Sweet iron
A bit with a sweet iron mouthpiece with added copper inlays
Sweet iron is a term for cold-rolled "mild steel" or carbon steel that has been work hardened, popular for use in bit mouthpieces used on horses in the western riding disciplines. It easily rusts, which supposedly encourages salivation from the horse and acceptance of the bit. This metal is used in many Western riding disciplines, and is not as popular in English riding.