3rd. The large conical furnaces, now in general use in the porcelain factories of Europe. They may be heated by either direct or by reversed flame.[13]
In China the fuel is generally pinewood, in billets of uniform size. In many European kilns wood is still used: birchwood, cut in lengths of fifteen to twenty inches, is the only fuel used at the present day at Sèvres. In England, however, the difficulties attendant on the use of coal appear to have been overcome.
The reader will find in the third volume of Brongniart’s great work (Traité des Arts Céramiques, Paris, 1877) several plates giving plans and sections of all these types of furnaces. From a careful examination of these engravings more is to be learned than from any amount of verbal description. A thorough grasp of the process of firing is of the greatest assistance in understanding the problems and difficulties that arise in the manufacture of porcelain, and we shall have to return to the subject when we come to treat of the several wares.
Whatever differences there may be in the shape of the furnaces, when it comes to filling the interior with the ware to be baked, there is one precaution which has been adopted in nearly every country.[14] The ware must be protected from the direct heat of the flame by means of a case of fireclay in which it is placed. These are the seggars (French cassettes; the process of filling and arranging them is called encastage), to the preparation of which so important a department has to be set apart in all porcelain works, and whose manufacture adds so much to the working expenses.
The seggar proper is a cylindrical pan of fireclay, in shape and size like a hatbox. They are piled, in the furnace, one over the other, and these piles or ‘bungs’ are arranged in the furnace so as to allow a free circulation of the hot gases between them, but otherwise they are packed as closely together as possible. These seggars may be used several times over. When broken, the fragments are ground up and mixed with fresh fireclay or argile-plastique to form new cases—without this addition the clay would be too plastic or ‘fat’ for the purpose. The greatest precautions are taken in the packing of the seggars in the furnace. The giving way of one pile from any inaccuracy in the arrangement may destroy the contents of the whole oven. So again infinite care must be taken in the arrangement and support of the objects in each seggar. The bottom is covered with ground flint or other infusible material, and the vessel is supported, when necessary, by various forms of struts, props, or crow-claws, which sometimes leave their mark on the base or side of the finished object. In spite of these precautions, a large quantity of defective pieces or ‘wasters’ are produced in all works, and these are usually cast aside. The finding of such fragments in after days is sometimes the only proof we have that porcelain or pottery has formerly been made at the spot. But the proof is final, for defective pieces and ‘crow-claws’ are not objects likely to have been imported from a distance. Again, the indelible marks left on the porcelain, either on the edge which rested directly on the seggar or at the points where the object was supported by the crow-claws, often give valuable hints as to the provenance of the piece in question.[15] In the case of valuable wares these rough edges and marks are removed as far as possible by grinding on a small wheel, and then polishing the surface with pumice or with putty.
CHAPTER III
GLAZES
BEFORE attacking the somewhat complicated subject of the nature and composition of glazes, it will be well to take up again the thread of the mechanical processes that are involved in the making of a piece of porcelain.
The materials that enter into the glaze are reduced to the finest powder in mills similar to those in which the china-stone and flint are ground for the preparation of the paste. If any substance soluble in water, such as borax or salts of the alkalis, enter into the composition of the glaze, these must be first partially fused in combination with the other materials to form a frit, a kind of imperfect glass. These frits, which enter so largely into the composition of soft-paste porcelain, are formed with the object of bringing the soluble constituents into an insoluble form before mixing with water to form the slip. There are indeed other practical reasons that render a preliminary partial fusion desirable.
The finely ground elements of the glaze, mixed in due proportion, are worked up with water to form a creamlike slip into which the vessel to be glazed is now dipped. In China, in many cases, the glaze-slip is blown upon the surface in the form of a spray. This is done by means of a bamboo tube, covered at one end by a piece of silk gauze, through which the liquid is projected by the breath of the operator (French, insufflation); in other cases the glaze may be painted on with a brush. In China, as we have mentioned, the glaze-slip is generally applied to the raw surface of the thoroughly dried but unbaked ware, but in other countries there is, almost without exception, a preliminary firing of greater or less degree to produce a biscuit.
We shall restrict the use of the word glaze to the vitreous coating applied directly to the surface of the raw paste or of the biscuit to enhance the decorative effect of the ware, and with the more prosaic object of allowing the surface to be easily kept clean. In the case of porcelain this coating is always more or less transparent.[16] There is here no necessity for concealing the natural white colour of the paste. In the case of many kinds of pottery, however, as in the ‘enamelled fayence’ of Delft and Italy, the glaze is rendered opaque by the addition of oxide of tin, so that the ill-favoured ground is concealed by a white shiny surface which may be made to resemble closely the natural surface of porcelain. A glaze of this kind is often called an enamel, but as we are not concerned with such an expedient we shall confine the use of that word to the various forms in which a vitreous decoration, whether translucent or opaque, is superimposed upon the glaze and fused into it, more or less thoroughly, by a subsequent firing in a muffle furnace.