Silk fabric has played an important role in the history of the textile industry, and in the wider global picture. The ancient silk road was an important conduit for people and ideas. As a natural material, silk has a long history of human use and is still used for many textile applications today.
As a natural material, it is sometimes seen as a preferable alternative to synthetic fabrics. However, the sustainability and ethics of how is silk made are by no means clear cut. As with other materials, it is vital to look at silk in more depth to better understand how and why it is used in the textiles industry and to build up a picture of how eco-friendly, ethical and sustainable this natural textile fibre really is.
How and Why Silk is Used in Weighted Blankets
Silk has long been highly prised for its fineness and its soft, silky skin feel. Unlike many synthetic fibres developed to mimic it, however, silk is not slippery. This is one of the strongest of the natural fibres, and yet, as you will discover below, a number of different factors can alter its mechanical properties. It can be a particularly durable fabric, and yet, as a natural fibre, will biodegrade at the end of its useful life.
The strengths of silk when considered as a fibre choice are clear. It will not only feel nice when used close to the skin, but also has low conductivity that can help to retain heat when it is needed, and breathability, high emissivity and good absorbancy which can also make things feel more comfortable when it is cool.
However, when considering silk as a material for use in weighted blankets, it is also important to consider its weaknesses for use in textiles applications. One downside is that it can be weakened if exposed to too much sunlight. It can also be prone to insect attack, especially if left dirty. As a poor conductor of electricity, silk can also be susceptible to static cling. It can shrink when washed and sometimes needs to be dry cleaned.
Of course, ethical and environmental concerns may also limit the utility of silk in sustainable weighted blanket applications. These concerns will be explored in more depth later in this article.
The Origins of Silk
Silk fibre is produced by certain insect larvae, which produce it to form their cocoons. A number of different insect species produce this fibre, though generally speaking only the silk of moth caterpillars is used in textile production. The best-known silk is that produced by the larvae of the mulberry silkworm, Bombyx mori. These silkworms are bred in captivity for the purpose of silk production in a process known as sericulture, which has been underway for at least 5,000 years (1).
There are three different types of Bombyx mori. The univoltine breed is generally linked to silk production in greater Europe. They have only one brood per year. A second type, bivoltine, is normally found in China, Japan and Korea, and breeds twice annually. In the tropics, a third type, polyvoltine, is found. Silkworms in this last group can lay eggs which hatch within 9-12 days and so can have up to eight broods throughout one year (2).
After eggs have hatched, the larval silkworms eat continually. While they will eat other plants, they have a preference for white mulberry (Morus alba). After moulting four times, larvae then prepare to enter their pupal phase. It is at this point that silk is created when it is extruded from the salivary glands of the larvae to create raw silk cocoons. It is at this point which silk is harvested – a process that we will explore in more depth later in this article.
Each cocoon is made of a raw silk thread which is around 300-900m long. Around 2,000-3,000 cocoons are required to make one lb of silk (0.4kg). Yearly production, which currently stands at around 70 million pounds per year, requires almost 10 billion cocoons (3).
Like other natural textiles like wool, silk is an animal-derived product. However, unlike wool, it involves killing living animals (silkworms) in its origins. For this reason, many people have raised ethical concerns over the origins of this fibre. In order to obtain the silk, the silkworms are boiled or gassed alive inside their cocoons. This causes the silk to begin unravelling so that workers can gather the silk threads. More details of how larvae and moths can suffer in the production of silk can be found in the ‘How Silk is Produced Today’ section later in this article.
The Structure of the Silk Fibre
Silk is a natural protein fibre. It is composed mostly of fibroin. The shimmering appearance of silk is due to the prism-like structure of the silk fibre. This prismatic shape allows silk cloth to reflect the light at different angles, thereby producing different hues.
Fibroin consists of layers of antiparallel beta sheets, and its primary structure consists of an amino acid sequence (Gly-Ser-Gly-Ala-Gly-Ala). In silk in its raw form, the fibroin is joined with the other main protein of the fibre, sericin, which forms a glue-like layer that coats two single filaments of fibroin, called brins (4).
The structure of fibroin that is naturally emitted from the silk glands of Bombyx mori is known as ‘silk I’. Spun silk, which is used in various commercial applications, is known as ‘silk II’. A newly discovered structure of fibroin, known as ‘silk III’ has recently been discovered (5).
