The Absolute Wonder of Merino Wool
The famous climber and author Andy Kirpatrick summed it up well in his blog: Merino wool fibres are less than a thousandth of an inch thick (1/4 of the size of a normal wool fibre) and is made up, like all wool, of three main layers. First there is a hydrophilic (water loving) core made of a protein called keratin, which is present in all skin and hair and designed to maintain the homeostasis (a stable metabolic rate) of the body. The core is surrounded by overlapping cuticle scales (like shingles on a house), which are very tough, waterproof and also self-cleaning (when the fibre moves the scales brush against each other) and UV resistant, with most clothes having a high UV factor of UPF50+, important if you're using the fibre to keep out the sun at high altitude. These scales are what can make normal wool itchy, but Merino wool being much finer (like silk) means that they're too small to irritate the skin. This scaly layer is then covered by a filmy skin called an epicuticle, which acts as the fibre's waterproof shell, causing water to run off and is the cause of water beading up on wool clothing as if it had been sprayed with DWR. Unlike synthetic fibres, most of which are produced at a considerable environmental cost; both in processing and in being part of the petrochemical industry, wool is a natural fibre and although is not without it's own eco damaging processing, is far more ecologically sound.
Sounds impressive doesn't it?
A waterproofed fibre. Well it isn't. Synthetic fibres are just as waterproof, as they are basically plastic and so can't really absorb anything (hollow fibres can absorb 4% of their weight). The difference is that whereas synthetic fibres are 100% waterproof they are also non breathable, with their wicking abilities coming from the way they are woven or knitted together to make the fabric (and what hydrophilic/hydrophobic chemicals they are treated with to boost their performance). The secret of wool is that amazingly the fibres themselves breathe, with the outer epicutile layer and cuticle scales featuring tiny pores that allow moisture through but not water. This means that when you sweat the actual fibres soak up the moisture, not just the surface of the fibres. This moisture is absorbed into the protein core of the fibre, with the fibre being able to absorb 30/35% of it's weight. This is not to say that your wool clothing will get to put on a couple of kilos then stop working, as the same processes are at work as with synthetic fabrics, namely the moisture wicks out towards the surface pushed by your body heat. Wool clothing will get heavier (as anyone who's fallen in a river as a kid wearing a wool jumper will know), but because we're only talking about very thin base layers then this isn't a problem, as the base layer is the fastest drying item of clothing you have. What's important is how warm the fabric is when wet - not how quickly it dries in a lab. The actual insulation properties of the Merino wool comes both from its super fine three dimensional 'sprung' structure (or crimp) which helps to maintain a very resilient dead air spaces, wet or dry (the fibre can be bent, flexed, and stretched in any direction 30,000 times or more without damage), which creates the perfect wet warmth environment to push the moisture in the fabric out across the temperature gradient on to the surface and eventually the atmosphere.
Fleece was the answer. The idea of imitating animal fur with a pile fabric began during the Second World War, but was not introduced for work wear by Helly Hansen until the 1960s. Those early pile jackets were functional but looked tatty very quickly. Fleece was different, it was both functional and looked good and, at a time when wool was relatively expensive, it took off. People wanted a change. The warmth: weight ratios are very similar for fleece and wool - providing you stay dry, if you don't fleece is lighter and dries more quickly so you can carry less. Not everyone abandoned wool - many people still use a Shetland woolly is a mid-layer. Now that the price of wool has fallen it is beginning to make a comeback in sports wools - those blends of wool and polyester that bring the best out of both wool and polyester.
Probably the most amazing attribute of wool is that the wool isn't dead like plastic and will actively try to maintain your comfort level, dynamically trying to achieve an equilibrium with the surrounding environment, absorbing and desorbing moisture vapour in order to maintain this equilibrium. It sounds hard to believe (and harder to understand), but this last fact explains a lot and may explain why wool base layers actually feel part of you, and there is even a study that shows it reduces lactic acid build up in the muscles. Wool also stands out from synthetic in that it does not stink after a day of wearing it and sweating in it.