Fiber Content is what a fabric is made of, all garments sold in the USA must have a fiber content label by law. If you are trying to determine fiber content of a fabric, a burn test is the most reliable method and this will give you some insight into how it will interact with flame as well. Below are the 3 types of fibers to look out for, for more detailed information about each one as well as burn test tips, check out the Fabric Glossary.
Flame Retardant and Resistant Fabrics– Very safe for spinning-
Such as Aramid (IFR) and Duvetyne (FR) etc. will not ignite when exposed to flame, nor do they continue to burn once the ignition source is removed. IFR fabrics can last a lifetime of washing whereas FR fabrics may lose their retardancy when washed.
Natural fibers-Safe to spin in-
Such as leather, wool, cotton, silk etc. are made from naturally occurring materials like plants (cellulosic) and animals (protein based). A garment made of natural fibers can catch on fire, it will turn to ash but it will not melt into your skin.
Synthetic materials- NOT safe for spinning!-
Such as polyester, Acrylic, nylon, spandex etc. are made from petroleum products; they are essentially made of fuel! A burn from a synthetic garment can easily cause second and third degree burns with the synthetic material embedded inside the blisters and scar tissue.