Shrinkage II: Felting

Since I couldn't sucker any of my readers into explaining shrinkage of wool, I forced the SockPuppetofDoom to do the work. Here is what he came up with:

(From an answer to Rebekah's question at the MadSci Network.)

Here is the response I recieved from Miss Helen Daily, who is a professional woolclasser and more than qualified to make this response. You can find more information via the Woolwise website:

"The problem lies in the professor's misinformed question. Wool does not shrink, it felts. And this is simply because of the raised scales of the cuticle layer of the fibre catching on one another. The fibres in a fleece on a sheep are all growing out of the follicles in the same direction, and generally speaking, they all grow at a similar rate. This means that the cuticle scales (which are a bit like the teeth on a saw, but not that pronounced) are all pointed in the same direction. They don't catch on one another. These scales can be seen clearly in electron micrographs. (some available on the Woolwise site).

After the fleece is shorn, the processing stages cause the natural fibre alignment to be completely disrupted. As the fleece is scoured, the "staple" structure is destroyed and the fibres no longer line up "tip to base" as they would in the fleece. They can be in all dimensions and also suffer entangling after scouring and drying, and the purpose of subsequent carding and combing is to remove extraneous matter and disentangle and align the fibres into a parallel arrangement. However the fibres will not necessarily be "tip to base". The scales now can be at 180degrees to one another, and can catch on one another.

When the fibres are spun, they come in close contact with each other, and the interlocking nature of the scales is what helps keep the yarn together (apart from the twist that is inserted). Felting usually occurs in the presence of heat, water and agitation, and this acts as a ratchet, tightening the contact between the fibres in the yarn, and then the yarns in the fabric.

Wool's propensity to felt is because of the scales on the fibre. Other animal fibres have cuticular scales also, but to different degrees. For instance, the scales on human hair are much flatter. I don't know much about dreadlocks, but I imagine this is caused by interrupting the usual parallel arrangement of the hair scales. Fine diameter wools are more likely to felt than broad diameter wools because they have a greater surface area, and hence more scales proportionately.

Shrink-proofing is a chemical treatment of wool, which uses chlorine to "burn" off the scales...this doesn't entirely remove them, but it does lessen their profile, and then the fibres are coated with a resin to smooth the fibre still further. This allows the wool to be machine washed without felting, and the shrinkage of the fabric associated with felting. So that is the story of wool felting in a nutshell. The wool proteins are very interesting, but really don't play a role in this part of the wool story!"

I felt it hat, er had, to be something like this (Rae Ann's answer was very close, but lacked pedantic certainty).