In the Wu and Yue areas of the Eastern Zhou period, there were bronze swords, spearheads and ge dagger-axes bearing on the surface a regularly-spread lozenge pattern, which, as seen on the famous sword of Yue Prince Gou Jian and spear of Wu Prince Fu Chai, is not inlaid by mechanical means and possesses high decorativeness. This splendid decoration has drawn much attention from Chinese and foreign archaeologists and scientists since the 1970s, but no explicit report had been released and no successful replica obtained in previous studies. A fragment of a lozenge pattern sword was examined and tested by our research group. It was discovered that the sword was cast in bronze and that the surface ore in the decorated zones a thin fine dendrite layer different from the body in structure and composition. Several methods for revealing the making technique of the layer were tested, and one of them, named tin-rich paste coating, was found to be able to produce a layer similar to that on the ancient sword in form, structure and composition and with a high corrosion-resisting quality suitable to protecting the sword body. Through acceleratd corrosion tests on lozenge pattern reproductions, the yellow and white colors on their surface changed into brigh-black and grayish-black similar to those on the sample. Thus the mystery of the ancient technique for producing the veiled lozenge pattern has been successfully solved.