As for the other constituent of porcelain, the petuntse or china-stone, we have called it a disintegrated granite, and this is the condition in which it is usually excavated. It corresponds to the French cailloux, the stony or gravelly material as opposed to the clay. In French works it is not generally distinguished from felspar, and indeed some varieties of petuntse may contain little else. However, if pure felspar is used, the second constituent in granite or in petuntse, I mean quartz, will have to be added to our porcelain paste in the form of sand or powdered flint. The third constituent of the china-stone, the mica, is usually neglected: in many cases the mother rock contains but little, and what there is is eliminated in the washing. Mica is more fusible than felspar; the white variety, muscovite, is practically free from iron, and only from granite rocks containing this variety can petuntse suitable for the manufacture of porcelain be obtained. The importance of mica as an element of the Chinese petuntse has only recently been recognised (Vogt, Comptes Rendus, 1890, p. 43). As much as 40 per cent. of muscovite has been found in samples brought from China. The pegmatite of the Limoges district, on the other hand, contains only 30 per cent. of this white mica, and of this only a small portion passes into the paste. We have here, perhaps, the principal cause of the greater hardness and the higher softening-point of European compared with Oriental porcelain.
We shall see later on that this softer Chinese paste has many advantages, especially in its relation to the glaze and the enamels, but for the present we will continue to take the more ‘severe’ European porcelain as our type.
Let us consider what takes place during the firing of a paste of this latter description. After all the water, including that in combination in the kaolin, has been driven off, we have, as the temperature rises, an intimate mixture of two silicates, one of which, if heated alone, would be unaltered by any temperature at our command—this is the silicate of alumina derived from the kaolin; while the other is a fusible silicate of alumina and potash. There is also present a certain amount of free silica. There is reason to believe that at a certain point a chemical reaction takes place between these constituents, accompanied by a local rapid rise of temperature in the materials, the rise being due to this reaction. As a result there is a rearrangement of the molecules of the mass, although no complete fusion takes place. It is now, says M. Vernadsky (Comptes Rendus, 1890, p. 1377)—we are now following the account of his experiments—that the sub-crystalline rods—the baculites of which we have already spoken—are formed. M. Vernadsky claims to have separated these rods from the glassy base by means of hydrofluoric acid, in which the former were insoluble. He found them to consist of a very basic silicate of alumina, containing as much as 70 per cent. of that earth, while the glassy base was chiefly composed of silica in combination with the potash and with a small quantity of alumina. In their optical properties the crystals or baculites resemble the mineral known as sillimanite, a natural silicate of alumina.
This is all that scientific research has so far been able to tell us of the intimate constitution of porcelain; but as far as it goes, it is evidence in favour of our claim that we are dealing with a definite substance, sui generis, and not merely with a casual mixture of certain superior kinds of clay, something, as we have said, between glass and stoneware.
There are certain other elements that enter at times into the composition of porcelain—magnesia, which may have been added to the paste in the form either of steatite or magnesite; and lime, derived either from gypsum or chalk. These additions generally tend to increase the fusibility of the paste, especially when accompanied by an additional dose of silica; but as their presence is not essential we are not concerned with these substances here.
The glazes used for porcelain are as a rule distinguished by their comparative infusibility and by their containing no lead. The composition of these glazes follows more or less that of the paste that they cover, with such modifications, however, as to allow of a somewhat lower fusing-point: as in the case of the paste, there is a harder and more refractory, and a softer and more fusible, type. The harder glazes are composed essentially of felspar, with the addition in most cases of silica, kaolin, and powdered fragments of porcelain. At Sèvres, a natural rock, pegmatite, consisting chiefly of felspar, has been melted to form a glaze without further addition. Of late years, however, the introduction of a milder type of porcelain has necessitated the use of a more fusible glaze, containing a considerable quantity of lime, and it is a glaze of this latter type that has with few exceptions found favour in other districts where porcelain is made.
We have attempted in this chapter to give some idea of the nature of porcelain from a physical and chemical point of view, and in doing so have taken as our type the hard, refractory paste of Europe. When we come to describe the porcelain of the Chinese, we shall notice some important divergences from this type. We say nothing here of the soft-paste porcelains, seeing that so long as we confine ourselves to the question of chemical composition and physical properties, they lie entirely outside our definitions. It is only from the point of view of its history and of its artistic qualities that this group has any claim to the name of porcelain.
CHAPTER II
THE MATERIALS: MIXING, FASHIONING, AND FIRING
IT would be quite foreign to the scope and object of this book to attempt to describe in any detail the different processes that come into play in the manufacture of a piece of porcelain. There is the less cause for any such detailed treatment, inasmuch as the operations involved in the preparation of the paste and in the subsequent potting and firing do not essentially differ in the case of porcelain from those employed in the manufacture of other classes of pottery. The differences are rather those of degree—greater care is necessary in the selection of the materials, and these materials must be more finely ground and more intimately mixed. Again, the great heat required in the kilns necessitates, in the firing of porcelain, many precautions that are not called for in the case of earthenware or fayence. Without, however, some slight acquaintance with the processes of the manufacture, it would be impossible to avoid an amateurish and somewhat ‘anecdotal’ treatment of our subject. There are, indeed, many intimate features, many delicate shades of difference that distinguish the wares of various times and places, both in Europe and in the East, which can only be rationally explained by reference to the details of the manufacture.
At the present day there is only one district in Europe where true porcelain is manufactured on a large scale. This district lies on the western and south-western border of the central granitic plateau of France, especially in the Limousin and in Berry. Again at Sèvres, for the last hundred years and more, a succession of able chemists has carried on a series of experiments on the composition and preparation of porcelain. It is no wonder, then, if we find that the literature concerned with these practical departments is almost entirely French. One result of this is a greater richness in technical terms than with us. We find in France names for the various implements and processes of the potter’s art, that are something better than the workshop terms of the local potter. Again, the little that has been written in England upon the technology of pottery has been concerned chiefly with earthenware of Staffordshire.[6]