Types of Silk and Their Characteristics

What is Silk?

Silk is one of the most luxurious natural fibers, prized for its luster, strength, and softness. Produced by both cultivated and wild silkworms. Silk is available in many forms, including cocoons, unprocessed fiber, and yarn. It originates from various species of silk moths and varies widely in quality, appearance, and characteristics depending on its source.

At this time, china produces more silk than any other country, surpassing even Japan in the late twentieth century. Most of that silk fiber production is from the domesticated Bombyx mori silk moth, which produces a pure white lustrous filament. The second most abundant form of silk fiber produced is from the tussah silk moth, also from China. In addition, there are several other varieties of wild silks from India, Southeast Asia, and Africa.

Different Types of Silk

Silk comes in a wide range of types, from the cultivated Bombyx to various wild silks like Tussah, Muga, Eri, and Cricula. Each type differs in color, texture, fiber diameter, and luster, reflecting the unique silkworm species, their diet, and rearing conditions. These differences define their quality, uses, and handling methods.

A. Cultivated Silk

Bombyx Silk:

Bombyx mori cocoons and fiber are bright white in color; the very fine fibers measure between 9 and 11 microns in diameter. Bombyx silk, produced by the Bombyx mori moth, is the fine, bright white fiber we think of as pure silk. This lustrous and shiny variety accounts for the majority of silk on the commercial market. Bombyx, as it is commonly called, is also known as mulberry silk, because of the silkworms’ exclusive diet of mulberry leaves.

Bombyx silkworms are raised on farms, where the farmer provides a secure and predator-free environment for the silkworms to grow, as well as a continuous supply of mulberry leaves. The life cycle of this esteemed silkworm is controlled at every step. The process of raising silk fiber starts with reserving a number of cocoons from the reeling process so that they may hatch, mate, and produce eggs for the next crop of silkworms and cocoons.

Bombyx silkworms are fed only mulberry leaves, from white, fruitless black, or fruiting black trees. Of these, white mulberry leaves produce the finest and best-quality fiber, followed by leaves from the fruitless black mulberry, then leaves from the fruiting black trees. Other varieties of mulberry leaves, grown in India and North America as either trees or lower growing bushes, are also fed to silkworms.

As noted, several moths from each batch are allowed to hatch, mate, and lay eggs. The female moths are examined to reduce the possibility of disease in the future crop—misshapen or discolored moths and eggs will be destroyed. Each fertilized female silk moth lays more than three hundred eggs, called “seed,” onto a gummy paper provided by the farmer to keep the eggs in place.

The moths are confined during the egg-laying process so that each batch of moths and eggs may be examined under a magnifier and isolated if they’re found to contain any type of disease or disability. The eggs are closely monitored until they hatch.

Upon hatching, the tiny worms grow through five stages—called instars—eating twice their weight in leaves and molting between each stage. After the fourth molting, they are placed in baskets or boxes provided by the farmer where they spin their cocoons by extruding fibroin covered by a gluey substance called sericin. The colonies are again housed under controlled conditions to ensure the safety of the cocoons as the worms transform into moths.

About five days after formation, the farmer collects the cocoons and stifles the worms. The timing is critical here—if the worm develops too far into the moth stage, the cocoons may be damaged or stained.

B. Wild Silk

Wild silkworms, including those we know as tussah silkworms, live less restricted lives. Some varieties are gathered from trees and bushes, then the caterpillar is stifled so that the cocoon remains intact and can be reeled. Other varieties are allowed to hatch, which renders the cocoons unreelable. In some cases, the farmer saves a number of cocoons for the next year’s crop and allows the moth to emerge and lay eggs (seed). In other cases, mating and egg laying occur in the wild—the farmers hunt for hatched caterpillars, which they house together for feeding and tending until the cocoons are spun.

Several varieties of wild silk moths feed on a number of different trees and are raised to varying degrees without human intervention. Just like their cultivated bombyx cousins, wild silk moths experience distinct growth phases—egg, larva, pupa, and moth. Farmers or silk producers can control some of these phases: cocoons can be gathered to hatch under controlled conditions away from predators, moths can be coerced to lay eggs in specific areas, and the caterpillars can be fed through their several molting processes so that the cocoons are easily secured. However, other than the country of origin, little information is available to handspinners from suppliers about the conditions under which wild silk fiber is produced and harvested.