The Physical Properties of Silk Fibre
Silk can vary considerably in its physical properties and in many instances, the links between its physical structure and characteristics are only poorly understood. Let us now take a look at the physical properties of the fibre that make it desirable for a range of textiles applications.
The Fibre Density of Silk
The equivalent diameters of Bombyx mori silk fibres range from around 10 to 15 µm. Silk today retains its position as the finest fibre for apparel and worldwide production of silk is increasing in spite of competition from newer synthetic fibres. Silkworm silk was used as the standard for the denier, a measurement of linear density in fibres. Silkworm silk, therefore, has a linear density of approximately 1 den, or 1.1 dtex and is the standard, in a sense, against which other fibres are measured.
The Thermal Properties of Silk
Silk’s relatively low conductivity means that it allows heat to be retained close to the skin. However, due to its high emissivity for infrared light, it will generally retain a cool feel to the touch, and its absorbency can make it comfortable to have close to the skin in warm conditions (6).
Water Absorption and Retention in Silk
The ability of a fabric to absorb and retain moisture from the surroundings can determine how well it performs in a range of different textile applications. Silk has relatively good moisture regain of 11%, though it will lose up to 20% of its strength when it is wet.
The Mechanical Properties of Silk
As mentioned above, silk fibre can vary considerably in terms of its physical properties and this is particularly the case when it comes to its mechanical properties. While it is generally considered to be a strong fibre with competitive moduli and failure stress compared to synthetic fibres, the variable cross-section of the fibre makes it challenging to measure (7).
One interesting thing to note is that silk has different mechanical properties depending on whether it has been ‘de-gummed’ and dying and other post-processing can also have a significant effect. Recent research has explored the possibilities of controlling the tensile strength of silk through removing sericine from fibroin and through the alternation of the physical fibre through processing. The mechanical properties of silk fibres can also be controlled by physical methods of forced silking. This process involves controlling the spinning rate of the fibres by stretching them directly after spinning. Increasing the rate increases the value of failure stress but decreased those of strain to failure (8).
Comparison with the silk of certain spiders (which is finer and has more useful mechanical properties than the silk fibres produced by Bombyx mori) suggests that there is great potential for new uses for silk in textiles and other fields. The biological nature of the fibres permits genetic modifications to the standard silk and also the regeneration of silk from silk solution. It seems that the mechanical properties of silk could be improved through various means to produce fibres with even more exceptional properties. This is an interesting area of research for the future, especially when such research is combined with efforts to improve the ethics and sustainability of silk textile production.
Biodegradability
As a natural fibre, silk is a naturally biodegradable polymer which can break down over time in a natural environment. Interestingly, however, while it will biodegrade, silk varies from other natural fibres like cotton in that it will take a lot longer to do so. The fibre begins to show signs of biodegradation after around four years, though scientists have proven that the use of acidic enzymes can speed up this process. The durability of silk fibre is one of its strengths during its lifetime and can add to its sustainability credentials as it will last longer in use and therefore require replacement less frequently. This can reduce the overall environmental and resource cost of textiles.
However, since silk will take a relatively long time to break down naturally when thrown away, this needs to be taken into account when determining how ecologically friendly this material is over its entire life cycle. Silk in landfill sites, for example, could increase strain on such systems, and thought must be given to how silk might impact the environment before it begins to break down. Of course, the impact of silk after its useful life will still be significantly less than the impact of petroleum-based synthetic fabrics, which will take significantly longer to break down and persist in natural environments for a very long time.
The History of Silk
Before we go on to explore how exactly silk is produced today, let’s take a look at the history of this interesting fibre. This will give us some context that will allow us to better understand the applications of silk, and the modern silk production industry.
Silk for use in the manufacture of textiles was first developed in ancient China over 5,000 years ago (9). The earliest examples of silk were found in tombs at the Neolithic site Jiahu in Henan province, and are believed to date back 8,500 years. Originally, silk fabrics were the preserve of the very wealthy and powerful – reserved for the use of Chinese Emperors. However, over time, the use of silks spread through the rest of Chinese culture and began to be spread to other regions of Asia through social connection and trade. It became a popular luxury fabric in many areas that were accessed by Chinese merchants, and due to its fine texture and lustre, was in great demand and a staple of pre-industrial international trade.
The first evidence of long-distance silk trading is silk fibres found in the hair of an Egyptian mummy dating from around 1070 BCE. The silk trade stretched from China as far as the Indian subcontinent, the Middle East, Europe, and North Africa. The well known and lucrative trade routes between Asia and Europe became known as the Silk Road.
