Felt paper is available in several grades, the most common being Type 1—commonly called 15-pound (15#) or No. 15 (#15)—and Type 2—commonly called 30-pound (30#) or No. 30 (#30). The weight designations originated with organic base felt weighing 15 or 30 pounds per 100 sq. ft. (6.8 kg or 14 kg per 9.3 m2). However, modern base felts are made of lighter-weight fibre, so the weight designations, though common colloquially, are no longer literally accurate.
Another basic designation is organic or inorganic. Organic felt paper has a base material made with formerly living materials such as rag fibre, hessian (burlap), or cellulose fibres (wood, or jute). Organic felt papers are now considered obsolete, only comprising five percent of the market in 1987. Inorganic base products are polyester, glass fibre developed in the 1950s, and historically, asbestos mat. Polyester mat is weaker and less chemically stable than fibreglass.qualiting felting materia