Made with special fireclays. Lettering is placed on this panel to make either a raised or depressed letter in the frog. Thomas Bull had one near his pot kiln. These were more efficient, but were hard to operate with high grade ore which crumbled more easily and jammed the kiln. Limekilns are like updraft furnaces or ovens.
Some sources use the singular, kiln, to denote the site, even though more than one kiln exists.
Handling the product is hazardous, as it is caustic, takes water from the flesh, and gives off heat. This heat is enough to char wood, and warehouses and schooners shipping lime were known to catch fire. This is similar to the process that creates flowstone, and stalagmites and stalactites.