Part B: Confirming the Dye Class on Cotton — The Second Diagnostic Journey
General disclaimer: This article is intended for educational understanding of confirmatory textile dye-class identification. It is not a substitute for official standards, institutional laboratory procedures, safety manuals, or professional chemical-handling training. Any actual testing should be performed only by qualified personnel using appropriate personal protective equipment, ventilation, supervision, validated methods, documentation, and waste-disposal practices.
In Part A, the dye was questioned through broad behaviour. We asked whether the colour stripped, bled, transferred to cotton, transferred to wool, responded to reduction, returned after oxidation, or behaved like a colour formed inside the fibre. That first journey gave us a probable dye class. Part B now takes the next step: it asks whether that suspicion can be confirmed by a more specific reaction.
This second journey is not a repetition of Part A. It is more like cross-examination. If Part A says, “This may be a direct dye,” Part B asks, “Can it behave like a direct dye under stronger confirmation?” If Part A says, “This may be sulphur dye,” Part B asks, “Can we detect the sulphur behaviour more specifically?” If Part A says, “This may be vat dye, azoic dye, pigment, oxidation black, or ingrain dye,” Part B gives separate confirmation routes for each possibility.
Why Confirmation Is Needed
Preliminary tests are useful, but they are not always final. Some dyes overlap in behaviour. A dye may resist stripping because it is chemically bonded, but another dye may resist stripping because of after-treatment. A black shade may look like sulphur black, vat black, or oxidation black. A pigment may not behave like a normal dye because it is held by a binder rather than absorbed into the fibre.
So the confirmatory stage asks a sharper question: does the suspected dye class give its own characteristic reaction? This is the logic of Part B. We move from general behaviour to class-specific proof.
Practical idea: Part A gives a probable direction. Part B checks whether that direction can stand up to a more specific chemical test.
1. Confirming Direct Dyes
If the preliminary test suggests a direct dye, the confirmation begins by checking whether the colour can be extracted and then re-applied to cotton in a controlled way. One route is to boil the specimen briefly in 5 percent sodium hydroxide solution, add a little mercerized cotton, and allow the extracted dye to dye the mercerized cotton for about 10 minutes. If the dye fixed on the mercerized cotton is not removed by 1 percent ammonium hydroxide solution, the behaviour supports the presence of a direct dye.
A second route uses cold ethylenediamine. The specimen is shaken with a small amount of ethylenediamine, and the coloured extract is diluted with water. White cotton is then introduced, heated to around 80°C, and a little sodium chloride is added. If the white cotton is evenly stained and the stain is not removed by boiling with 1 percent ammonium hydroxide solution, this again supports direct dye behaviour. This route is especially useful for certain pale blue dyeings that may not respond strongly to the sodium hydroxide extraction route.
The logic is simple. Direct dyes should be extractable under suitable conditions and should show affinity for cotton. The confirmation is not just that the colour comes out, but that it can again attach to cotton and remain there against a mild stripping challenge.
2. Confirming Formaldehyde After-Treated Direct Dyes
Sometimes the suspicion is not merely “direct dye,” but direct dye after-treated with formaldehyde. This is a more specific situation because the dye has been modified after application to improve performance. The confirmation uses 12 N sulphuric acid extraction for about 5 minutes. Then 1 to 2 ml of concentrated sulphuric acid and 4 to 5 drops of chromotropic acid are added. A reddish violet colour supports the presence of formaldehyde after-treatment.
The logic here is important for commercial textiles. An after-treated direct dye may not behave like an ordinary direct dye in the preliminary test. The confirmatory test therefore looks not only at the dye, but also at the chemical history of the fabric. In other words, it asks: was the direct dye modified after dyeing?
3. Confirming Basic Dyes
If the preliminary behaviour points toward a basic dye, the confirmation begins by extracting the colour with alkali and then changing the medium. The specimen is treated with 1 ml of 5 percent sodium hydroxide solution and boiled briefly. Then 4 ml of 5 percent ammonium chloride solution is added, and the mixture is boiled again. This extract becomes the basis for further confirmation.
