The reign of Chêng Tê, though not mentioned in the _Po wu yao lan_ and but briefly noticed in the _T’ao shuo_, must have been an important period in the history of Chinese porcelain. The _yü ch’i ch’ang_ (Imperial ware factory) was rebuilt[88] and the direct supervision of a palace eunuch renewed. The porcelain, we are told in the _T’ao lu_, was chiefly blue painted and polychrome, the finest being in the underglaze red known as _chi hung_. An important factor in the blue decoration was the arrival of fresh supplies of the Mohammedan blue.[89] The story is that the governor of Yunnan obtained a supply of this _hui ch’ing_ from a foreign country, and that it was used at first melted down with stone for making imitation jewels. It was worth twice its weight in gold. When, however, it was found that it would endure the heat of the kiln, orders were given for its use in porcelain decoration, and its colour was found to be “antique and splendid.” Hence the great esteem in which the blue and white of the period was held.[90] The merit of this new Mohammedan blue was its deep colour, and the choicest kind was known as “Buddha’s head blue” (_Fo t’ou ch’ing_). Its use at this period was not confined to the Imperial factory, for we read that the workmen stole it and sold it to the private manufacturers. In the following reign a method of weighing the material was instituted, which put an end to this pilfering. Some account has already been given[91] of this material and its use in combination with the commoner native mineral blue. It was, no doubt, the blue used on Persian, Syrian and Egyptian pottery of the period exported by the Arab traders. One of the oldest routes[92] followed by Western traders with China was by river (probably the Irrawady) from the coast of Pegu, reaching Yung-ch’ang, in Yunnan, and so into China proper. This will explain the opportunities enjoyed by the viceroy of Yunnan. There were, of course, other lines of communication between China and Western Asia by sea and land, and a considerable interchange of ideas had passed between China and Persia for several centuries, so that reflex influences are traceable in the pottery of both countries. Painting in still black under a turquoise blue glaze is one of the oldest Persian methods of ceramic decoration, and we have seen that it was closely paralleled on the Tz’ŭ Chou wares (vol. i, p. 103).