Coal slag, also known as boiler slag, is a recycled product produced by coal-burning plants. When coal is “fired” at over 2,500 degrees, it melts and produces a by-product called slag. The slag is cooled through a process called vitrification which causes it to solidify and form sharp, angular granules in a variety of sizes. This slag is then reclaimed, dried, crushed, and the remaining granules are sorted into seven different grades of blasting abrasives.