The modern celadon glaze is made by mixing ferruginous clay with the ordinary feldspathic glaze and adding a pinch of cobalt (the mineral from which the blue colour is obtained) to give it the requisite tone[161]; and it is certain that the colour of the old celadons is due to the presence of oxide of iron, whether assisted or not by oxide of cobalt. Possibly the earliest celadons were the accidental result of the iron in a strongly ferruginous clay escaping in the heat of the kiln and imparting a green tinge to an otherwise colourless glaze. The conditions in the Lung-ch´üan district would have specially favoured such an accident, for the local clays were of the ferruginous kind, as is shown by their peculiarity, which we have already noted, of turning red or reddish brown when exposed without protection to the heat of the kiln. The presence of iron in greater or less quantity is a common feature of potter's clays all the world over, and it is usual in modern potteries to pass the clay over strong magnets in order to remove this disturbing element when a pure white ware is in view. This fact alone will explain the prevalence of green tints of the celadon type among the earlier Chinese wares, and observation of these results would naturally lead to the discovery that a certain quantity of particular clay mixed with the ordinary glaze would produce a beautiful green colour, resembling jade. The reddish brown spots occasionally observed in old celadon glazes are no doubt due to flaws in the glaze-covering, which allowed a partial exposure of the body, or to a local excess of iron oxide in the material. Like a great many other accidental effects, these were turned to account by the Chinese, and in some examples we find patches of brown which evince a deliberate intention (Plate 21). These effects are highly prized by the Japanese, who call the ware _Tobi seiji_ or "spotted celadon." The manufacture of celadon must have been very extensive in the Lung-ch´üan district. Besides the principal factories at Liu-t´ien Shih, there were minor works at Chin-ts´un already mentioned, and according to the _T´ao lu_[162] at Li-shui Hsien[163] in the Ch´u-chou Fu, the latter already operative in the Sung dynasty. Its wares were included in the comprehensive term _Ch´u yao_, and "the material was coarse and thick, the colour similar to that of Lung-ch´üan ware, both dark and light, but the workmanship was coarser." At the beginning of the Ming dynasty, we are told[164] that the Lung-ch´üan factories were removed to Ch´u-chou, and that the ware made on the new site was green (_ch´ing_), with a white body which, like the older ware, assumed a red colour in the exposed parts, but that the ware was not so good as the old. Local tradition asserts that the celadon industry in the district came to an end with the Ming dynasty.[165] [Illustration: Fig. 1 Fig. 2 Fig. 3 Plate 21.--Three examples of Lung-ch´üan Celadon Porcelain.