Tussah Silk:

Tussah cocoons are large and darker in color than bombyx; the fibers measure between 20 microns and 30 microns in diameter. Natural colors in tussah varieties range from very light to dark beige and brown. Some tussah fibers are a natural light honey color. Commercially bleached tussah can be very light in color. Tussah fibers that will be dyed are often bleached to a lighter color first.

The largest group of wild silks is from the family Antheraea, commonly called tussah, or Tassar, silk. Antheraea pernyi, from China, Antheraea mylitta, from India, and Antheraea yamamai, from Japan, are the most common wild silks. Distributors rarely know the particular variety of tussah silk they receive. Most of the tussah available for spinning comes from China, with a lesser amount from India.

Tussah silk cocoons generally are large, reelable, and are treated in much the same manner as bombyx cocoons. Depending on the variety of the silk moths and what they ate as worms, tussah fiber is golden or honey in color; but it can have a darker, cinnamon tone, as well. This variety of silk has a larger diameter and a less uniform surface than bombyx. Indentations and irregularities on tussah fibers give the lustrous fibers a more sparkly than shiny appearance.

In China, tussah silk is cultivated. The moths are most likely to eat leaves from a variety of oak trees. The silkworms are bivoltine in most cases (in some colder climates, they’re univoltine), and the first crop each year is the finest and softest.

In India, tussah cultivation remains second to bombyx because of the difficulties in rearing and protecting the silkworms from predators and disease. As farmers take a more active role in production, the conditions are more controlled, and tussah production has increased. The farmers secure cocoons to be raised in open sheds, rather than outdoors on trees, and have developed handling processes that ensure greater productivity and higher-quality fiber.

C. Other Wild Silk

Muga (Antheraea assama)

Muga fibers are more golden in color than tussah. Laps (muga shown here) are produced in the early stages of processing fibers for spinning. The fibers are less uniform in length than in tops and may have more impurities.

Muga is the fabled golden silk raised and cultivated in the northeastern Indian state of Assam. For centuries, fabric woven from this silk, prized for its luster and golden color, was reserved for royalty and nobility. Recently, the Indian government has supported an economic development program for rural employment, which includes encouragement and assistance for farmers raising muga silk as fiber and turning it into fabric. This type of silk is increasingly available to spinners, both in laps and as top. Muga takes dye beautifully, with an undertone of its natural golden color.

Eri (Samia cnynthia ricini)

Because the moth has escaped and left a hole in the cocoon, eri cocoons are not reelable. The fibers are fine and soft. The red color in red eri silk is produced by sericin, which is removed in the silk degumming process. The fibers in this lap preparation are less uniform and contain impurities that the spinner will have to remove.

There are two types of eri silkworm, differentiated by color—one white and one red. The caterpillars feed on the castor-oil, or “era,” plant in Assam, from which the silkworm derives its name. It comes also from the northeastern part of India. Although attempts have been made to transplant this variety to other parts of the world, they have been unsuccessful so far.

Eri cocoons are elongated and have an opening at one end, which prevents them from being reeled. I’ve found white eri fiber both in laps and as top, but red eri only as laps. In general, eri fibers feel less smooth (more cottony) than bombyx, and the surface of spun yarn shows less of the luster typically associated with bombyx. Although I haven’t found any reports on eri fiber size, to me, the fibers feel finer than bombyx.

Cricula Trifenestrata

Cricula cocoons are riddled with small holes. The degummed fibers are short and spin like cotton. Another type of Asian silk comes from the curious many-holed cocoon of the family Saturnidae. The Cricula trifenestrata silk moth inhabits India, Sri Lanka, Indonesia, and Myanmar. Because of their lacy and hole-studded nature, these cocoons are not reeled, but only spun. The sericin is gold; however, once removed, the silk is light tan. The fibers are short and spin into a fine yarn. Because of the decorative nature of the cocoon itself, it’s often left unspun and used as decoration in clothing and for jewelry.

