Process used in production of fiber

The cleaning process used in fiber production, chemicals used and the temperature of the water used in cleaning all impact the quality and felting ability of the wool. Some harsh processes and chemicals damage the wool and the resulting felt can be more hairy.

Small amount of lanolin left in the wool after cleaning may help the felting process. It is of course possible to felt in the grease, without any cleaing as well, commonly used for felting fleeces. The lanolin in the wool works both as a type of resist to retard the felting but also helps the felting by allowing the fibers move around better.

The carding process to produce roving or batting may keep long fibers or break them down into short fibers like in short fiber merino or kap wool in the extreme. Fibers with many short ends will felt differently than long roving. Batts organize fibers in many random directions, impacting the resulting felt quality. On the other side, sheep that molt their wool will have fewer raw ends. Vikings used this property to build woven sails for their sheep (Source [1]) and would produce different quality of felt.

Chemical processes like coating and dyeing after wool is cleaned involves adding a coating to make it more hydrophobic. I personally experienced this with DHC fibers when I tried to overdye them. This does not impact their felting ability and likely results in a less hairy felt. In the extreme, the superwash process coats all scales of the wool in a chemical, making them impossible to wet felt.

The color of dyes impact the underlying chemicals used, which can impact the felting ability of the wool. The black dyed wool is often hardest to felt.