Acid dyes. Acid dyes are needed to dye nylon, wool, and other animal fibers, such as mohair, angora, etc. (Silk is a special case and can be dyed like wool or like cotton.) Acid dyes will not dye cellulose materials such as cotton. Acid dyes normally require the same sort of heating as described above for all-purpose dye, but sometimes they are used in hot tap water.