Practical Test Procedures for Preliminary Identification of Dyes on Wool, Silk and Other Protein Fibres
General disclaimer: This article is intended for educational and general technical understanding only. The procedures discussed involve hazardous, corrosive, toxic, flammable, reducing, oxidizing and environmentally sensitive chemicals. They should be performed only by trained personnel in a properly equipped laboratory with suitable personal protective equipment, ventilation, supervision, documentation and waste-disposal systems. This article should not replace official standards, laboratory manuals, safety data sheets, institutional protocols or professional textile-testing advice.
Wool, silk and other protein fibres are dyed with several classes of dyes. The colour on the fabric may look simple, but the dye chemistry behind it may be quite different. A red silk, a black wool fabric or a blue protein-fibre yarn may be dyed with acid dyes, basic dyes, direct dyes, metal-complex dyes, mordant dyes, vat dyes or azoic dyes.
The purpose of these practical tests is not to identify the exact commercial dye name. The purpose is to identify the broad application class of dye. This is useful because different dye classes behave differently during washing, perspiration, rubbing, light exposure, steaming, finishing and chemical treatment.
The testing logic is based on behaviour. Does the dye bleed? Does it strip from the fibre? Does it stain cotton? Does it form a precipitate? Does it respond to EDTA? Is metal present? Does the colour disappear under reduction and return on oxidation? Each observation becomes a clue.
1. Preparation of the Test Specimen
Objective
The objective is to select a representative coloured portion of the material for testing. If the sampling is wrong, the test conclusion may also be wrong.
Procedure
If the material is a fabric, take a small representative piece from the coloured area. If the material is yarn, take the coloured yarn separately. If the fabric is multicoloured, each colour should be tested separately because different colours in the same fabric may have been dyed with different dye classes.
This is especially important in silk sarees, wool shawls, embroidered fabrics, printed fabrics and jacquard fabrics. The body, border, pallu, motif, extra-weft design, embroidery thread and printed portion may not have the same dye chemistry.
Use clean specimens and avoid contamination from dirt, oil, finishing agents, detergent residue or loose colour from another area. Where a test requires a fresh specimen, do not reuse a previously treated sample because earlier reagents may already have changed the dye behaviour.
2. General Solvent Stripping Test
Objective
The objective of this test is to observe whether the dye can be stripped from wool, silk or another protein fibre by selected hot solvents. The strength of bleeding gives the first indication of the possible dye class.
Reagents Required
The reagents used are 50 percent dimethylformamide, concentrated dimethylformamide, and a mixture of glacial acetic acid and rectified spirit in the ratio 1:1 by volume.
The mixture ratio may be written as:
\[ \text{Glacial acetic acid : Rectified spirit} = 1 : 1 \]
Procedure
Take the dyed specimen and treat it successively with 50 percent dimethylformamide, then concentrated dimethylformamide, and finally with the glacial acetic acid and rectified spirit mixture. Each treatment is carried out at boil for about 3 to 4 minutes.
Between treatments, wash the specimen with water and squeeze it gently before moving to the next reagent. Observe whether the colour bleeds into the liquid. Record the degree of bleeding as strong, slight or almost absent.
Interpretation
Strong bleeding in hot dimethylformamide suggests the possibility of acid dyes. Slight bleeding may indicate metal-complex dyes. No bleeding may suggest mordant or chrome dyes, especially if later tests support the presence of metal.
This test should be treated as a first clue and not as the final answer. Shade depth, finishing chemicals, after-treatment, dye mixtures and poor washing-off may affect the observation.
3. Test for Basic Dyes
Objective
The objective is to check whether the dye behaves like a basic dye. Basic dyes are cationic dyes and can form coloured complexes with certain reagents.
Reagents Required
The reagents used are glacial acetic acid, water, tannin reagent, rectified spirit, sodium hydroxide and acetic acid.
Procedure 1: Tannin Precipitation Test
Take a test specimen. Add 1 ml of glacial acetic acid and warm the specimen. Then add 5 ml of water. To the extract, add tannin reagent and observe whether a coloured precipitate is formed.
Procedure 2: Rectified Spirit Extraction Test
Take another test specimen and boil it with rectified spirit. Observe whether a coloured extract is obtained. A coloured extract supports the possibility of a basic dye, especially when read along with the tannin reagent test.
Procedure 3: Alkali and Acid Colour-Change Test
Take a test specimen and boil it in glacial acetic acid. Then add 30 percent sodium hydroxide until the solution becomes alkaline. Observe whether there is a change in colour or complete decolourization.
