Many insects and spiders produce silk, which is essentially a solidified, viscous liquid secreted from specialised glands or orifices. The only significant source of silk for textile use is the silkworm larva, also known as the silk moth due to the two glands on either side of its body.
As a textile material, silk can be found in two distinct forms: continuous filament yarn and staple yarn. There is a wide variation in the length of the filament, from about 300 metres to about 1 200 metres. Glycine, serine, alanine, and tyrosine are the primary amino acid constituents of this polymer.
Collecting the raw silk
The first step in making silk is to kill the insects inside the cocoons. The cocoons are steamed or baked to kill the insects inside. Reeling is a delicate process used to extract the silk fibre from the cocoons. The delicate cocoons that result from an early harvest are easily destroyed if the process is interrupted. However, the moth’s emergence after a delayed harvest could ruin both the cocoon and the filament.
Following collection, cocoons are sorted by a variety of criteria, including colour, size, fibre diameter, and uniformity. The size and consistency of the cocoon are two of the most important factors in determining its quality. Similarly, the finer the fibre, the better. When heated in boiling water, the gummy substance that keeps the cocoon filament in place is released. In order to create the thread that is wound on a reel, the filaments from four to eight cocoons are heated, joined, and twisted. Whenever a cocoon is unwound, it is immediately replaced by a new cocoon. When processed, the resulting thread is known as “raw silk”, and it typically contains 48 individual silk fibres. The thread is continuous, and its fibres are much longer than those used to spin cotton or wool, for example.
Silk composition
Silk is made up of long chains of amino acids — glycine and alanine, in particular linked together via peptide links and hydrogen bonding between parallel chains.
Raw silk’s chemical make-up
Raw silk, in addition to sericin and fibroin, has the other components listed in Table 1, including mineral matter, traces of fat, colouring matter, and water.
Physical characteristics
Table 1: Composition of raw silk.
Strong and resilient, silk has a tensile strength of 0,34 N/Tex to 0,39 N/Tex and an elongation at breaking point of 20% to 30% (Tenacity is the standard way of assessing the strength of textile products such as yarn and fibre ropes. Tenacity is measured by dividing the breaking load of the component by its mass per unit length and is expressed in N/tex in the International System of Units. Tex is a mass per unit length of 1g/km). It has a high natural resistance to creases and can bounce back quickly after being folded or stretched.
Silk is an extremely hygroscopic fibre, regaining between 10% and 15% of its original moisture content after being dried. It has a poor electrical conductivity and it’s easy to carry, keeps you warm, and drapes well. Its cross section is triangular, and it is a smooth, translucent fibre. The high sheen of continuous-filament silk is one of the reasons for its prestige.
(Source: Pixabay by butterflyarc)
Chemical characteristics
Cold, highly concentrated mineral acids dissolve silk quickly and easily.
Nitric acid, even in its diluted state, discolours silk yellow. Short periods of contact with cold, concentrated solutions of caustic alkalis appear to have no effect on silk. Strong zinc chloride solutions can break down silk. When exposed to sodium chloride for a short time, there is no effect.
The identification of fibres
The lustre of silk is exceptional. The triangular cross-section can be examined under a microscope to confirm its identity. Scales do not form on silk as protein-based fibre. Silk can also be identified by dissolving it in a 59,5% to 70% sulphuric acid solution.
If you put a cocoon that has a pupa inside it into boiling water, the pupa will die. The pupa is an excellent source of protein and can be sold locally as a protein source to generate additional income. (Source: pexels by Quang Nguyen Vinh)
Collecting cocoons from the frame and delivering them to the buyer
The harvesting of cocoons from the mountage is an important task and the quality of the cocoons is affected by the season and method. Harvesting takes place 6 to 8 days after mounting. If cocoons are harvested before the young silkworms inside have fully pupated, the delicate pupa inside may be damaged by the force of the harvesting tool, transportation, or the removal of the floss (the floss of the cocoon is the thread that the silkworm first expels when making the cocoon, and is the cotton-like cocoon filament covering the outside of the cocoon). Because of this, the cocoon’s interior becomes tainted and loses quality. The harvesting process is not complete until the defective cocoons have been removed through cocoon sorting. Often only double cocoons, thin shelled cocoons, pierced cocoons, and unshaped cocoons are discarded in the silk reeling process.
Scores for cocoons
To protect the interests of both the farmer (the seller) and the silk reeler (the buyer), cocoons must be graded fairly and accurately. Better-quality cocoons result in more money in the farmer’s pocket, and the correct evaluation of cocoon quality is essential. For the silk reeling side, cocoon quality is also important from the perspective of securing profit, as it directly influences reeling efficiency and the quality of the raw silk.
Cocoons in the process of being hand-sorted. (Source: pexels by Quang Nguyen Vinh)
Drying the cocoons
Cocoon drying reduces the humidity of the pupa and cocoon for long-term storage to prevent mould and discoluration, and equalises the thread’s loosening and tension when silk reeling, revealing the bivoltine cocoon’s characteristics. Cocoon drying temperature, time, and degree of drying are all important for raw silk production. Proper cocoon drying affects reeling efficiency and raw silk quality. The drying conditions may vary, but ideally, by increasing the temperature to 110 °C within 30 minutes, maintaining it for 2 to 2,5 hours, and then decreasing it to 60 °C for 3 to 3,5 hours, the cocoon can be dried and reduced to 45% of its original weight in 5 to 6 hours.
On certain farms, it is difficult for the drying cocoon equipment to increase the room temperature to 110°C, and sometimes drying takes longer than a day and is confirmed by touch, which is not proper drying. The drying facilities’ structure and external chimney height make cocoon insertion and removal difficult. Inadequate drying causes cocoon discoloration and mould, lowering raw silk quality. These risks need to be overcome for successful silk production.
Boiling water releases the cocoon filament’s gummy substance to make reel-wound thread. (Source: pexels by Quang Nguyen Vinh)
References
Starovoytova, D. (2015). Mulberry and Silk Production in Kenya. Textile Science & Engineering. Volume 5. Pages 1-7.
SMEDA. (2020). Sericulture / Silk Production – Small Business. Ministry of Industries and Production (MoI&P) Government of Pakistan. Available at: https://smeda.org/phocadownload/OTC_Documents/ Sericulture%20-%20Silk%20Production%20for%20Small%20Business.pdf
Duraiswamy, D. (2019). The Origin of Silk Production. Silk- Road Universities Networks Online Journal.
CGSpace. (2007). Sericulture in East Africa. Japan Association for International Collaboration of Agriculture and Forestry. Available at: https://www.jaicaf.or.jp/fileadmin/user_upload/publications/FY2007/report-2007_1_e.pdf
