Natural Bronze (NB)

Solid bronze is an alloy of copper and tin. The alloy has a reddish colour and a brown oxide layer will build up as it reacts with oxygen, resulting in the dark bronze shade (AZ). If it is allowed to oxidise further, a green-brown oxide layer forms, known as green bronze (VI). Places that are more exposed to oxygen will become darker, while places where there is more friction (for instance due to human contact) will be lighter.