Rock wool in the process of fiber whether adopt high pressure air injection into fiber or centrifugal into fiber, mineral wool melt from melt into fiber viscosity range in a flash, the city of rose to brittle viscosity range, in the moment, most of the melt is drawn as fibrous mineral cotton fiber, but there are still a fraction of the melt could become the fibers and particles, that is the residue of mineral wool ball.

Rock wool content of slag ball is refers to in mineral wool (or its products), the particle diameter greater than 0.25 mm of the ratio of slag ball quality with the sample quality, expressed as a percentage of the national standard GB/T11835-1998, the residue of mineral wool raw cotton ball content requirement for less than 12%.