Felting is a innate characteristic of wool fibre. This means that unless the wool is treated in some way (more later), whether they are spun and knitted, crocheted, woven or just felted on their own, wool fibres tend to felt. Our distant ancestors in Europe and Asia knew this through experience, but the clever scientists of the 20th century found out why.
This is important because once we know how to felt, we can either encourage or avoid this tendency in our woolcraft items.
Soooo...the wool industry has brought out Superwash wool, mostly because the invention of easy-wash fabrics has gutted the wool textile industry over the last fifty years. Let's face it, coming home from work at 6 p.m. and doing the woolies has never been that attractive a prospect! Polar fleece, in particular, was a strong influence on the downturn in the sale of woollen garments, because we could now be kept warm in things that were cheap and could be thrown into the washing machine. Millions of dollars have been spent inventing alternatives to a fibre that was already plentiful and easily washed - as long as you stuck to the rules!
But I digress...Superwash is a process that strips the tiny scales off the wool fibres and applies a chemical coating to the fibre, preventing dirt and liquids from penetrating. This also prevents stains from getting in, of course, and allows the fabric to be machine washed. These are fantastic innovations, but they cut right across the characteristics needed to make felt, which is why you must always ask "Is this fibre Superwash treated?" if you want to make felt from it.
The three things needed to make felt from wool fibre are:
Wool will felt if these things are combined. The two most powerful factors are moisture and friction; heat just speeds the process. Now why does this happen? the answer lies within the fibre structure.
Each fibre consists of an outer layer which is made of overlapping scales, and an inner core which consists of two separate compartments, one which accepts and one which repels moisture.
If the wool becomes wet, the moisture-accepting core swells while the other core doesn't, forcing the fibre into a curve. In this state the wool becomes very prone to felting, because as the fibre curves, the scales one one side open up and can easily become enmeshed with other fibres. If any friction is then applied to the wool, the scales become even more enmeshed, and a further property of the wool is activated.
Each fibre will shrink up to 50% of its length, in one direction only, from root to tip. This is an irreversible process. Heat accelerates the shrinkage of the fibre, and this is why most modern feltmakers prefer to use hot water. the addition of soap or detergent to the water merely make the water penetrate more quickly into the fibre, as well as providing a useful lubricant to allow the scales to slide together more easily.
From what I've just said, it's easy to see why careless handwashing or machine washing can completely ruin wool jerseys.
A word of warning: felting doesn't stop when you want it to! Any felted item must be treated the same as fine knitwear - that is, it must be gently washed with an appropriate detergent, avoiding sudden temperature changes. It should be dried flat away from harsh light (particularly when dyed - no dye is entirely unaffected by strong UV light), and stored where insects can't attack it.
Felting (or fulling) has been applied to woven fabrics to make them more stable, warm and durable since time immemorial, and the first felt appears to predate the Roman empire. How exciting to be able to take part in a craft that spans a large part of the existence of humanity!