Fulling involves two processes: scouring and milling (thickening). Originally, fulling was carried out by the pounding of the woollen cloth with a club, or the fuller's feet or hands. In Scottish Gaelic tradition, this process was accompanied by waulking songs, which women sang to set the pace. From the medieval period, however, fulling was often carried out in a water mill, followed by stretching the cloth on great frames known as tenters, to which it is attached by tenterhooks. It is from this process that the phrase being on tenterhooks is derived, as meaning to be held in suspense. The area where the tenters were erected was known as a tenterground.
In Roman times, fulling was conducted by slaves working the cloth while ankle deep in tubs of human urine. Urine was so important to the fulling business that it was taxed. Stale urine, known as wash, was a source of ammonium salts and assisted in cleansing and whitening the cloth. By the medieval period, fuller's earth had been introduced for use in the process. This is a soft clay-like material occurring naturally as an impure hydrous aluminium silicate. It was used in conjunction with wash. More recently, soap has been used.
The second function of fulling was to thicken cloth by matting the fibres together to give it strength and increase waterproofing (felting). This was vital in the case of woollens, made from carding wool, but not for worsted materials made from combing wool. After this stage, water was used to rinse out the foul-smelling liquor used during cleansing. Felting of wool occurs upon hammering or other mechanical agitation because the microscopic barbs on the surface of wool fibres hook together, somewhat like Velcro.