China guarded the secrets of sericulture closely, and Chinese Emperors strived to keep its silk trade secrets. Chinese style sericulture, however, did spread – to Korea in around 200 BCE, the ancient Kingdom of Khotan by 50 CE and India by 140 CE. (It is believed, however, that sericulture employing wild silk threads from native silkworms was already carried out as early as 2450 BC-2000 BCE in the Indus Valley).
China was still responsible for most trade in silk, though it was not only the Chinese who profited from the lucrative silk trade. Ancient Persians, along the Silk Road, for example, also benefited greatly from the trade. As time went on, other silk industries emerged in Thailand, Vietnam, Bangladesh, and other Asian countries, and the Roman Empire, while not involved in silk production themselves, knew of and traded in silk. Chinese silk was the most highly prized of the luxury goods they imported (10).
The secrets of sericulture finally also spread to Europe via the Byzantine Empire, in around 550 CE. Legend states that monks working for Emperor Justinian I smuggled silkworm eggs from China to Constantinople in hollow canes. Silk production then started inside the Great Palace Complex found in that city. Later, Muslim Moors brought silk with them to Spain during their conquest of the Iberian Peninsula.
By the Middle Ages, sericulture in Europe was much more widespread. During the 11th Century, Catanzaro, in the Italian region of Calabria, emerged as the most important area of silk production in Europe. The silk from Catanzaro supplied almost all of Europe and became the lace capital of the world. Their large silkworm breeding facility produced all the laces and linens used in the Vatican and the city gained worldwide renown (11). Lucca, Genoa, Venice, and Florence were also involved in silk production in Italy from the 12th Century.
In Spain, silk was produced and exported from the Granada region, especially the Alpujarras area, until those who made it, the Moriscos, were expelled from Granada in 1571. In France, from the 15th Century, silk production in France centered around Lyon. Many mechanical tools for mass production were later first introduced here in the 17th Century.
King James I tried to introduce silk production to England, but the attempts were unsuccessful. Silk production in England did, however, develop around Stockport, Macclesfield, and Congleton in the 18th Century. These three towns remained the centre of the English silk throwing industry until silk throwing was replaced by silk waste spinning. Though unsuccessful in his attempts to start sericulture in England, King James I did introduce silk production to the British colonies in North America in around 1619. Shakers in Kentucky adopted the practice.
From the 1830s, Manchester in Connecticut became the early centre of the silk industry in America. The Cheney brothers became the first to properly raise silkworms on an industrial scale – their former mills are now a Historic District where visitors can explore the fascinating history of the production facility. Other smaller areas of production took off in the vicinity of Northampton, Massachusetts, and neighboring Williamsburg. By 1911, Skinner Fabrics, in the city of Holyoke, had become the largest manufacturer of silk satins in the world and had the world’s largest silk mill under one roof (12). Paterson, New Jersey, became another 19th Century centre for silk production in the United States, and gained the nickname ‘Silk City’.
The Second World War interrupted the silk trade from Asia and prices rose significantly. During this time, alternatives to silk were sought and this led to the development and increased use of synthetic silk-like fabrics such as nylon and later, semi-synthetic fabrics like lyocell. These fabrics mimicked many of the beneficial characteristics of natural silk, and are sometimes difficult to distinguish from it.
How Silk is Produced Today
Today, in spite of the proliferation and success of synthetic fibres, as mentioned above, silk production continues to increase, though it still only accounts for a small fraction of the overall textile industry. China is by far the biggest reliable producer of silk globally, producing around 54% of the total amount of silk globally (13). The next largest producer is India. Other smaller silk producing industries are found in, in descending order of production by weight: Uzbekistan, Brazil, Iran, Thailand, Vietnam, North Korea, Romania, and Japan.
In order to better understand how raw silk from Bombyx mori cocoons (and from other natural sources) is turned into textiles, let’s take a look at the processes involved in creating silk fabrics today:
Mulberry Cultivation
The extraction of raw silk starts with the process of cultivating silkworms on mulberry leaves. Unfortunately, this cultivation stage often has a huge environmental toll. Silk production has a large carbon and water cost, largely due to the high levels of fertilizer and water that are required per unit of fibre produced when compared to certain other natural or semi-natural fibres (14). Mulberry trees can be grown organically, however, and organic mulberry tree growing and silk production can be a preferred fabric in the context of sustainability. Looking for organic silk can, therefore, make this material a far greener option.
