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.