Clothing Answers

Why wool shrinks when washed in hot water while silk does not?

Wool does not shrink, it felts. This is caused by the raised scales of the cuticle layer of the wool fiber catching on one another. The fibers in a fleece on a sheep are all growing out of the follicles in the same direction and they generally grow at a similar rate. This means that the cuticle scales (which are a bit like the teeth on a saw blade) are all pointed in the same direction. They don't catch on one another. These scales can be seen clearly under a microscope.

After the fleece is shorn, the processing stages cause the natural fibers alignment to be completely disrupted. As the fleece is cleaned and scoured, the "staple" structure is destroyed and the fibers no longer line up "tip to base" as they would in the fleece. The fibers end up in all dimensions and suffer entangling after scouring and drying. The purpose of washing and scouring is to remove unwanted materials and to disentangle and align the fibers into a parallel arrangement for spinning yarn. However the fibers will not necessarily be "tip to base". The scales now can be at 180 degrees to one another, which can cause them to catch on each other. After the fleece is shorn, the processing stages cause the natural fibers alignment to be completely disrupted. As the fleece is cleaned and scoured, the "staple" structure is destroyed and the fibers no longer line up "tip to base" as they would in the fleece. The fibers end up in all dimensions and suffer entangling after scouring and drying. The purpose of washing and scouring is to remove unwanted materials and to disentangle and align the fibers into a parallel arrangement for spinning yarn. However the fibers will not necessarily be "tip to base". The scales now can be at 180 degrees to one another, which can cause them to catch on each other.

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

Wool tends to felt is because of the scales on the fibers. Other animal fibers have cuticle scales too, but to varying degrees. For instance, the cuticle scales on human hair are much flatter. Fine diameter wools are more likely to felt than broad diameter wools because they have a greater surface area, and hence more scales proportionately.Wool tends to felt is because of the scales on the fibers. Other animal fibers have cuticle scales too, but to varying degrees. For instance, the cuticle scales on human hair are much flatter. Fine diameter wools are more likely to felt than broad diameter wools because they have a greater surface area, and hence more scales proportionately.




Silk on the other hand Silk on the other hand

Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from cocoons made by the larvae of the mulberry
silkworm Bombyx mori reared in captivity (sericulture).


And what of the shrinkage question? Silk is a strong natural fiber, a protein actually, that does not itself shrink. It is the fashion in which the separate fibers are wound together during the weaving process that causes shrinkage. Silks that are highly twisted or very loosely woven will bunch up, as water will create a twisting energy within the fibers. However, silks of poor quality will also shrink at the dry cleaners. It is advised that you always purchase an extra 1/8 of a yard for silk sewing projects to accommodate any shrinkage during pre-washing. The maximum shrinkage you can expect is usually 5% to 10%, although some loosely woven silks could shrink as much as 15%. Again, pre-washing before beginning your project is very important.
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