Why Choose Silk Thermal Underwear?

The thought of silk is just so luxurious that it seems like choosing it for thermal underwear seems like it might be overkill. However, when you look at the properties of silk, there are a number of reasons why silk thermal underwear might be the perfect thermal choice for both women and men.

Firstly, there are a number of things about silk that I only learned when I started researching this article, some good – like its many valuable properties, some not so good – like the domesticated life of the Mulberry Silkworm. Having encountered silk as a material used in thermal base layers intended for domestic use I started this research as an attempt to understand why, unlike wool, it’s not a prominent natural material used in athletic baselayers.

All About Silk

Silk is a protein-based natural fiber consisting mainly of fibroin and sericin. Silk comes from multiple sources, however, the primary source for commercial fabric use is from the cocoons of insects undergoing the transformation from the larval phase to the adult phase of their lifecycle. Although not commonly used for fabrics, there are some other insects and arthropods which spin silk throughout their lives.

Arachnids such as spiders are prime examples of life long silk spinners producing very fine and super-strong fibres.

A few fun facts about Spider Silk:

• It was reported that spider silk was even used for wound dressings in the Greek and Roman times!
• Spider silk is still studied by material scientists today looking to understand its unusual properties of strength and toughness. Check out this scientific paper exploring the Engineering Properties of Spider Silk if you’d like to know more about the properties of spider silk.
• There are some goats that have been genetically engineered to produce spider silk protein in their milk. This protein is extracted, spun into fibers. It has been used to created Bulletproof Skin by Dutch artist and Entrepreneur Jalila Essaïdi which she suggests may have uses as a cruelty-free leather substitute.

Commercial production of silk comes from reared silkworm pupae, the majority is from the domesticated Mulberry Silkworm (Bombyx mori). The Bombyx mori has undergone generations of selective breeding to produce a blind, unpigmented, flightless moth. With a lifecycle of 45 to 55 days the Mulberry Silkworm spends 9 or 10 days in an egg phase, 24 to 28 days as a larvae, 8 to 10 days as a pupae (within the cocoon), and 3 to 4 days as a moth.

In standard commercial silk production, the pupae are killed by means of boiling (and the worms themselves eaten) to preserve the full length of the silk fiber, this has the added advantage of softening the sericin which is binding the cocoon together to allow unraveling of the cocoon without damaging the silk fiber. A silk fiber produced by the standard production method which can be over a mile in length and produces the most valuable silk due to its strength. It takes the fibers from five to ten silkworms cocoons to make one silk thread. Several of the silk threads are then wound together to form a yarn that can be woven or knitted into fabric.

Historically the wild silk used for generations has been made from the cocoons of Bombyx mandarina (the wild silk moth) discarded after the pupa has emerged from the chrysalis cycle. There are ethical sources of silk called peace or vegetarian silks that follow the wild silk methods and allow the silkworm to emerge from its cocoon before the thread is harvested.

If the moth is allowed to emerge from the cocoon, it escapes by producing a silk destroying enzyme to make a hole in the cocoon. This enzyme breaks the silk fibers down to random lengths. Because the cocoon no longer consists of one long fiber due to the damage caused by the moth emerging, the fibers are spun into a thread in the same manner as cotton. This results in a slightly weaker fabric but it is warmer and softer then commercially produced silk.

How Silk Thermals are Made

Silk fibers have up to 20% natural stretch, but there is a risk of breakage over time as its elasticity gradually reduces in line with standard wear. This is why most silk thermals are made with an interlock knit pattern. This is a reversible compound fabric which is made by interlocking two ribbed fabrics. It stretches well in the crosswise direction, with minimal lengthwise stretch. This type of knit only unravels from the end where it knitting finished.

As a result of this knitting pattern expect your thermals not to have a great amount of vertical stretch, but to accommodate your standard exercise requirements from the lateral or cross body fit perspective. Silk thermals are not particularly common as athletic base layers although they have a number of properties that make them ideal for selected sports, such as hiking where multiple layers are the key to achieving comfort. I really wouldn’t  choose them for running though.

Attractive Properties of Silk

Silk is a lightweight, thin material that layers well. Because silk thermals are so light and thin they are really easy to wear under normal clothes. Currently, the predominant use of silk thermals is for domestic purposes (around the house), they are considered excellent additional warmth and barrier layers under wool sweaters or jumpers and are particularly prized by those who find their wool layers itchy as the silk, with its smooth but not slippery texture, feels great against the skin. There are many reports from people who have developed wool allergies layering their expensive wool garments over silk underlayers to continue to get the benefit from the expensive wool clothes.

Silk fibers are some of the strongest natural fibers in existence and resistant to stretching and deformation. Silk has excellent thermal properties and is good insulation keeping you warm in the winter and cool in the summer. The interlock knit pattern used with silk thermals allows the skin to breathe and one of the stand out features of silk thermals is with their wicking and odor avoidance properties they transition well from outdoors to indoors without the overheating effects that can be found with some synthetic thermals.

Downsides of Silk Thermals

The downsides of silk base layers are all directly or indirectly related to the process of growing the silk. The labour intensive process (both in terms of the human and silkworm labour) results in an expensive product, it takes approximately 3000 cocoons to make one yard of silk fabric. The ethical issues of standard commercial silk production coming from an insect without a demonstrable quality of life will also be a barrier to the adoption of silk as a material for some companies and individuals.

Care of Silk Thermals

As always, check the washing label, but in general knitted silk thermals can be machine washing on a delicate cycle at a cool temperature with like colours. Use an enzyme-free detergent and as an added precaution they can be washed separately, inside out and/or in a delicate or lingerie bag. However, hand washing is also an option.

They should be dried at a low temperature, generally laid flat to avoid stretching the wet garment out of shape and out of direct sunlight to protect the colour and fibers.

Conclusion

Silk has been harvested throughout recent history, however traditional sources are both expensive and come with intrinsic ethical dilemmas. I believe this has been part of the reason for minimal adoption of silk as a natural material used in athletic base layers.

However, some of the silk thermals designed for domestic use might be the perfect solution for specific activities. Ramblers, hill walkers and hikers in particular could benefit from exploring silk underlayers as the ideal, lightweight and comfortable base of their layering systems.

I’d love to hear your opinions in the comments below – have you ever tried a silk base layer? What about other natural materials such as wool or bamboo?

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