Chemical Identification of Silk

Please see the following link for details on chemical identification of silk.

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Weighting of Silk

Weighting of Silk

Silk is sold by weight. By means of weighting the manufacturer can increase the weight of silk by 3 to 4 times. The weighting substances in the silk includes tannins such as salts or iron, tin (mostly used), chromium, sodium, magnesium and barium. Also sugar (mostly used), glucose, gelatin, glycerin and paraffins are used.

How weighting is done

Silk has great absorptive power. It can take upto 50% of tannin. Once that is applied the tannin itself can attract salts of iron and tin by another 50% without any visible indication of being changed in character. For dark colored silks, iron salts are used, for light-colored silks, tin salts are used. 

For applying weighting, the silk is first degummed. During degumming process, silk loses approximately one-fifth of the weight. This is done prior to weaving. Then it is immersed in a solution of catechu or some other substance rich in tannin. Then it is tranferred from tannin vat to iron or tin baths. After this cloth is taken out and washed in pure water. 

Effects of Weighting

Weighting causes the fabric to lose its strength as soon as the weighting is applied. Heavily weighted silk must be made into garments as soon as it is made. Spots develop in the dyes. Saltswater, perspiration and tears cause spots to be formed which seems as if the silk is eaten by acids. Sunlight also attacks weighted silk and can cause silk to fall to pieces.


How to Detected Weighting in Silk :

Weighting of silk can usually be detected by the burning test. Separate threads from the warp and the weft are set on fire with a burning match. Pure silk burns very badly and stops burning as soon as the burning match has been removed. Practically no ash is formed (less than one per cent), and the end of the fiber left unburned takes the shape of a little bulb.

Weighted fibers, when burned, leave a considerable amount of ash, and the entire thread may keep its shape after being burned. When only the filling or the warp is weighted, applying the flame to a sample of the cloth seems to consume only one set of threads, the unweighted ones, the others keeping their form because of the heavy ash content.

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FAQ about Mercerization

This Article written in early 1900s is a superb treatise on everything about Mercerization:

Here is a list of FAQ that this articles seeks to clarify:

1. Why Mercerization ?

Mercerization was evolved to get over the limitation of silk. It is the high price of silk and low production. On the other hand, a product was needed which could imitate the high lustre, steel-like strength, its attractive smoothness and softness, its elasticity and and its quality of taking the most delicate tints and shades in the dyeing process.

2. What is Mercerization

Mercerization is a process applied to cotton yarn or fabrics which gives to the cotton fiber a silk-like luster, greater strength than ordinary cotton and a greater affinity for dyes.

3. How it is done

The cotton is soaked in strong caustic soda or caustic potash solution for a few moments under stretch and then washing in pure water to remove the caustic.

4. What happens to the structure of cotton during Mercerization

In natural condition the cotton fiber is a flat, twisted, ribbon-like filament. When immersed in caustic solution it swells out and takes on a round and a hair like appearance, and becomes plump instead of flat.

5. What happens to the chemical structure of cotton during Mercerization

The cellulose is changed into hydro-cellulose or cellulose-hydrate.

6. Why mercerized cotton takes dyes so quickly. 

Cellulose cannot be dyed so easily. Hydro-cellulose on the other hand, absorbs almost any kind of dye readily. Mercerised cotton takes dyes so fast, that chemicals are added in the dye bath to check the process in order that the dyes may not enter so rapidly as to render the shading uneven.

7. What is role of stretch during mercerisation.

Stretch causes the luster, the more the stretch the more the lustre. However, after a certain point, the stretch causes a decrease in strength. 

8. How mercerization is done actually ? What are the chemicals added and other process parameters.

9. Apart from Caustic Soda, what other chemicals can be used for Mercerisation, what are their limitations. 

10. Why sometimes Carbon disulphide is added in mercerisation. 

11. Should bleaching be done before or after mercerisation

12. What pre-processes ensure better luster in Mercerisation processes

Please refer to the article.

13. What type of Cottons are suitable for Mercerisation

Longer cottons are more suited to get as much natural luster as possible. Similarly combed cotton lend themselves better to Mercerisation than carded cottons. 

14. What is part Mercerisation. How it is used in produces various fabrics.

Taking a cotton blend, and then mercerising will produce an effect called as crepon effect. Similarly, mercerisation can be used to produce seersucker effect by Mercerising only certain stripes on the warp direction by covering the rest of the cloth by suitable means.

15. How to identify Mercerised Cotton

(This method has not been tried by me. Please take all precautions including consultation from a chemical scientist before attempting)


Mercerized cotton may be determined as follows: A solution is prepared by dissolving 140 gms of potassium iodide in about 475 ml of water. To this solution add 30-60gms of iodine, and mix with another solution made by dissolving 850 gms of zinc chloride in 350 gms of water. The cloth sample should first be soaked in water, immersed in this prepared solution for three minutes, and then rinsed in water. Mercerized cotton will have a deep blue color, while unmercerized cotton will wash out white. The blue of this solution on mercerized cotton will show through quite heavy dyes.

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Twill Variations-1

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