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 5% of the fibers are active at any one moment, but the process is continual, and so different 'sets' of fibers become activated and then 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 sometimes 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-17th to the mid-20th 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 peeled off and passed through wet rollers to cause the fur to felt. These 'hoods' were then dyed and blocked to make hats. This toxic solution and the vapors it produced resulted in widespread cases of mercury poisoning among hatters, which may have been the origin behind the phrase "mad as a hatter" and the character of the the Mad Hatter in Alice in Wonderland. The United States Public Health Service banned the use of mercury in the felt industry in December 1941.
Felting differs from fulling in the sense that fulling is done to fabric that is constructed before continuing with the felting process as noted above.
Knitted woolen garments which shrink in a hot machine wash can be said to have felted ( it is actually "fulled") — an example of how the fibers bond together when combined with the movement of the washing machine, the heat of the water, and the addition of soap. Therefore, woolen clothes should only be hand-washed or machine-washed in cold water.
Cheaper felt is usually artificial. Artificial felt, if made using the wet method, has a minimum of 30% of wool fibers combined with other artificial fibers. This is the minimum required to hold a fabric together with the fibers alone. It would be difficult to achieve a stable fabric by hand at this ratio. All other wholly artificial felts are actually needle-felts.
Loden is a type of felt originally worn in the Alpine regions, which has recently gained worldwide acceptance as a textile for fine and durable clothing.