After this, acidify the solution with 5 percent acetic acid and observe whether the original colour is restored.
Interpretation
If the extract gives a coloured precipitate with tannin reagent, the dye may be a basic dye. If rectified spirit gives a coloured extract, this further supports the possibility. If the colour changes or disappears in alkali and then returns after acidification, this also supports the basic dye indication.
The practical logic is that basic dyes respond strongly to changes in ionic environment. Their interaction with tannin reagent is useful because it gives a visible precipitate.
4. Test for Direct Dyes
Objective
The objective is to find out whether the dye can leave the wool or silk specimen and stain cotton under alkaline conditions. This is useful because direct dyes have affinity for cellulosic fibres such as cotton.
Reagents Required
The reagents and materials used are 5 percent sodium carbonate solution, bleached cotton pieces and 1 percent ammonium hydroxide solution. For silk dyeings, 5 to 10 percent sodium hydroxide may be used instead of sodium carbonate solution.
The alkali concentration may be written as:
\[ \text{Sodium hydroxide solution for silk dyeings} = 5\% \text{ to } 10\% \]
Procedure
Take a test specimen and boil it with 5 percent sodium carbonate solution for about half a minute in the presence of a few pieces of bleached cotton. After boiling, remove the cotton and observe whether it has become stained.
Then treat the stained cotton with 1 percent ammonium hydroxide solution and observe whether the stain is removed or remains. For silk dyeings, use 5 to 10 percent sodium hydroxide solution instead of sodium carbonate solution.
Interpretation
If the cotton becomes stained and the stain is not much affected by 1 percent ammonium hydroxide, the result suggests the presence of a direct dye. The idea is simple: the dye leaves the protein fibre and shows affinity for cotton.
There is an important caution. Some dyes that are chemically close to substantive azo dyes may stain cotton lightly and may also be reduced under alkaline hydrosulphite conditions. Therefore, this test should not be interpreted alone.
5. Ammonium Hydroxide Extraction and Re-Dyeing Test
Objective
The objective is to check whether the dye can be stripped in dilute ammonium hydroxide and whether the stripped dye can re-dye cotton or wool under different conditions.
Reagents Required
The reagents and materials used are 1 percent ammonium hydroxide solution, sodium chloride, bleached cotton, scoured wool and 10 percent sulphuric acid.
Procedure
Take a fresh test specimen and add 5 to 10 ml of 1 percent ammonium hydroxide solution. If the extract becomes coloured, remove the stripped specimen and divide the extract into two portions.
To the first portion, add about 30 mg of sodium chloride and 10 to 30 mg each of bleached cotton and scoured wool. Boil the mixture and observe whether the cotton or wool becomes stained.
To the second portion, neutralize and then acidify using 10 percent sulphuric acid, adding a few drops in excess. Then add bleached cotton and scoured wool and boil. Again observe which fibre becomes stained.
Interpretation
This test gives information about the behaviour of the extracted dye under alkaline and acidic conditions. If the dye stains cotton, it suggests affinity for cellulose. If it stains wool under acidic conditions, it may indicate a dye class with affinity for protein fibre.
The strength of this test lies in comparison. The same extract is observed in two conditions, one without acidification and the other after acidification. The difference in staining behaviour becomes a useful clue.
6. Tests for Acid, Metal-Complex and Mordant or Chrome Dyes
Objective
The objective is to distinguish among acid dyes, metal-complex dyes and mordant or chrome dyes. These dye classes are especially important for wool and silk.
Test 1: Bleeding in Hot Dimethylformamide
Take a test specimen and boil it with dimethylformamide. Observe the degree of bleeding. Strong bleeding indicates acid dye. Slight bleeding indicates metal-complex dye. No bleeding indicates mordant dye or chrome dye.
Test 2: EDTA-Glycerine Test
Heat a test specimen in a solution of EDTA in glycerine at about 140°C and observe the colour change. No change suggests acid or mordant dyes. A rapid change within 1 to 2 minutes suggests 1:1 metal-complex dye. A slow change within 10 to 15 minutes suggests 1:2 metal-complex dye.
At about 160°C, no change indicates acid dye. The principle is that EDTA is a chelating agent. If metal is important in the dye structure or dye-fibre complex, EDTA may disturb that arrangement and produce a colour change.
