In the north and east of Thailand, many kinds of coins circulated from the 14th century onwards. Many of them were quite simply little silver ingots, which occur in many shapes and sizes. Such bars are called lat money and known among collectors as "tiger-tongue coins." How their characteristic warty surface was produced is not entirely clear. One theory says that it was done by scattering ants of a particular species on the hot metal and that the formic acid caused the typical raised bumps.