Structure and Formative Mechanism of Black Patina on Bronze Mirrors

The formation of the black patina induced by soil humic asid under different conditions was studied, the black, grey and brown colored shiny surfaces prepared in laboratory were analyzed. As a result the formation of tin-rich shiny black surface on bronze mirrors is the product of soil corrosion of the bronze mirrors. The patina is formed by reection between the alloy and the humic acid in soil, and leaching of copper and lead oxides through a long period time when the mirrors were buried under the ground leaving a dense compact corrosion-resistant layer consisted predominantly of stannic oxide microcrystal.