This may be represented simply as:
\[ \text{Metal-dye complex} + \text{EDTA} \rightarrow \text{Disturbed complex} + \text{Colour change} \]
Test 3: Dilute Hydrochloric Acid and Hydrosulphite Test
Take a test specimen and boil it with dilute hydrochloric acid. Then take out the specimen and warm it with 10 percent sodium hydrosulphite solution. Observe whether the colour is destroyed.
Most after-chrome dyes are not stripped easily. This resistance to stripping should be taken as a clue for mordant or chrome dyes.
Interpretation
If the dye bleeds strongly in dimethylformamide and does not show metal-complex behaviour in EDTA, acid dye is indicated. If the dye bleeds slightly and changes in EDTA-glycerine, metal-complex dye is indicated. If the dye does not bleed and later metal-related tests support the observation, mordant or chrome dye is indicated.
7. Ash Test for Presence of Metal
Objective
The objective is to detect the presence of metal in the dyed fibre. This is especially relevant when mordant, chrome or metal-complex dyeing is suspected.
Reagents and Materials Required
The materials used are a porcelain crucible, sodium carbonate, sodium nitrate and suitable reagents for metal detection. A flux made from equal parts of sodium carbonate and sodium nitrate is used.
The flux composition may be written as:
\[ \text{Sodium carbonate : Sodium nitrate} = 1 : 1 \]
Procedure
Take a test specimen of about 5 g and ash it completely in a porcelain crucible. Add about 200 mg of flux made from equal parts of sodium carbonate and sodium nitrate, and fuse the residue. Then test the fused material for the presence of metal.
The presence of chromium or cobalt supports the possibility of metal-complex dyes or mordant/chrome dyes, depending on the earlier observations.
Interpretation
If metal is detected and the dye was difficult to strip, mordant or chrome dyeing becomes likely. If metal is detected and the dye showed slight bleeding with EDTA response, metal-complex dye becomes likely.
This test should be treated as supporting evidence. The presence of metal alone is not enough; it must be interpreted along with bleeding, stripping and colour-change behaviour.
8. Test for Vat Dyes
Objective
The objective is to identify vat dye behaviour through reduction and oxidation. Vat dyes can be reduced to a soluble leuco form and then oxidized back to the coloured form.
Reagents Required
The reagents and materials used are 10 percent sodium hydroxide, sodium hydrosulphite, sodium chloride, bleached cotton, sodium nitrate and acetic acid solution. Hydrogen peroxide may also be used in additional differentiation tests.
Procedure
Take a test specimen of about 200 to 300 mg. Add 2.5 ml of 10 percent sodium hydroxide and boil until the specimen is dissolved. Add 25 to 30 mg of sodium hydrosulphite, 20 to 50 mg of sodium chloride and 10 to 15 mg of bleached cotton.
Keep the mixture near boil for about 2 minutes and then cool. Remove the cotton and place it on filter paper for 1 to 2 minutes. Oxidize the cotton with sodium nitrate and acetic acid solution.
Interpretation
If the cotton is dyed and the colour returns on oxidation, vat dye behaviour is indicated. The chemical logic is reduction and oxidation. Under reducing alkaline conditions, vat dyes form a soluble reduced form. On oxidation, the coloured insoluble form is regenerated.
The simplified logic may be written as:
\[ \text{Vat dye} \xrightarrow{\text{reduction}} \text{Leuco form} \xrightarrow{\text{oxidation}} \text{Original coloured form} \]
9. Additional Tests for Vat and Azoic Dyes
Objective
The objective is to distinguish vat dyes from azoic dyes when reduction-oxidation behaviour creates doubt.
Procedure 1: Paraffin Wax Heating Test
Warm some paraffin wax in a white porcelain crucible until faint vapours appear. Hold the test specimen in the molten wax for about one minute. Remove the specimen. After cooling, observe whether staining of the paraffin wax is seen against the white background of the porcelain.
Procedure 2: Blank Vat Solution and Oxidation Test
Take a test specimen and treat it with a blank vat solution at about 50°C in a test tube. Then oxidize the specimen with 3 percent hydrogen peroxide.
If the colour changes and the original colour is restored on oxidation, vat dye is indicated. If the colour changes and the original colour is not restored on oxidation, azoic dye is indicated.
Procedure 3: Ethylenediamine and Hydrosulphite Test
Warm a test specimen with ethylenediamine. Add aqueous sodium hydrosulphite solution to the ethylenediamine extract. If the coloured extract is decolourized readily and permanently, this observation is used in the differentiation of vat and azoic dyes.
