Part A: Practical Test Procedures for Preliminary Identification of Dyes on Cotton
General disclaimer: This article is intended for educational understanding of preliminary textile dye-class identification. It is not a substitute for official standards, laboratory protocols, institutional safety procedures, or professional chemical-handling training. The tests discussed may involve toxic, corrosive, flammable, volatile, reducing, oxidizing, and environmentally hazardous substances. Any actual testing should be performed only by trained personnel in a properly equipped laboratory with suitable personal protective equipment, ventilation, supervision, documentation, and waste-disposal systems.
In the previous part, we understood the logic of dye identification: do not judge the dye only by its colour; judge it by its behaviour. A red colour, a blue colour, or a black colour may be produced by different dye classes. The real question is: what happens when the dyed cotton is treated with solvents, acids, alkalis, reducing agents, oxidizing agents, or auxiliary fibres such as white cotton and wool?
This part explains the tests more practically. The purpose is to show how each test is performed, what quantities are used where specified, what temperature condition is used, and what observation should be made. This should be treated as an educational explanation, not as a substitute for a trained laboratory procedure. Several reagents mentioned here are hazardous and should be handled only in a proper laboratory.
1. Preparation of the Test Specimen
Before any dye identification test is started, the specimen has to be selected correctly. If the material is fibre or yarn, take a tuft of about 3 cm length. If the material is fabric, take a test piece of about 3 cm × 3 cm. For multicoloured woven fabrics, each coloured yarn should be tested separately. For printed fabrics, the specimen should be taken from the printed portion, not from the plain ground. Finished textiles may need pretreatment twice with 1 percent hydrochloric acid at boil for 5 minutes.
This specimen preparation stage is not a minor formality. If the wrong portion is tested, the result may be misleading. For example, in a printed cotton fabric, the ground may be reactive dyed while the print may be pigment printed or azoic printed. Testing the wrong area may identify the wrong colour system.
2. Solvent Stripping Test for Strongly Fixed or Ingrain Dyes
The first practical question is: can the colour be stripped out by strong solvent treatment? The specimen is treated successively with three solvent systems. First, it is treated with 50 percent dimethylformamide. Then it is treated with concentrated dimethylformamide. Finally, it is treated with a 1:1 mixture of glacial acetic acid and rectified spirit. Each treatment is done at boil for 3 to 4 minutes, with intermediate washing in water and squeezing between treatments.
The observation is simple but important. If there is no stripping, or only partial stripping, the dye may belong to the group of reactive dyes or ingrain dyes, except azoic dyes. The logic is that reactive dyes are chemically fixed to cellulose, and ingrain colours are formed within the fibre system. Therefore, they resist ordinary stripping. However, this is only a preliminary indication, because some basic dyes may also resist stripping.
3. Mild Alkali Bleeding Test
If the first test does not establish strongly fixed or ingrain behaviour, a fresh test specimen is taken and boiled in 1 percent ammonium hydroxide solution for 1 to 2 minutes.
This test asks whether the dye bleeds into a mild alkaline medium. If the solution becomes distinctly coloured, the dye has been extracted from the fibre to some extent. But extraction alone is not enough for identification. The next step is to see whether the extracted dye can re-dye another fibre.
4. Direct Dye Re-Dyeing Test on White Cotton
If the specimen bleeds and the solution becomes distinctly coloured, remove the original test specimen. Then add a few pieces of white bleached cotton and 25 mg of sodium chloride. Boil this for 2 minutes. After boiling, cool and rinse the added bleached cotton.
If the added white cotton is dyed to approximately the original shade, the indication is direct dye. The reason is that direct dyes have affinity for cotton, especially in the presence of salt. The dye leaves the original specimen, enters the solution, and then dyes fresh cotton. In practical language, the dye repeats its own dyeing behaviour in miniature.
