What is meant by oxidised?

Most of my jewellery are oxidised. Copper and sterling silver will oxidise naturally on contact with air producing that familiar dark patina which can easily be removed by polishing. This natural oxidation process normally takes place over a long period of time but I can achieve this instantly by using liver of sulphur. Liver of sulphur is a mixture of potassium sulphides. It works by turning copper black and sterling silver dark grey which I clean up using fine wire wool. Any recessed areas that I have hammered, stamped, coiled or etched are left darker creating a contrast in the metal and that gorgeous antique patina.

The photos below show examples of oxidized copper pieces.