Sweet iron is a term for cold-rolled "mild steel" or carbon steel that has been work hardened, popular for use in bit mouthpiece

Bit mouthpiece

The mouthpiece is the part of a horse's bit that goes into the mouth of a horse, resting on the bars of the mouth in the sensitive interdental space where there are no teeth. The mouthpiece is possibly the most important determinant in the severity and action of the bit. Therefore, it should be...

s used on horses in the western riding

Western riding

Western riding is a style of horseback riding which evolved from the ranching and warfare traditions brought to the Americas by the Spanish Conquistadors, and both equipment and riding style evolved to meet the working needs of the cowboy in the American West...

 disciplines. It easily rusts, which supposedly encourages salivation from the horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

 and acceptance of the bit. This metal is used in many Western riding

Western riding

Western riding is a style of horseback riding which evolved from the ranching and warfare traditions brought to the Americas by the Spanish Conquistadors, and both equipment and riding style evolved to meet the working needs of the cowboy in the American West...

disciplines, and is not as popular in English riding.