Rock wool is made from natural minerals like basalt or diabase.
Slag wool is made primarily from iron ore blast furnace slag. Rock and slag wool insulations are produced by a centrifugal wheel process. Natural rocks or iron ore blast furnace slag are melted, and the hot, viscous material is spun into fiber by pouring a stream of molten material onto one of several rapidly spinning wheels. The fiber is then collected and formed into batts or blankets for use as insulation, or baled for use in other products, such as acoustical ceiling tile and spray-applied fireproofing, insulating, and acoustical materials. The mineral wool form of MMVFs was initially developed in the mid-1800s by melting slag and spinning it into insulation for use in homes and industry.