Because of the directional scaly nature of a wool fibre, it will move when it is rubbed. You can feel the difference in roughness/smoothness by stroking fibre wool between finger and thumb fibre one way and then stroking it the other way. This difference allows each fibre to move, in one direction only, amongst other fibres also moving. As they move, so a tightening network is formed, and the travelling fibres move through the spaces unable to unlock themselves because of the scales.
Hot water has the effect of: 1. swelling the fibres; therefore the scaliness is more pronounced. 2. Stretching and making the fibres more elastic; therefore they bend and twist making a more inter-locked network.
All lengths of wool fibre will felt, although if very short there is less twisting and interlocking possible and the felt may pull apart under stress; if very long then the rubbing action is less effective on the fibre. Wool fibres have crimp . The less the amplitude of the crimp, the better for felting, the greater the frequency of the crimp, the better for felting. The softer and finer the fibre, the softer and finer the felt. Experimenting with different wools and blends will produce different qualities of felts.
Pressure while rubbing, pushes the network together, closing the gaps. Too little pressure and the holes in the network are too big, and the fibres can be pulled back out. Too great a pressure (hardly possible by hand) and the holes are forced shut and felting is not possible.
Methods of Felting.
Hand rolling as practiced traditionally in the Middle East:
The wool is laid upon a cotton cloth or reed mat, design face down. (Many felters here now use bubble wrap instead of a reed mat). The fleece is sprinkled with hot water (addition of soap as lubricant is an advantage) and then tightly rolled up, possibly round a dowel. The cylinder is then bound with string and rolled backward and forward.
Pressure on and friction inside the roll as the layers move past each other causes the wool to felt.
An adaptation of the Industrial method. For hand felters. Hardening
The wool is laid up as above, and covered with a cloth, sprinkled with soap and hot water. Using flat hands, massage with pressure and small circular movements. This causes the fibres to twist and drill from top to bottom and bottom to top of the thickness of the felt. It shrinks in this one direction. Continue hardening until the wool holds together well as a material. It is possible to stop the felt at this point it has a smooth finish but little lateral strength.
Action is required to cause sideways shrinking and strengthening by pushing together the fibres. In Industry the felt is bunched up and pounded in such a way that every part is acted upon. Hand felters use several methods of Milling. One is to roll up again round a dowel, either with or without a carrier cloth. Rolling and pressure pushes the fibres together and causes shrinking.
To finish; wash out the soap lay the felt flat and roll out creases with a dowel or heavy roller. Dry flat, rolling the surface with a rolling pin or dowel occasionally as it dries.