eighth smaller after firing than before. The shrinkage is also modified by the degree of heat to which the clay has been subjected in firing, and there will sometimes be a perceptible difference in the size of two pieces of ware, made from the same clay, and of equal size before firing, after having been fired at different temperatures.
The length of time consumed in firing, varies with the qualities of the different wares. In the manufacture of white earthenware, the firing lasts from thirty to forty hours, while the more commonkinds of ware require less time. In kilns of ordinary size, a thousand dozen pieces of ware are frequently fired at once. Of course, considerable time is required to place the seggars containing the ware in the kiln. Some hours must elapse before the kiln and its contents will become cool enough to permit its being opened with safety, so that the firing of hard-baked pottery may require two or three days for its completion.
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.
The glaze used upon the finer kinds of earthenware consists of materials similar to those of which the body of the ware is made, with the addition of boracic acid (a powerful flux), which with a little lead renders the glaze fusible. 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 materials of which the glaze is made, are combined in the proper proportions and diluted to form a liquid of about the consistency and the appearance of cream. The piece of ware is then dipped carefully into the liquid, and so skillfully manipulated that it is completely covered witha coating of the glaze of the necessary thickness.
After having been covered with the glaze, which, before firing, has the appearance of an opaque white paint, the ware is ready for the second firing. This is done in what is called the “gloss” kiln, in which the heat is not brought to such a high degree as in the biscuit kiln, but is sufficient to fuse the glaze, and cause it to form a glassy, transparent surface, which should completely cover the body of the ware, and present an equally brilliant appearance in every part.
If the glaze is not suited to the body of the ware, and does not shrink equally with it, its surface will soon present a network of fine cracks. A glaze in this condition is said to be “crazed.” This, among potters, is considered a serious fault, and in ware intended for cooking or table use, is certainly very undesirable.
Old Japanese crackle-ware, in which this condition is a distinguishing feature, is, however, much esteemed, and many, indeed, the majority of the Japanese earthernwares, both ancient andmodern, display a surface of fine-crackled glaze, which may not be said to detract from their value as articles of ornament.
A similar effect is also seen in old pieces of glazed ware which have seen long service as cooking utensils, in which case it has been the natural result of the usage to which they have been subjected.
In some modern wares, both French and English, decorated under the glaze, this defect is seen. In these cases it has been caused by the use of a softer glaze than the body of the ware demanded. This expedient has been resorted to in order that the brilliancy and beauty of the colors might be preserved. These articles being intended only for ornament, beauty is more of an object than durability, and this defect of glaze may be permissible as the means of obtaining more brilliant effects. In certain methods of decoration this may be a necessity, but these cases are exceptional and experience leads me to believe that it is possible to obtain beauty of coloring with a glaze which will remain intact. Itis to be hoped, however, that with the improvements in making colors the time may come when it will be possible to obtain colors which will retain their beauty under the degree of heat necessary to the production of an article which a practical potter would call a perfect piece of ware.
Pottery can be decorated eitherunderthe glaze,with the glaze, as in the case of majolica, in which the color is effected by the use of colored glazes and in the decoration of soft porcelain (pâte-tendre), where the painting is executed upon the unbaked glaze, and, lastly,overthe glaze. We will concern ourselves only with the first of these methods. A few words, however, may not be out of place here, upon the differences between over and under glaze painting, which are frequently compared and as frequently mistaken for each other.
Over-glaze painting, as is well known, is executed upon the glazed surface of the finished ware with vitrifiable colors having enough flux or fusible material in their composition to cause them tofuse at a comparatively low temperature, and so become attached to the glaze.Under-glaze decoration is effected by the use of colors which contain less flux, and consequently require a higher temperature for their fusion. It can be executed either upon the ware in the “green” state, or, as is more commonly the case, in the biscuit. These two methods of painting differ materially in their effects. Any one familiar with these effects can readily discover whether a piece of pottery has been decorated over or under the glaze. Those who are not practically conversant with the two methods can be easily misled, and mistakes are frequently made by those who should know better, as to the method employed in the case of a piece of ware in question.
A certain delicacy of tint and firmness of outline characterizes overglaze painting, which is easily recognized by experts, but those who can not distinguish between the two methods by this means may ascertain to which class the decoration belongs by looking aslant the surface. The glaze upon the painted portions of a pieceof ware decorated over the glaze, will not, even after the most perfect firing, equal the glaze upon the uncovered portions in brilliancy, and there will probably be certain inequalities of surface between the painted and unpainted parts which will be revealed to the touch.
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. On the other hand, overglaze painting, while not subject to the changes produced by the fire, to so great an extent, is more difficult as regards the manipulation of the painting upon the glazed surface. The facilities for the practice of overglaze painting are greater than those afforded for painting under the glaze, which last, requires not only the handling of an artist whohas acquired facile use of the brush and some experience of pottery clays and colors, but also the assistance of an intelligent and skillful potter.
Let us not, however, laud one method of painting at the expense of the other. Both are good in their way, and confined within their proper limits, have beauties peculiarly their own. Overglaze painting possesses a delicacy of effect and a variety of color which the underglaze decoration can not rival. The latter is, however, the most artistic as well as the most effective, and lends itself more readily to the uses of decorative art and to the modern taste in color.
It is this latter method of decorating pottery which we will now consider in some of its various forms.