1. Why Sheep Stay the Same in Rain but Wool Sweaters Shrink in the Wash
On a rainy farm, sheep get completely soaked. Yet, their wool does not shrink or harden easily. [6] A scientific explanation from Harvard University lists two reasons for this. First, on living sheep, the surface scales point away from the body. Second, natural oil called lanolin prevents the fibers from sticking and tangling. [6] Light rain is very different from the pushing and kneading in a washing machine, which may involve hot water or soap. [6] This explains why wet sheep’s wool resists shrinking, though cut wool behaves differently.
What happens in the washing machine? A report from the Ibaraki Prefectural Industrial Technology Center states that the general view is that irreversible shrinking and felting occur because the scales on wool create directional friction, causing fibers to tangle. [5] Thus, the shape of the wool surface, which is usually helpful, causes shrinking when specific conditions are met. So, what are these scales?
2. How Overlapping Scales on Wool Fibers Rub Differently Depending on Direction
According to Woolmark, the outer shell of wool, called the cuticle, consists of overlapping scales like roof tiles. [1] Although this is a microscopic view, imagining overlapping tiles helps. The number of scales is high; a dictionary notes that fine Merino wool has 1000 to 1200 scales per 1 cm (about 0.4 inches) of length. [2] The diameter of Australian Merino wool is 0.018 to 0.023 mm. [2] Converted to a per-millimeter basis, this is 100 to 120 scales, with an average spacing of about 8 to 10 μm (0.008 to 0.010 mm). This is an example of fine wool, not a fixed number for all wool.
Just as tiles catch if rubbed upward but not downward, wool requires more force to rub against the scales. It is stated that wool has directional friction due to these scales, a property called the directional friction effect. [5] A detailed paper on human hair shows that overlapping scales are the root cause of this difference; without overlap, the difference does not occur. Even if surface lipids are chemically removed, the difference remains. [3] So, why does this directional rubbing lead to "irreversible" shrinking?
3. How Water, Heat, and Rubbing Make Wool Scales Interlock
Woolmark explains that heat and moisture lift the scales. When mechanical kneading is added, the scales of neighboring fibers begin to interlock like a ratchet. [4] A ratchet is a mechanism that moves easily in one direction but is hard to return in the reverse. It is often described as claws catching. [4]
Felting is the phenomenon where fibers continue to move in one direction (toward the root) and tangle. [4] They slide easily in the direction that does not catch, but struggle to return in the reverse. Repeating this movement hundreds of times gradually shrinks the bundle of hair. Another factor is that finer wool has more crimp. A dictionary explains that felt forms because reverse scales interlock and the crimp tries to return to its original shrunken state. [2] Water, heat, and kneading are easily present in a washing machine, creating conditions different from those for a sheep in the rain. This is why this property is a friend to those who want to make cloth from wool.
4. How People Used Wool's Shrinking to Make Felt Cloth
Felt is a cloth-like material made by shrinking animal hair, such as wool, using steam, heat, and pressure. No glue is used. [7] Its feature is that it does not need to be woven or knitted. Traditional methods involve laying wool on a bamboo mat, sprinkling water or soapy water, and stepping on it to apply pressure. Then, the mat is tightly rolled and rolled along the ground. [7]
A dictionary states that the oldest felt was excavated from the Pazyryk site in Russia (5th to 4th century BCE) and includes large items with human and animal patterns. [7] According to the Hermitage Museum’s explanation in Silk Road materials, the Pazyryk tomb artifacts are dated to around the 3rd century BCE after re-evaluation, with a range in dates. The tomb contained felt rugs, saddle covers, and decorations for a felt covering on a carriage. [8] It is safer to accept these as "among the oldest" rather than declaring them "the world's oldest." Thus, people have used the "shrinks and does not return" property as a tool for at least 2000 years. Is there a way to harden wool without water and soap?
5. How Needle Felting Works and How Wool Is Treated Not to Shrink
Needle felting is a technique where wool is pierced with special barbed needles to tangle fibers into a dense mass. This is different from wet felting, which uses water and soap. [9] Some explain that the needle barbs catch the scales. Here, it is introduced as a method of tangling fibers by piercing wool.
Conversely, there is shrink-proof processing for people who do not want wool to shrink in the wash. The Ibaraki report lists processing that removes scales, processing that coats the surface with resin, and a combination of both. Removing scales tends to lower fiber strength, and chlorine treatment poses environmental burdens. [5] Among methods for machine-washable wool, one of the most widely used is Halco-set. It smooths the scale surface with chlorine to create footholds for resin, then applies polyamide resin. This is a two-step process. [4] Shrink-proof methods generally combine removing or smoothing scales and reducing friction between fibers. [4] Those who needle felt and those who coat with resin look at the same scales. They use the same scale properties: utilizing them for felting and suppressing them for shrink-proofing.
6. How to Use Wool Labels at Home to Understand Scales
Look at the labels on wool sweaters or socks at home and find washing instructions. If you find "hand wash," "use a net," or "washable processing," see what washing method is recommended and compare it with the explanation in Section 3.
If you have felt accessories, gently look at the surface. You may see no weave, but fibers tangled and clumped together. Do not use fire or blades; only look and touch. [7] Official information on scales and shrink-proof processing is available from Woolmark and the Ibaraki Prefectural Industrial Technology Center research report. [1][5]