5. Acid Dye Transfer Test on Wool
Sometimes the specimen bleeds into the alkaline solution, but the added white cotton remains undyed or only slightly stained. In that case, neutralize the solution with acetic acid. Then add 1 ml of 10 percent acetic acid, introduce pieces of undyed wool, and boil for 1 minute. After boiling, cool and rinse the wool pieces.
If the wool becomes dyed, the indication is acid dye, provided direct and basic dyes are absent. The logic is based on fibre affinity. Acid dyes generally prefer protein fibres such as wool and silk. So if the extracted dye does not properly dye cotton but dyes wool in acidic conditions, the behaviour points towards an acid dye.
6. Basic Dye Test Using Mordanted Cotton
If the test specimen does not bleed, or bleeds only slightly, a fresh specimen is treated differently. Add 1 ml of glacial acetic acid and warm the specimen. Then add 3 to 5 ml of water and boil. Remove the original specimen, add 25 mg of mordanted cotton, and boil for 2 minutes.
If the mordanted cotton becomes dyed, the indication is basic dye. Basic dyes do not necessarily show strong affinity for untreated cotton, but mordanted cotton can attract them. The mordant acts like a bridge between the dye and the fibre. This test is therefore not simply about whether the dye comes out; it is about whether the dye is captured by a specially prepared receiving material.
7. Test for Direct Dyes After Resin Treatment
Sometimes a direct dye may have been after-treated with resin or fixing agent. Because of this, it may not bleed or transfer like an ordinary direct dye. To uncover this possibility, the specimen is treated with 1 percent hydrochloric acid and then tested for direct dye behaviour. If the specimen responds to the direct dye test after acid treatment, the indication is direct dye after-treated with resin.
This is a very practical commercial point. A buyer or merchandiser may see a cotton fabric that behaves better in washing because the dye has been fixed after dyeing. The dye class may still be direct dye, but the after-treatment masks its ordinary behaviour.
8. Acid Pre-Treatment Before Moving to Reduction Tests
If direct, reactive, ingrain, acid, and basic dyes are absent, take a fresh specimen and add 10 to 15 ml of 1 percent hydrochloric acid. Boil for 1 minute, discard the acid solution, and repeat once or twice.
This step prepares the specimen for the next stage of testing. By this point, simple extraction and fibre-transfer behaviour have not given the answer. The identification now moves toward dyes that reveal themselves through reduction and oxidation.
9. Sulphur Dye Reduction and Reoxidation Test
For sulphur dye behaviour, take a fresh specimen and add 2 to 3 ml of water, 1 to 2 ml of 5 percent sodium carbonate solution, and 500 mg of solid sodium sulphide. Boil the mixture for 2 minutes. Remove the specimen. Then add 25 mg of sodium chloride and a few pieces of white bleached cotton. Boil again for 2 minutes. Place the original specimen and the white cotton on filter paper and allow reoxidation.
If the white cotton is dyed and, after reoxidation, the white cotton is redyed to approximately the original shade while the test specimen also restores its colour, the indication is sulphur dye. This test is based on the reduction–oxidation nature of sulphur dyes. Under reducing alkaline conditions, the dye becomes mobile. On exposure to air or reoxidation, the colour returns.
Diagnostic idea: Sulphur dye behaviour can be understood as:
\[ \text{Insoluble coloured dye} \xrightarrow{\text{Reduction}} \text{Soluble leuco form} \xrightarrow{\text{Oxidation}} \text{Insoluble coloured dye} \]
10. Oxidation Black or Aniline Black Test
If cotton is not redyed from the sodium carbonate–sodium sulphide solution, a fresh sample is taken in an evaporating dish. Add 2 to 3 ml of concentrated sulphuric acid and shake just enough to extract the dye. Pour the extract into a test tube, add 25 ml of water, and filter. Wash the filter paper with water. Then spot the filter paper with 10 percent sodium hydroxide solution.
If the spot turns red-violet, the indication is oxidation black, also called aniline black. This is especially relevant for black shades. Not every black is sulphur black or vat black. Some blacks are produced by oxidation chemistry on the fibre, and this test is designed to detect that behaviour.
