The term mineral wool historically refers to two materials: rock wool and slag wool. (Fiberglass is also included in some references to mineral wool.) Rock and slag wool fall within a group of
materials historically referred to as man-made mineral fibers (MMMFs) or synthetic vitreous fibers (SVFs); however, a more appropriate name is man-made vitreous fibers (MMVFs), reflecting the glassy, noncrystalline nature of these materials.
The mineral wool form of MMVFs was developed initially in the late 1800s by melting slag and spinning it into insulation for use in homes and industry. Over the past century, mineral wool manufacturing has evolved into a large and diversified industry as more and more products have been developed.
Rock wool and slag wool each use different raw materials in their manufacture. Rock wool is made from natural minerals, made primarily from natural rock such as basalt or diabase. Slag wool is made primarily from iron ore blast furnace slag. Slag wool accounts for roughly 80 percent of the mineral wool industry, compared with 20 percent for rock wool, in the United States. These proportions are reversed in European countries.18
While mineral wool was at one time the most common type of insulation, its market share was lost largely to fiberglass in the 1960s and 1970s. In the past few years, however, the product appears to have been making a comeback. There are currently several manufacturers of mineral wool in the United States and about eight plants that produce it.18
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 or several rapidly spinning wheels. As droplets of the molten material are thrown from the wheel(s), fibers are generated. As the material fiberizes, its surface generally is coated with a binder and/or dedusting agent (e. g., mineral oil) to suppress dust and maintain shape. The fiber is then collected and formed into batts or blankets or baled for use in other products, such as acoustical ceiling tile and spray-applied fireproofing, insulating, and acoustical materials.19
Because of the manufacturing process and the differing performance characteristics of specific products, rock or slag wool insulation materials are comprised of a wide range of fibers with varying thicknesses or diameters. Typically, individual fibers range between 1 and 15 pm in diameter, with an average diameter of 3 to 7 pm. (A micron is 1/1,000,000 of a meter or 1/25,400 of an inch.) By comparison, a human hair is about 70 pm in diameter.19
When viewed under a microscope, rock and slag wool fibers resemble single rods. Because they are noncrystalline in nature, these fibers break across their long axis, resulting in shorter fibers of the same diameter. They do not split lengthwise into thinner, smaller-diameter fibers, as do many crystalline fibers such as asbestos, an important factor when considering potential exposures.