Patina Chemistry

A patina is a chemical reaction that forms on the surface of a metal. Patina is synonymous with aging, as it forms from copper being exposed to the elements over time. Patinas form when copper (or other metals) reacts with oxygen in the presence of water. They create an oxidized layer that protects the copper from further weathering.

The baseline process stems from the reaction of copper and oxygen to form copper oxide, through the equation:

4Cu + O₂ → 2Cu₂O

Copper, oxygen and water will continue to react to form a patina:

2CuO + CO₂ + H₂O → Cu₂CO₃(OH)₂ 

These are the base equations for a patina, but other compounds could be in the mix. Different concentrations and ratios of components will result in different colors in the end. For example, the minerals malachite, azurite, and brochanite form colors from dark green to dark blue, and even purple.

Here are some of the compounds I experiment with to achieve my patinas:

• Copper Nitrate (Cu(NO₃)₂)

• Copper Acetate (Cu(OAc)₂)

• Ammonium Chloride (NH₄Cl)

• Sulfurated Potash (HK4O₃S₃+)

• Sodium Tetraborate (Na₂H₂₀B₄O₁₇)

• Acetic Acid (CH₃COOH)

• Salt (NaCl)