11. Vat Dye Reduction and Developer Test
If sulphur dye and oxidation black are absent, take a fresh specimen and boil it with sodium sulphoxylate formaldehyde-glycol solution containing a few drops of 44 percent sodium hydroxide solution. The specimen may become decolourized or show a marked shade change. The solution may become yellow, bluish red, or show another characteristic colour. Then test whether the original colour is restored by treatment with a vat-dye developer.
If the original colour is restored, the indication is vat dye. Vat dyes are identified through their reversible reduction–oxidation behaviour. In dyeing, a vat dye is reduced to a soluble form, enters the fibre, and then is oxidized back to its insoluble coloured form. This test reproduces that principle in a diagnostic way.
12. General Reduction Test for Group III and Group IV Behaviour
There is also a broader reduction observation in the sequence. If dyes of the earlier group are absent, a fresh test specimen is boiled for 1 to 2 minutes in 5 to 10 ml of water containing 10 to 30 mg of sodium hydrosulphite, to which 4 to 6 drops of 44 percent sodium hydroxide solution are added.
The observation separates two broad behaviours. Group III dyes may decolourize or change shade radically, and the colour may be restored on exposure to air or with vat-dye developer. Group IV dyes are destroyed and do not restore to the original colour on reoxidation. This is a key distinction: reversible colour change suggests one type of dye chemistry, while irreversible destruction suggests another.
13. Chromium Salt After-Treatment Test
For direct dyes after-treated with chromium salts, take a fresh test specimen of about 6 g and ash it in a porcelain crucible. Add 200 mg of flux, made from equal parts of sodium carbonate and sodium nitrate, and fuse.
If the fused mass is orange-yellow when hot and permanent greenish-yellow when cold, the indication is direct dye after-treated with chromium salts. Here the test is no longer looking only at the dye. It is looking for evidence of metallic after-treatment.
14. Copper Salt After-Treatment Test
If chromium is absent, ash the specimen as above. Dissolve the ash in a few drops of concentrated nitric acid. Add 2 ml of water, boil, and cool. Then add 2 ml of concentrated ammonium hydroxide.
If a blue colour appears, the indication is direct dye after-treated with copper salts. This is another example where the fabric’s after-treatment history becomes part of dye identification. The original dye may be direct dye, but metal salt treatment changes its performance and laboratory behaviour.
15. Formaldehyde After-Treatment Test
If chromium- or copper-treated direct dyes are absent, take a fresh specimen and treat it with 5 percent boiling sulphuric acid. Cool the solution and add 1 percent carbazol solution dropwise.
If a blue precipitate appears, it indicates the presence of formaldehyde, and the dye is interpreted as direct dye after-treated with formaldehyde. Again, the test is not simply detecting the shade. It is detecting a chemical after-treatment associated with the dyeing process.
16. Pyridine Test for Azoic and Developed Dyes
For azoic and related developed dyes, take a fresh specimen, add 2 ml of pyridine, and boil. Repeat the treatment using 2 to 3 fresh portions of pyridine.
If the specimen bleeds and continues to bleed in subsequent treatments, the indication is azoic dye. If the specimen does not bleed, or bleeds only slightly, and the bleeding decreases or usually stops, the indication is diazotized and developed dye. This test is useful because azoic colours are often formed within the fibre through coupling reactions. Their behaviour is therefore different from ordinary absorbed dyes.
Practical Flow of the Tests
The test sequence starts with the least specific but very revealing question: can the dye be stripped? If not, reactive or ingrain behaviour is suspected. If the dye can be extracted in mild alkali, the next question is whether it can re-dye white cotton. If it can, direct dye is suspected. If it cannot dye cotton but can dye wool in acid medium, acid dye is suspected. If mordanted cotton takes up the colour, basic dye is suspected.
