Utagawa Kunisada (Japanese, 1786 – 1865), Otomo no Kuronushi, ink on paper, 14 1/2 x 9 7/8 inches, Museum Purchase, 1923.2701. Reading Public Museum, Reading, Pennsylvania.

Here at the Bruce Museum, the exhibition Floating Beauty: Women in the Art of Ukiyo-e recently went on display. In this show, viewers can see a beautiful mix of historic Japanese fashions, such as kimono and kosode. A kimono can be made from just about any textile, and some modern kimono have even been made of lace or denim. However, the highest formality and most traditional kimono are made from silk.

Just like kimono have a long and rich history in Japan, silk has a long and rich history across the entire globe. Now, we are gaining a deeper understanding of the science of what makes silk so special too.

Silk Science

Silk is a natural fiber, produced mainly by spiders and insects, but also by a few other types of invertebrate. Spider silk is hard to harvest in great quantities, so has seen only occasional use by humans throughout history. The silk that made shimmering garments from Japan to Italy was made instead from the protective cocoons weaved by insect larvae as they prepare to undergo metamorphosis. Though there are several species used in the silk industry, it is the mulberry silkworm (or silk moth), Bombyx mori, which sees the most use.

The mulberry silkworm has been farmed for over 2,000 years, making it the only fully domesticated invertebrate animal. The silkworms of today have diverged quite far from their wild ancestors. One of the most obvious changes is in color. As adult moths, most are white and have small, stunted wings, no longer to either fly or camouflage themselves. Their cocoons are different as well, and they produce more silk in more regular quality than wild moths do.

Like all domesticated animals, there are many varieties of domesticated silkworm, and different silk varieties come from different regions. The silkworms that are closest to their wild ancestors live in China, indicating a Chinese origin for silkworm domestication.

If you hold a piece of silk in your hands, admire its shimmering luster, and run your hands over its smoothness, it’s easy to see why it’s been one of the most desirable fabrics to ever be invented. Though some of this beauty certainly comes from human artistry, most of it is inherent to the silk itself and comes from the chemistry and structure of the original substance.

Silk gets its start in the salivary glands of a silkworm larva. It is secreted as a liquid, which hardens upon exposure to air. Completed cocoons consist of a single thread that can be over half a mile long. Each silk thread has a triangular cross-section, creating a prism-like effect which reflects light at different angles, giving silk its shimmer. If the silkworm is allowed to emerge as an adult moth, it secretes enzymes to dissolve a hole in the cocoon, which results in many smaller threads of various sizes. It is much easier to work with longer thread, so most cocoons are boiled, killing the developing larva and allowing the silk thread to be recovered intact.

Boiling also breaks apart the protein which glues strands of the cocoon together, which makes the cocoon easier to unwind. This glue-like protein is called sericin, and is one of the two major protein components of raw silk. The other dominant protein is fibroin, which forms the filament structure. Fibroin is very tough and resilient, and has attracted the attention of the medical community. Silk is readily accepted by the human body, and fibroin proteins are being tested for use as scaffolding to help create new tissues. Silk has shown promise in the creation of artificial tendons and ligaments as well.

In the future, you could be wearing silk while having it inside your body as well!

Silk cocoons from different parts of Assam, India. The left three cocoons are from “wild” species. Photo by Biswarup Ganguly.

Wild Silk

Across the world, other species of insects are used to produce silk. These fabrics are sometimes called “wild silk,” to indicate that they weren’t produced by domesticated silkworms (though wild-type species are still farmed). Some regions of India and China produce wild silk, and one notable variety is tussar silk. Tussar silk comes from the cocoons of moths in the Antheraea genus. Traditionally, tussar silk was left undyed, in the golden color produced naturally. However, modern synthetic dyes have opened up new options. Now tussar silk can be found in an array of different shades.

Tussar silk may be beautiful and unusual, but the rarest type of silk in the world is sea silk, Sea silk is quite different in origin from other silks. Rather than coming from an insect, or even a spider, raw sea silk is produced by the Pinna nobilis species of fan mussel, a shellfish.

Fan mussels grow 1 – 4 feet long, and anchor themselves to rocks with silky threads called a byssus. Though many species of mussels use bysusses (even those native to Long Island Sound!), the byssuses of P. nobilis mussels are exceptionally fine and strong. When harvested, these fibers can be woven into golden sea silk, a warm and shimmering fabric.

Sea silk was one of the greatest treasures of ancient times, with records existing across the Mediterranean region and beyond. Unfortunately, modern pressures have pushed populations of fan mussels to the brink, and now P. nobilis is critically endangered. This has likewise caused a collapse in the sea silk industry, and only a few women on a single island off the coast of Sardinia still continue to make it. As they grow older and retire, we may see the sad end to this ancient craft.

Just as some silks decline, others may rise in the coming years. Spider silk is a fascinating material, with amazing strength and durability. Scientists have been developing ways to mass produce it, and already are testing out its use in different products. We might not have sea silk in a hundred years, but might see kimono made from spider silk instead!

- Kate Dzikiewicz, Science Curatorial Associate and Seaside Center Manager