Additional Note on Azoic Dyes
Many azoic dyeings on wool may yield slimy residues of the same intense colour as the original dyeing when boiled in 5 percent and 10 percent sodium hydroxide solution. Many yellow dyeings and prints may change to orange or red colour.
Interpretation
If the colour disappears under reduction and returns after oxidation, vat dye behaviour is suggested. If the colour changes and does not return after oxidation, azoic dye behaviour is suggested. If special residue formation or characteristic colour changes occur in alkali, azoic dyeing becomes more likely.
10. Ether Extraction Test for Metal-Complex and Mordant Dyes
Objective
The objective is to help distinguish metal-complex dyes from mordant dyes when earlier observations point toward metal involvement.
Reagents Required
The reagents used are 1 percent ammonium hydroxide, hydrochloric acid and ether. Ether is highly flammable and volatile, so this test should only be performed under strict laboratory safety conditions.
Procedure
Strip the dye in hot 1 percent ammonium hydroxide. After cooling, acidify the solution with hydrochloric acid. Shake the extract with ether. Observe whether the ether layer becomes coloured.
Interpretation
If the ether becomes coloured, metal-complex dye is indicated. If the ether is not coloured, mordant dye or chrome dye is indicated.
The practical idea is that some stripped metal-complex dye material may move into the ether layer, while mordant or chrome dye behaviour may not show this response in the same way.
11. Practical Observation Record
A laboratory record should not simply say “positive” or “negative.” It should record the exact behaviour observed at each stage. A suggested format is given below.
| Stage of Test | Observation to Record | Possible Interpretation |
|---|---|---|
| Hot dimethylformamide stripping | Strong, slight or no bleeding | Acid, metal-complex or mordant/chrome indication |
| Glacial acetic acid and water extraction | Whether extract is coloured | Useful for further basic dye testing |
| Tannin reagent test | Whether coloured precipitate forms | Basic dye indication |
| Rectified spirit boiling | Whether coloured extract forms | Supports basic dye indication |
| Alkaline boiling with cotton | Whether cotton is stained | Direct dye indication |
| Ammonium hydroxide extraction | Whether extract is coloured | Used for re-dyeing test |
| Re-dyeing with cotton and wool | Which fibre is stained | Indicates dye affinity |
| EDTA-glycerine treatment | Rapid, slow or no colour change | Metal-complex or acid/mordant indication |
| Ash test | Whether metal is detected | Supports metal-complex or mordant/chrome indication |
| Reduction and oxidation | Whether colour disappears and returns | Vat dye indication |
| Special alkali behaviour | Slimy residue or colour shift | Azoic dye indication |
12. Practical Precautions
These tests are qualitative and require experience. A faint stain, slight bleeding or slow colour change can be interpreted differently by different observers. Therefore, where possible, the unknown sample should be compared with known samples dyed with authentic dye classes.
The sample should be tested colour by colour. In a multicoloured silk saree, the body, border, pallu, motif and extra yarn may all behave differently. In a wool fabric, the ground yarn and decorative yarn may also differ. In printed fabrics, the print and ground should be treated as separate systems.
Finishing agents, softeners, after-treatments, optical brighteners, metallic salts, poor washing-off and mixtures of dyes can interfere with interpretation. A shade may not be produced by a single dye class. Black, navy, maroon and brown shades are especially likely to be mixtures.
Conclusion
The practical identification of dye classes on wool, silk and other protein fibres is a step-by-step diagnostic exercise. The tester observes how the colour behaves during solvent stripping, acid treatment, alkali treatment, re-dyeing, tannin precipitation, EDTA treatment, metal detection and reduction-oxidation testing.
No single observation should be treated as final. Strong bleeding, slight bleeding, cotton staining, tannin precipitation, EDTA colour change, metal detection and oxidation behaviour are all clues. When several clues point in the same direction, the dye class can be identified with greater confidence.
For textile students, these procedures teach the chemistry behind colour. For laboratories, they provide a practical path for preliminary dye-class identification. For merchandisers and quality professionals, they explain why a fabric may bleed, stain, fade or behave differently during use.
Acknowledgement
This practical explanation is based on the dye-identification procedures for wool, silk and other protein fibres described in IS 4472 Part II.
Goyal, P. Practical Test Procedures for Preliminary Identification of Dyes on Wool, Silk and Other Protein Fibres. My Textile Notes. Available at: https://mytextilenotes.blogspot.com/2026/05/practical-test-procedures-for_0283956059.html
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