Tinning Copper Cookware

Copper is popular for cookware because it conducts heat very well. However, bare copper reacts with acidic foods. This reaction creates compounds that are not safe to eat in large amounts. For centuries, coppersmiths have solved this problem in a simple way: they line the inside of copper pots with a thin layer of tin. Tin does not react with food the way copper does. To apply the tin, workers first clean the copper down to bare metal. Then they coat it with a flux, usually a chloride compound. The flux stops oxidation and helps the tin stick evenly. Next, the pot is heated until the inside reaches about 230 degrees Celsius, the melting point of tin. Melted tin is poured in and spread across the surface, often by hand with a thick cotton pad, because the pot is too hot to touch. The bond between the tin and copper is strong for a specific reason. The two metals do not just sit on top of each other. At the point where they meet, copper and tin atoms combine and form new compounds. These compounds have much higher melting points than tin alone. This creates a bond almost like welding, down at the microscopic level. Over time, heavy use wears the tin layer away, especially scrubbing with metal tools or frequent high heat. When this happens, tinned copper cookware must be re-tinned. Today, only a few skilled craftspeople still know how to do this.