Exposure to heat, air and humidity can all change the appearance of copper.  As kitchens are often hot and humid, this can occur without even using your cookware.

The process is called oxidation and happens when the copper combines with the oxygen in air and involves the transfer of electrons.

This can cause the color of the copper to change and can give rise to areas of discoloration.  Copper may turn a deeper orange, a pinkish shade or can even develop purple or blue streaks.

Such discoloration is only on the surface and can be removed with any polish suitable for copper. Alternatively, rubbing half a lemon and salt on the surface is an effective organic method of cleaning copper cookware.