D. African Wild Silk

Gonometra rufobrannea aurivillius is native to the continent of Africa. The fibers spin fine and smooth. Although bombyx was imported centuries ago and used by Moorish weavers and embroiderers in North Africa, Madagascar, West Africa, and Ethiopia, several varieties of wild silk are indigenous to Africa. Many are considered communal brown silk moths.

Anaphe infracta moths live on tamarind leaves and form communal colonies or clusters of cocoons within a casing. Their fiber is rustic, coarse, and strong and is said to resist dye, but I’m skeptical of this last claim. Anaphe moloneyi moths also form clusters of cocoons, but they’re not enclosed in a casing. They produce a coarse, rough fiber that is lighter in color than A. infracta.

Properties / Characteristics of Silk

Despite the many varieties of silk—from cultivated bombyx to the many wild types—and their diverse origins, they hold many properties in common.

1. Appearance: One of the finest natural fibers, silk has a diameter as small as 9 microns for cultivated bombyx and up to 25 microns for wild varieties. In terms of length, it’s the longest natural fiber, with filaments typically 1,000 to 1,300 yards (914 to 1,189 meters) long and reaching up to 3,000 yards (2,743 meters) long. In cross section, the fiber is triangular, which allows it to reflect light with its characteristic shine.

2. Strength: Silk is known for its strength (but not durability). A cocoon needs to be strong enough to hold the silkworm for the duration of her metamorphosis into a moth. However, the moth has to be able to escape the cocoon once the change has occurred. To do so, she secretes an alkaline substance that dissolves a hole through the cocoon. As a result of this necessary property, silk fibers can be dissolved in a number of chemicals. Alkaline-based liquids or mineral acids will burn holes right through the fibers, while sunlight and even oxygen can degrade them.

3. Moisture absorption: Silk is able to absorb about 30 percent of its weight in moisture. This greatly aids the fibers in their ability to absorb dyes and makes silk one of the most pleasing fibers to dye. However, silk will also absorb impurities, such as metal salts, which can damage the fibers. “Weighted” silk fibers have been subjected to a process that intentionally adds metal salts to give the fiber a crisper hand but compromises its durability.

4. Elasticity: Silk has good elasticity, but not quite as good as wool. Silk fibers can stretch up to 20 percent when dry and up to 35 percent when wet. Dry silk will break when stretched more than 20 percent. Silk’s ability to stretch puts it in high demand, especially for such items as parachutes.

5. Heat reaction: At temperatures above about 300°F (150°C), the molecular structure of silk starts to break down and the fiber begins to turn yellowish. To avoid discoloration, it’s important to iron silk on a medium heat setting or use a damp pressing cloth between the iron and the fabric. Silk burns with a sputtering flame, leaving a crisp brittle ash and an odor much the same as burnt hair or feathers.

6. Susceptibilities: Chlorine bleach will dissolve silk. Silk is damaged by perspiration, which will weaken and discolor the fibers. It’s also harmed by aluminum chloride, a chemical commonly added to antiperspirants. In addition, ultraviolet rays can destroy silk fibers. As little as six hours of direct exposure may damage the fibers as much as 60%. Oxygen is also harmful, and unless kept in sealed containers, silk will eventually break down. Although silk is resistant to mildew and moths, it’s vulnerable to carpet beetles.

7. Shrinkage: Silk shrinks with washing; the degree of shrinkage determined by the variety. Take this into account when you combine two or more silks in the same fabric. Some of this shrinkage can be attributed to yarns relaxing during finishing. With forceful pressing and tentering (stretching on a frame), many silk fabrics can regain most of what’s lost. However, the restretching process will reduce some of the fabric’s elasticity, because the fibers will be stretched to their full extent in the process. Woven fabrics will shrink more in the warp-wise direction, but this shrinkage is also attributable to the weaving process, which keeps these yarns under tension and is accounted for in the take-up measurement when calculating warp lengths.

Conclusion

Silk, whether cultivated or wild, remains one of the most valued natural fibers in the world. Each variety—from the lustrous white Bombyx to golden Muga, soft Eri, and rugged wild silks—offers unique properties that affect its texture, strength, luster, and applications. Understanding these differences helps in selecting the right types of silk for weaving, dyeing, or crafting, ensuring the best quality and durability.

References

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