To most insects and worms copper is a potent toxin. By impregnating the protein with certain metallic compounds the protein becomes toxic to most life forms which would use it as a food source. Since the metal lic compound is insoluble and chemically bonded to the protein it will not be toxic to the user or wearer. Until recently, the mineral pigment dyes and mordant dyes using a metallic compound were the only means available to protect wool and silk against insect attack. Mineral pigment dyes are noted for extreme durability.
Sunlight has no effect on the final colors.
Only acid wash water will leach the mineral pigment dyes from protein fabrics. Protective versus decorative. When used in small amounts, mineral pigment dyes, such as copper acetate, would be protective and not really decorative. When used in larger amounts, the dye becomes both protective and decorative. Pickling in a weak copper acetate solu tion insured that a hide would not sour nor would fly larvae eat holes in the hide.