The most common metal used to make bits for horses today is stainless steel.
However, some horses find the metal stainless steel bits cold and tasteless and do not salivate when being ridden in them.
In contrast using a sweet iron bit is said to encourage salivation in the horse's mouth - this can help to keep a horse or pony's mouth soft and encourage it to take the bit.
The sweet taste of sweet iron, which is what encourages salivation, is produced as the metal oxidises or rusts. As the rusting is what produces the sweetness you shouldn't worry if you see this!
Early sweet iron bits got a bad reputation due to inferior manufacturing. But now most sweet iron bits are well made and safe - they are often also made with copper as well - for example with a copper lozenge in a jointed snaffle, or a sweet iron copper roller snaffle.
Many types of horse bit are available in sweet iron for example french link snaffle bit, pelham or gag.
Although relatively new to the UK sweet iron has been used in bits for many years by american cowboys.