Chemical Dyeing and Its Effect on Surface of Freshwater Cultured Pearls

Freshwater cultured pearl from Zhuji, Zhejiang Province, is coloured by the chemical dyeing. The rosy red and orange-yellow dyed pearls are obtained by using C₂₈H₃₁ClN₂O₃ and C₁₄H₁₄N₃NaO₃S respectively. The changes of the hue, luster and micromorphology of the pearls during the initial stage of the chemical treatment and the dyeing process are analyzed by using JEY-PS(China) and field emission scanning electron microscope (FEI-SIRION, Holland).The results indicate that the parti-colours are removed and the micropores are generated at the same time on the pearl surface after the chemical treatment, being advantageous to control the dyeing process. The reflectivity of the pearl decreases with the deepening of colour in the initial stage of the dyeing process. However, the reflectivity increases gradually in the later period of the dyeing process due to repairing of the micropores, although negative effects on the luster of pearl appeared with the formation of micropores during the initial stage of the treatment. The luster of the dyed pearl therefore reaches its optimal level due to reducing the pearl surface roughness. Consequently, keeping the dyeing time of 6~7 days, the optimal rosy red and orange-yellow dyed pearls can be obtained.