That said, silk production still uses a very large quantity of resources in order to produce only a relatively small quantity of silk. Some estimates suggest that as little as 35 lbs of silk can be harvested from one acre of mulberry trees. This can raise issues surrounding best-practice in land management, and the best ways of obtaining a yield from a given piece of land. Some argue that large areas of land given over to silk production might be better used for alternative raw materials for textiles, or for food production.
Another issue raised with regard to growing mulberry trees and cultivating silkworms for silk production is that the process is very labour intensive. It requires many workers. Unfortunately, this raises a number of issues, especially in regions where exploitation of the labour force is widespread.
Unfortunately, the silk industry is embroiled in a range of problematic and unethical practices, such as bonded and child labour, low wages, and inequality. In China, and other main producing nations, silk production can be important rural employment – preventing migration to big cities and allowing rural communities to be sustained. Unfortunately, many of those employed in the industry work for minimum wages (which are often not sufficient to support a basic standard of life).
In India, much has been done by charities and NGOs to reduce child labour since such issues came to the fore in the 1990s. Unfortunately, however, in India’s main silk producing regions of Uttar Pradesh, Karnataka and Tamil Nadu, bonded children as young as five still work more than 12 hours a day in the silk industry, suffering an array of shocking human rights abuses (15).
Silk Harvest
Most silk is harvested while the Bombyx mori pupae are still in their cocoons. This is so that the process of their emergence does not damage the fibres and the silk threads can be collected intact. Many have spoken out against the cruel practices involved. Vegans will, of course, avoid silk as an animal-derived fabric, however, even those who are not fundamentally opposed to animal-derived fabrics still condemn the methods commonly used in silk harvesting (16). Many questions whether the killing of around 10,000 silkworms can be condoned to produce, for example, just one silk sari. According to PETA, some 6,600 silkworms are killed to make just 1kg of silk.
Usually, silkworms are boiled, steamed or gassed alive inside their cocoons before they emerge. Those who argue that these are ‘only’ worms should remember that these are living creatures. They produce endorphins and have a physical response to pain. In other words, they can ‘feel’.
Those who abhor the killing of silkworms to obtain silk often turn to ‘Ahimsa silk’ or ‘Peace silk’. Sellers claim that these materials have been produced from cocoons that were collected after the moths naturally emerged. This type of silk, influenced by the Ahimsa philosophy of Mahatma Gandhi, is often held up as a more ethical alternative.
Unfortunately, no certification authorities exist to determine that these standards of animal welfare and ethics are upheld. Conventional silk is sometimes sold as ‘peace silk’ and even when silkworms are allowed to emerge naturally from their cocoons, these creatures can still suffer. Sometimes, they are not given sufficient food or are forced from their cocoons prematurely. After they have laid their eggs, female moths may be crushed to death. Male moths are sometimes refrigerated and brought out only for breeding, before being discarded once they are no longer able to mate. It is not uncommon to see birds pecking at baskets full of male moths left outside silk production facilities. Even with so-called ‘Peace silk’, there is little transparency in the process and no guarantee that the silk truly is cruelty-free (17).
Silk Fabric Manufacturing
Sadly, the ethical issues surrounding the production of silk do not end there. Once silk has been extracted from the silkworm cocoons, the long fibres are unraveled and fed onto spinning reels. Workers (sometimes even young children) often scald their hands in the vats of hot water to palpate the cocoons to free the threads. Their hands can become raw and blistered. Those who wind the threads into strands often suffer from cuts that go untreated and can become infected. There is a range of human rights and worker safety issues throughout the process of turning silk cocoons into thread and then finished fabric.
Raw silk is twisted into strands sufficiently strong for weaving or knitting. This process is known as ‘throwing’ and prevents the thread from splitting into its constituent fibres. There are four different types of silk thread that may be produced: crepe, tram, thrown singles and organzine. Which of these four threads is required will depend on which type of silk fabric is being produced. In general, organzine is used for the warp threads of materials, tram threads for the weft, crepe thread for waving crinkly fabrics and a single thread for sheer fabrics. Broken or waste filaments are retained, treated to remove the sericin, then combed and processed into yarn and marketed as ‘spun silk’, a cheaper and somewhat inferior product.
