Hardcore Luxury®

The Science of Wool

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Heat of Adsorption

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Wool hates water, but loves mist. The outermost layer (epicuticle) of a woolen fiber is made of overlapping scales that shed liquid water very much like the shingles of a roof. However, Nature's design is that the extremely small spaces between the scales welcome water vapor to slowly enter into the fiber.
   
Wool adsorbs water.  Once inside the fiber, there is a temporary chemical bond (hydrogen bond) attaching water molecules to the surfaces of inner structures of the fiber.  All adsorbtion is exothermic, meaning that it releases heat. Breaking the hydrogen bond and freeing the water molecule, desorption, requires heat.  That is how wool can be cooling as well as heating! Both adsorption and desorption tend to happen very slowly.
   

With WeatherWool, we have done everything we can (tried to stay out of the way!) to maximize wool's natural behavior with respect to water:

  • We use fine (thin) but strong fiber in a Jacquard weave, to increase surface area of the fibers, thereby enabling more adsorption
  • We entirely avoid the use of liner fabrics ... our garments are almost entirely pure wool. Where necessary we use small amounts mil-spec wool/nylon blend, such as at the back of a pocket flap
  • Because superwashing detracts from wool's ability to handle water, we also do not use any superwashed fiber in our woven Fabric, from which almost all our offerings are made. We do use superwashed fiber in our knits (the Watch Cap, Neck Gaiter, and cuffs and waistband-ribs of our Hoodie)

Regarding how wool handles water in its various forms, I haven't done lab tests, and I don't claim to understand the research I've read. But I, and our Advisors (such as Mike Dean!), have been out in the weather -- THE REAL TEST -- and know something very interesting and counter-intuitive is going on. The behavior of wool in wet conditions is wildly different and wildly better than the behavior of cotton and synthetic garments that are so much more widespread and upon which people tend to base their assumptions about the behavior of wool. Here are some of the published explanations for why wool handles water so well:

  1. Typical wool can adsorb 30% of its weight in water without feeling wet because the adsorbed water is trapped inside the fibers.
  2. And again (sorry) the outside of a wool fiber sheds water ... so even when wool is literally soaking wet, it will quickly dry off on the outside, and feel dry to the touch, because the water is only held inside the fiber. (But if you submerge some WeatherWool in water for a few minutes, almost none of the water will be taken inside the fibers -- it won't soak -- and the wool will be very quickly dry.)
  3. Wool is also a hygroscopic material.  That is, when wool is completely dry it will adsorb water from the air until it comes to an equilibrium with the surrounding air.  This process of ‘equilibration’ with the surrounding air is called ‘conditioning’.
  4. Inside the wool fiber, water vapor binds to the cortex, producing the heat of adsorption.  Again, all adsorption is exothermic ...heat generating.
  5. The body burns a lot of heat to turn liquid water into vapor ... using up excess heat is the reason for perspiration. When perspiration vapor is adsorbed within a wool fiber, and then condenses, it slowly releases the heat of condensation ... the heat the body expended to create the warm vapor.
  6. Water molecules are polar, having both a positive and negative side. Once inside the wool fiber, the poles of the water molecules form new bonds with the wool, and the creation of these new bonds releases small amounts of heat ... the heat of adsorption. (Pretty much the same as #4.)
    These are some reasons why wool is famously warm even when wet. Probably there are others. This behavior is radically different from synthetics, and from cotton, which absorbs water like a sponge. 
       
    In 1858, French Scientist Coulier was the first to observe that when dry wool was moved to a humid room – when it adsorbed water -- it produced heat ... the small amounts of energy known as the "heat of sorption". Experiments have shown that human subjects can perceive the heat of sorption of water vapor by wool garments in typical winter conditions, particularly if the woolens are thoroughly dried before use. Some claim a kilogram of merino wool can release as much heat over 8 hours as an electric blanket (this seems high to me). This is just one of the reasons wool keeps you warm even when it is wet. So woolens are the rare exception to the clothing rule - they can generate heat. You'll really notice this in real life when going out on a cold, humid day. Your woolen outerwear will prevent the humidity of the air from chilling you ... the wool will dry the air near your body, creating a lower-humidity, warm micro-environment. Wet (high humidity) air pulls heat from the body much more quickly than dry air.

    The process of internal adsorption is very gradual, and is relatively impervious to liquid water. This is important in cold weather, where heat and energy must be conserved. Physical exertion, even on a cold day, causes the body to produce warm water vapor.  Wool will adsorb the warm vapor, trapping the moisture and the heat inside the fibers. Wool does this even on a cold, rainy day!   It's remarkable to be working in the rain and just feel warm and comfortable inside your wool. The wool can keep the rain from reaching you, while at the same time grabbing the perspiration vapor and heat. Work hard in rain gear and you just get soaked from the inside, even in the clothing that claims to breathe.
                  
