What Is Silk and What Is It Made of? An Introduction to Silk

What is silk and what is it made of? An introduction to silk

 

 

When you think of silk, the saying ‘smooth as silk’ may come to mind. What is this popular saying based on? What exactly is silk made of, and why is it so shiny and smooth? These are the core questions we answer in this article.

What is silk made of?

When we hear the word ‘silk’, we often think of smooth and shiny silk fabrics. Technically speaking, silk does not refer to a fabric, but to the silk fibers that are used to make these fabrics.

Silk is a natural protein fiber. Natural means that it is derived from animals, such as moths, spiders, and even a type of clam. While many insects are capable of producing silk, the silk we are most familiar with is made by the larvae of moths, also known as caterpillars.

To be even more specific, silk fibers are found in the cocoons of these moth larvae. The larvae make cocoons to grow in before they pupate and emerge as moths. These cocoons are each made of one long silk fiber that the larvae produce using their saliva. On average, a strand of silk from a cocoon ranges anywhere between 300 to 900 meters long. However, a silk fiber can measure up to 1600 meters in length. To put that into perspective, that’s about five times the length of the Eiffel Tower!


The poet makes silk dresses out of worms.
– Wallace Stevens, American Pulitzer Prize for Poetry recipient


We can extract silk fibers by unwinding the cocoons. First, we have to soften the cocoons and sift out defective ones. The cocoons are softened either by soaking them in hot water or by steaming them. After unwinding, the long strands of silk are spun together to form a thread, which can be used in the manufacturing of silk fabrics. Depending on the desired thickness of the yarn, the fibers from one or more cocoons can be combined to form one thread. Processing cocoons requires a lot of time, skill, and craftsmanship, which is one of the reasons silk is costlier than other fabrics.

To summarize, silk is made of the silk fibers that moth caterpillars produce when spinning cocoons. To put it in even simpler words: silk is derived from caterpillar spit. Isn’t nature amazing?

Collection of silkworm cocoons

What is a silkworm?

Most of the silk in the world is derived from one single type of moth caterpillar: the silkworm, also known by its Latin name Bombyx Mori. Silkworms are the larvae of the domestic silkmoth. Because of its domestication, the silkmoth can no longer fly, and you can no longer find it in the wild. The moths have become dependent on humans.

The life of a silkworm begins when a grown silkmoth lays its eggs. Each silkmoth can lay about 300 to 500 eggs at a time. The eggs will hatch and the silkworm will emerge from the eggs once the temperature is right. Grown silkworms and silkmoths are white in their appearance, as they’ve lost their colors over centuries of being bred for the silk production industry. After hatching, the silkworm will continue to grow and feed on mulberry leaves until it has shed its skin four times. Then, it is time for the silkworm to start spinning its silken cocoon using its special glands.

It is interesting to note that although the silkworm contains the word ‘worm’, the creature is actually a caterpillar and not a worm. In fact, caterpillars and worms are entirely different animals, as caterpillars are insects and worms are invertebrate. This makes the Bombyx Mori’s nickname of silkworm all the more confusing!

Silkworms eating mulberry leaves

What other animals can produce silk?

Silkmoths are not the only animals that are capable of producing silk fibers. Some spiders produce silk when weaving their webs. Some types of ant and bee larvae, such as the honeybee, produce silk when pupating. Others, like weaver ants and raspy crickets, make silk when building their nests. Since silk is a naturally strong fiber, it serves its purpose of protection very well for these animals. The list of silk-making insects goes on, and some other honorable mentions include fleas, flies, and beetles.

Most of the animals that can produce silk are insects, but not all of them are. One of the most interesting silk-producing animals is a seashell. This clam lives in the Mediterranean Sea and is known by the name of Pinna Nobilis. The Pinna Nobilis produces silk-like threads (also known as byssus) to help attach itself to rocks. These threads can be harvested and woven into a rare material called sea silk. Unfortunately, sea silk will likely continue to be a scarce material, as the Pinna Nobilis is now an endangered species.

Spider silk as seen in a spiderweb

Why is silk so shiny?

One of silk’s most appealing characteristics is its shimmering appearance. But where does silk get its shine from? To find out, we need to take a look at the composition of silk.

Just like our hair, silk made by silkworms is made up of proteins. Silk is composed of two proteins, fibroin and sericin. Fibroin is responsible for silk’s sheen. It is the main component of silk, as it makes up about 75% of a silk fiber. Fibroin molecules stack in sheets, following a structure similar to a folded leaflet. These sheets then stack together to form long tubes that resemble a triangular shape. The flat and smooth surfaces of this triangular prism-like structure make silk good at reflecting light. When light hits a silk fiber, it is reflected in many different directions. This produces different colours, which in turn makes silk cloth appear shiny.

Composition of a raw silk fiber

Why is silk so smooth?

Unlike cotton, wool, and other natural fabrics, silk fibers are smooth. To learn where silk gets its smooth texture from, we need to look at its composition again.

We learnt that fibroin protein forms the structural center of silk. Silk is also made up of about 25% sericin protein. The sericin in silk has a gummy substance and acts as a glue. Sericin coats fibroin filaments so that they can stick together. One of the main factors that give silk its smooth texture is the removal of sericin from the silk fiber. This process is called degumming and makes silk soft and lustrous.

When silk is not degummed, it can feel coarse rather than smooth. This also makes the silk harder to dye, which is not ideal for silk garment manufacturing. One example of a type of silk fabric that still has some sericin present is silk chiffon. Silk chiffon is not completely degummed. Because of this, it has more of a grainy and coarse texture when compared to silk types that are fully degummed, such as silk charmeuse (also known as silk satin).

Another factor that can determine the smoothness of silk fabric is how much the silk yarn is turned or twisted. The more the yarn is twisted, the crispier the texture of the silk. Silk crepe, for example, is made using silk thread that is tightly wrapped. This gives silk crepe fabrics a texture that is rougher than say, silk charmeuse, which is made of thread with little to no twist.

A comparison of natural fibres under a microscope

An interesting fact about degumming is that sericin does not have to be wasted once removed. The sericin produced by silkworms is rich in antioxidants and has anti-inflammatory properties. Thanks to these valuable characteristics, sericin is used in the food, cosmetics, and even biomedical industries. It has many useful applications, including the treatment of tumors and the improvement of colon health.

It is thanks to the fascinating properties and behaviors of the silkworm, and the people that have learnt how to harvest and process silk, that we get to enjoy silk fabrics today. To learn more about the history of silk and the silk production process, read our other articles linked below.