Felt is made by a process called wet felting, where the natural wool fiber is stimulated by friction and lubricated by moisture (usually soapy water), and the fibers move at a 90 degree angle towards the friction source and then away again, in effect making little "tacking" stitches. Only five percent of the fibers are active at any one moment, but the process is continual, and so different 'sets' of fibers become activated and deactivated in the continual process.This "wet" process utilizes the inherent nature of wool and other animal hairs, because the hairs have scales on them which are directional. The hairs also have kinks in them, and this combination of scales (like the structure of a pine cone) is what reacts to the stimulation of friction and causes the phenomenon of felting. It tends to work well only with woolen fibers as their scales, when aggravated, bond together to form a cloth.Felting is done by a chemical process in industry. It is also done with special felting needles, which grab individual fibers and drag them against their neighbors, thereby binding them. Felting may also be done in a domestic washing machine on a hot cycle.From the mid-seventeenth to the mid-twentieth centuries, a process called "carroting" was used in the manufacture of good quality felt for making men's hats. Rabbit or hare skins were treated with a dilute solution of the mercury compound mercuric nitrate. The skins were dried in an oven when the thin fur at the sides went orange-carrot color. Pelts were stretched over a bar in a cutting machine and the skin sliced off in thin shreds, the fleece coming away entirely. The fur was blown onto a cone-shaped colander, treated with hot water to consolidate it; the cone was peeled off and passed through wet rollers to cause the fur to felt. These 'hoods' were then dyed and blocked to make hats.The toxic solution and the vapors it produced resulted in widespread cases of mercury poisoning among hatters. (Some suggest that this may have been the origin behind the phrase "mad as a hatter" and the name of the character of the the Mad Hatter in Alice in Wonderland. However, others argue that the Mad Hatter's character was more likely based on someone who was not a hatter and did not exhibit signs of mercury poisoning.) The United States Public Health Service banned the use of mercury in the felt industry in December 1941.