    This is all separate from the insulation properties of wool created by trapping air in the crimp of the fibers.
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    Click here for great information from Woolmark, the Australian Wool trade organization.
       

    Water-Resistant Properties
    The water-resistant properties of wool can be life-saving. If you fall in a freezing river and reach dry land within minutes, your wool will be wet on the outside, like your hair, but will not have taken on any water internally. Because the wool cuticle is hydrophobic, your wool will dry quickly and you will be relatively unaffected, assuming your base layers and socks are also wool. The behavior of cotton is the opposite. Typical cotton garments will immediately soak up as much water as they can hold, and wet cotton fiber is wet on the inside and the outside, so it pulls immense amounts of heat from your body. Cotton Kills. There is no cotton whatsoever in WeatherWool garments. Fleece is very bad news in a dunking situation, and so is down, unless it is inside a waterproof shell. Some synthetics will dry pretty quickly, but none of them will stop pulling heat away from your body as quickly as will wool.

    Click here to see me jump in a river in winter, and then simply carry on, comfortably, and enjoy the day ... no problemo. 

    Pure Wool
    One important factor separating WeatherWool from inferior woolen garments is that WeatherWool Fabric is pure wool. It's very common that other woolen garments contain significant amounts of cotton. A garment label stating “100% Virgin Wool” means that the wool in the garment has not been recycled, but it does not mean that the fabric is pure wool. WeatherWool Fabric is always pure wool because any other material will degrade performance. We use no fillers or lower grade fibers in our yarn or weaving. You'll often see other makers claim they've added different materials to their woolens in order to get the best of both worlds ... as far as I'm concerned, all they are really doing is saving money on the cost of materials.

    Heat Conduction
    When you touch wool on a cold day, it feels warm because wool is a poor conductor of heat. That is, wool does not absorb heat from your body. There are two reasons for this. First, when you touch wool, you don't touch very much of it. The structure of wool is not flat … it is very curly and kinky and on a microscopic level, its surface is very rough. So when skin comes in contact with woolen fabric, it simply does not touch as much of the wool as other fabrics. Secondly, the cellular, chemical and biophysical structures of woolen fiber are all fine insulators.


     

    Why We Get Cold!
    Our skin radiates heat into the air. That warming air, which is lighter than cold air, rises up and away from the body. This convection increases with wind. Even in mild temperatures hypothermia can set in.

    Heat loss is also exacerbated by metals and water, which conduct heat quickly. Moisture greatly quickens our loss of heat. Wet skin loses heat up to 25X faster than dry skin. (Wool can keep our skin dry by capturing the vapor of perspiration before it condenses into what we generally think of as sweat.) In the cold, avoid getting sweaty and avoid jewelry. Sit on a fallen tree rather than a stone or cold ground. And rest your feet on a large branch or thick moss if possible.

    Heat is also lost when cold air is inhaled. Your nasal passages heat the air going into your lungs. Breathe through your nose, not your mouth.

    The body prioritizes where to maintain essential heat. Blood flow to arms and legs is reduced in order to maintain heat around vital organs and the head. The head receives about 25% of human blood flow. A head without a hat funnels body heat away. If you feel cold ... even if it is your feet (the classic!) that are cold -- put on a warm hat!

    Dress in Layers
    Start with a wool base layer next to the skin. Synthetic base layers that so many wear today for protection from the cold are designed to wick moisture away from the skin. And that's good, but it's not as good as capturing the vapor (and heat) of perspiration before it condenses into what we think of as sweat. Try to regulate your activity so that you do not detect any wetness of perspiration on your skin. Once you skin is wet, you lose heat much more quickly than if you skin is dry. Cool and cold-weather base layers -- and of course we recommend only wool -- will also insulate and capture the heat released from the body.

    Mid-layers add extra insulation of air. In cold weather with low activity level, a thick layer is needed.  Mid-layers should also act in concert with moisture (sweat) transpiration. If your mid-layer creates a barrier between your wool base layer and wool outer layer, the moisture is trapped,  This is why we use no liners. Wearers of WeatherWool working hard in serious cold have observed moist air escaping their garments and freezing on the outside.

    Top layers help insulate, regulate heat and moisture loss, and provide overall protection from elements. Outer layers also should handle abrasion, resist flame, odors, stains, dirt and electric-arc, and be absolutely silent. The ideal outer layer will also feel luxurious, provide concealment in nature and be admired in social settings. WeatherWool can handle a huge variety of weather conditions, but we do recommend a storm shield for extreme wind and rain conditions.

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    8 May 2020