Dyes and Treatments
Today acid, metal complex and reactive dyes are widely used for silk dyeing. Reactive dyes have become very popular due to their brilliance, variety of hues, high wet fastness, convenient usage, and high applicability. But such dyes come with a range of problems, such as low dye-ability, requirements of a large amount of auxiliary agents, a high volume of discharged wastewater, and other environmental concerns which always exist in the application of reactive dyes for silk fabrics.
What is more, some properties of silk fibre such as crease recovery, wash and wear properties, photo-yellowing, water and oil staining resistance, dyeability, and colour fastness are weak and chemical or mechanical means can often be employed to improve them. Some of these post-processing treatments also come with a range of environmental and health concerns.
Many conventional silk producers treat silk with heavy metal salts and synthetic resins to make it heavier – as this will allow them to achieve higher prices on the world market.
For these reasons, it is best, if considering silk fabrics, to look for textiles that adhere to certain standards or certifications.
The Future SIlk?
Standards & Certifications in the Silk Industry
As mentioned above, organic silk is better than most silk produced as fabrics marked with an organic standard or certification will not have used pesticides or herbicides in the growth of the mulberry trees required for raw silk production. The Global Organic Standard is one such certification that consumers should look for if choosing textiles made from silk (or other natural fibres). In addition to certifying that silk production has not required harmful and polluting chemicals, organic standards can also certify that silk fabrics have not been exposed to or treated with harmful chemicals throughout the rest of the supply chain.
As mentioned above, ‘Peace silk’ is a claim that is not backed up by a true certification authority, and so the claims of cruelty-free silk that are made cannot be guaranteed. It is only when there is true transparency in the supply chain, and you know exactly where your silk comes from, that you can say with any certainty what exactly was involved in the harvesting and production process.
However, when it comes to both environmental concerns and human rights, Oekotex standards and certifications can offer a certain level of peace of mind. These and other sustainable textile certifications can tell consumers that no harmful chemicals were used in silk production and that workers were not harmed or treated poorly throughout the process.
While silk is not the most sustainable nor the most ethical choice for textiles, and you may choose to eschew it altogether, looking for Organic certification or Oekotex standards can help you to choose the most sustainable and ethical silks if you do decide to go for something made from this natural material.
References
(1) J. W. Barber (1992). Prehistoric Textiles: the Development of Cloth in the Neolithic and Bronze Ages with Special Reference to the Aegean. Princeton University Pres
(2) Trevisan, Adrian.”Cocoon Silk: A Natural Silk Architecture”. Sense of Nature
(3)Food and Agriculture Organisation of the United Nations, www.fao.org
(4) Hakimi et al., “Spider and mulberry silkworm silks as compatible biomaterials”,Composites Part B: Engineering, vol. 38 (3), pp. 324–337
(5)Valluzzi, Regina; Gido, Samuel P.; Muller, Wayne; Kaplan, David L. (1999). “Orientation of silk III at the air-water interface”. International Journal of Biological Macromolecules. 24 (2–3): 237–242.
(6) Venere, Emil (31 January 2018). “Silk fibers could be high-tech ‘natural metamaterials'”. Phys.org.
(7) ‘Handbook of Tensile Properties of Textiles and Technical Fibres’, The Textile Institute, Woodhead Publishing, 2009
(8) Shao Z, Vollrath F (2002), ‘Surprising strength of silkworm silk’, Nature, 418, 741.
(9) Vainker, Shelagh (2004). ‘Chinese Silk: A Cultural History’. Rutgers University Press. pp. 20, 17.
(10) Garthwaite, Gene Ralph (2005). The Persians. Oxford & Carlton: Blackwell Publishing, Ltd. p.78
(11) “Office of Tourism – Italy – Calabria, Catanzaro”
(12) “The Largest Silk Mill in the World; The Story of Skinner Silks and Satins”.Silk. Vol.5 no.6. New York: Silk Publishing Company. May 1912
(13) “Mulberry Silk – Textile Fibres – Handloom Textiles | Handwoven Fabrics | Natural Fabrics | Cotton clothes in Chennai”. Brasstacksmadras.com
(14) Astudillo, Miguel F.; Thalwitz, Gunnar; Vollrath, Fritz (October 2014). “Life cycle assessment of Indian silk”. Journal of Cleaner Production. 81: 158–167.
(15) Ajay Phogat, http://childlabourintheworld.blogspot.com/p/indian-silk-industry-and-child-labour.html
(16) Stancati, Margherita (4 January 2011). “Taking the Violence Out of Silk”. Wall Street Journal.
(17) https://www.peta.org.uk/issues/animals-not-wear/silk/