If these routes do not identify the dye, the testing moves into reduction and oxidation behaviour. Sulphur dyes are checked through sodium sulphide reduction and reoxidation. Oxidation black is checked through strong acid extraction and alkaline spotting. Vat dyes are checked through reduction with sodium sulphoxylate formaldehyde-glycol solution and restoration with vat-dye developer. After-treated direct dyes are checked through chromium, copper, and formaldehyde-related reactions. Finally, azoic and developed dyes are examined through repeated pyridine treatment.
Summary Table of Practical Test Conditions
| Purpose of Test | Main Reagents and Quantities | Heating / Time | Positive Indication |
|---|---|---|---|
| Solvent stripping | 50% dimethylformamide, concentrated dimethylformamide, glacial acetic acid:rectified spirit 1:1 | Boil, 3–4 min each | No/partial stripping: reactive or ingrain dye |
| Mild alkali bleeding | 1% ammonium hydroxide | Boil, 1–2 min | Dye bleeds into solution |
| Direct dye check | White cotton + 25 mg sodium chloride | Boil, 2 min | White cotton dyed to near original shade |
| Acid dye check | Neutralize, add 1 ml 10% acetic acid + undyed wool | Boil, 1 min | Wool dyed |
| Basic dye check | 1 ml glacial acetic acid, 3–5 ml water, 25 mg mordanted cotton | Warm, then boil 2 min | Mordanted cotton dyed |
| HCl pre-treatment | 10–15 ml 1% hydrochloric acid | Boil, 1 min; repeat | Prepares specimen for further tests |
| Sulphur dye test | 2–3 ml water, 1–2 ml 5% sodium carbonate, 500 mg solid sodium sulphide, then 25 mg sodium chloride + white cotton | Boil 2 min + boil 2 min | White cotton redyed; colour restored on reoxidation |
| Oxidation black test | 2–3 ml concentrated sulphuric acid, 25 ml water, 10% sodium hydroxide spotting | Extraction, filtration, spotting | Red-violet spot |
| Vat dye test | Sodium sulphoxylate formaldehyde-glycol solution + few drops 44% sodium hydroxide | Boil | Colour restored by vat-dye developer |
| General reduction check | 5–10 ml water, 10–30 mg sodium hydrosulphite, 4–6 drops 44% sodium hydroxide | Boil, 1–2 min | Reversible or irreversible colour change |
| Chromium after-treatment | About 6 g specimen, 200 mg flux | Ash and fuse | Orange-yellow hot, greenish-yellow cold |
| Copper after-treatment | Ash + conc. nitric acid, 2 ml water, 2 ml conc. ammonium hydroxide | Boil and cool | Blue colour |
| Formaldehyde after-treatment | 5% boiling sulphuric acid, 1% carbazol solution | Boil, cool, add dropwise | Blue precipitate |
| Azoic dye test | 2 ml pyridine; repeat with 2–3 fresh portions | Boil | Continued bleeding indicates azoic dye |
Important Laboratory Safety Note
These procedures involve hazardous reagents such as concentrated sulphuric acid, concentrated nitric acid, sodium sulphide, sodium hydrosulphite, pyridine, dimethylformamide, sodium hydroxide, and ammonium hydroxide. Some are corrosive, toxic, volatile, or strongly reducing/oxidizing. These tests should be carried out only in a properly equipped textile or chemical laboratory with fume extraction, gloves, goggles, lab coat, trained supervision, and proper waste disposal.
For educational understanding, the most important lesson is not to memorize every chemical first. The main lesson is to understand the diagnostic logic: strip, bleed, transfer, reduce, oxidize, restore, destroy, or detect after-treatment. That is the practical grammar of dye-class identification.
Acknowledgement: This practical blog is based on the preliminary identification sequence given in Annex A of IS 4472 Part 1:2021.
Goyal, P. Part A: Practical Test Procedures for Preliminary Identification of Dyes on Cotton. My Textile Notes. Available at: https://mytextilenotes.blogspot.com/2026/05/practical-test-procedures-for.html
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