But most curious of all were the Chinese views on the subject of "furnace transmutations" (_yao pien_) and the fables which sprang from them. At the present day the strange behaviour of metallic oxides, notably copper, under certain firing conditions, is well known and turned to good account. But in early times, when the unexpected happened, and a glaze which contained an infinitesimal quantity of copper oxide was accidentally subjected to an oxidising or reducing atmosphere in the kiln (by the admission of air or smoke at the critical moment), instead of coming out a uniform colour, was streaked and mottled all over with red, green and blue, or locally splashed with crimson or mixed colour, the potters saw in the phenomenon something supernatural. It was a terrifying portent, and on one occasion, we are told, they broke the wares immediately, and on another they even destroyed the kilns and fled to another place. However, the irregular formation of the Chinese kilns greatly favoured these accidental effects, and in time they became comparatively common, so that these true "furnace transmutations" were taken for granted; and though they were not clearly understood before the end of the K´ang Hsi period, fairly rational explanations of them were offered by some of the late Ming writers. Thus the curious splashes of contrasting colour which appeared on the Kuan, Ko and Chün wares were attributed to the "fire's magical transmutation." In these cases only a partial transmutation had taken place, affecting the glaze alone. But the idea of transmutation in the fire was carried farther in the Chinese imagination, and stories grew of cases in which "the vessel throughout was changed and became wonderful." Su Tung-p´o has, for instance, left a poem on a vase organ, in the preface of which it is related[288] that in the year 1100 A.D., "while they were drinking at a farewell banquet to Liu Chi-chung, they heard the sounds of an organ and flute," and that on investigation "it was discovered that the sounds came out of a pair of vases, and that they stopped when the meal was over." Another story of the Sung dynasty tells of a wonderful basin in which the moisture remaining after it had been emptied displayed, when frozen, a fresh pattern every day. At first it was a spray of peach blossom, then a branch of peony with two flowers, then a winter landscape, "with water and villages of bamboo houses, wild geese flying, and herons standing upon one leg." The story of the "self-warming cups" told by an early Sung writer[289] evidently belongs to the realm of pure fiction: "In the treasury of T´ien Pao (742 A.D.) there were green (_ch´ing_) ware (_tz´ŭ_) wine cups with markings like tangled silk. They were thin as paper. When wine was poured into them it gradually grew warm. Then it had the appearance of steaming, and next of boiling. Hence the name 'self-warming cups.'"