The bulk of the Sung wares, at any rate, and among these the best of them, were either wholly undecorated--that is, wholly dependent on form and glaze, or else ornamented by such methods as moulding, stamping, application of clay reliefs, carving, or etching with a fine point. All these processes were applied while the clay was still unfixed, and the glaze was afterwards added and the ware finished once and for all in a single firing. It follows, then, that the glaze must be capable of standing the fierce heat required to bake the body, and as the Sung bodies are mostly of a high-fired porcellanous nature, the glazes used on them were limited to the refractory kinds composed largely of petuntse or porcelain stone. It follows also that any impurity, any particle of iron, for instance, in the clay would make its presence felt in the glaze and influence the colour of the latter, locally at any rate. There is a striking contrast between the characteristic coloured glazes of the Sung and the T'ang periods. The latter are, as a rule, comparatively soft lead glazes, resembling in their colour, texture, and their minute crazing the latter glazes on Ming pottery. The former are thick and hard, and the crackle where it exists is positive and well defined. Mr. W. Burton[90] makes some interesting comments on these high-fired glazes: "There are certain technical points of great interest to be drawn from a study of the Sung productions. In the first place, they prove that the Chinese, from a very early period, had learnt to fire their pottery at a much higher temperature than the contemporary potters of the West were using.... A third point of even greater interest, which seems to have escaped the notice of every previous writer, is that the method of firing used by the Chinese naturally produced glazes in which the oxide of iron and oxide of copper were present in the lowest state of oxidisation; and this is the explanation of the seeming paradox that the green glazes, known to us as celadon, and the copper-red glazes, were amongst the earliest productions of the Chinese porcelain-makers, while in Europe they have been among the latest secrets to be acquired."