The first confirmation is fibre affinity. A small amount of the extract is taken, a few pieces of undyed wool are added, and the solution is allowed to cool. If most of the dye is taken up by the wool, it supports basic dye behaviour. The second confirmation uses tannin reagent after acidifying the extract with 10 percent acetic acid. A coloured precipitate supports the presence of basic dye. The third confirmation uses 1 percent ferric chloride solution after acidification; a black precipitate is another supporting reaction.
The sequence makes sense. Basic dyes are cationic in nature and can form characteristic interactions with mordants and reagents. So the confirmation does not rely on one sign only. It looks at extraction, wool uptake, tannin precipitation, and ferric chloride reaction.
4. Confirming Sulphur Dyes
If the preliminary test suggests sulphur dye, Part B confirms it by looking for sulphur-related behaviour more directly. A specimen is boiled with stannous chloride solution in a test tube. The mouth of the test tube is covered with filter paper moistened with lead acetate solution. Brown staining on the filter paper indicates sulphur dye behaviour, with deep brown stains being especially significant.
There are also supporting checks. The specimen may be boiled with ethylenediamine, in which case sulphur dye is readily stripped. Another test treats the specimen with sodium hypochlorite solution; sulphur dyeings may bleach to white or buff colour. However, some special black dyeings may not behave in the same way, so these observations must be interpreted with care.
The logic is that sulphur dyes are not confirmed merely by their dark shade or by their reduction behaviour. The confirmatory route looks for evidence associated with sulphur chemistry and its response to specific stripping and bleaching conditions.
5. Confirming Vat Dyes
If the preliminary test indicates vat dye, confirmation again depends on reduction and reoxidation. A specimen is boiled with 5 to 10 ml of sodium sulphoxylate formaldehyde-glycol solution containing a little 44 percent sodium hydroxide solution. A distinct colour change is observed. The specimen is then removed and washed with fresh water. If the original colour returns, or returns after treatment with vat dye developer or hydrogen peroxide, vat dye behaviour is supported.
A second confirmation uses ethylenediamine and glucose near the boiling point. Under this treatment, the colour is more or less completely removed. This provides another way of testing the characteristic reducible nature of vat dyes.
The logic is straightforward. Vat dyes live between two chemical states: a reduced soluble form and an oxidized insoluble coloured form. The confirmation asks whether the dye can enter that reversible cycle and return to the original shade.
Vat dye confirmation can be understood as:
\[ \text{Oxidized coloured vat dye} \xrightarrow{\text{Reduction}} \text{Reduced soluble form} \xrightarrow{\text{Oxidation}} \text{Original coloured form} \]
6. Confirming Azoic Dyes
If preliminary testing suggests azoic dye, the confirmation begins with extraction. The specimen is boiled with a sufficient amount of ethylenediamine for a few minutes, and a considerable amount of dye is extracted. The extract is then divided into two parts. To one part, a little sodium hydrosulphite is added and warming is done if needed. Permanent decolourization supports azoic dye behaviour.
The other part of the extract is diluted with water and boiled. If the liquid becomes turbid and coloured pigment flakes settle on standing, that is another supporting sign. Additional confirmation may use sodium sulphoxylate formaldehyde-glycol solution with 44 percent sodium hydroxide, where many azoic dyeings reduce to colourless or yellow compounds. If reduction does not appear after one or two minutes, boiling in 5 percent sodium hydroxide solution with a little sodium hydrosulphite may reduce azoic dyeings to pale yellow or white.
Another practical confirmation uses liquid phenol. The specimen is dipped in phenol, lightly squeezed, placed between filter papers, and pressed with a hot iron or on a steam pipe. Staining of the filter paper supports azoic dye behaviour. This is a very physical-looking test, but the principle is still the same: coax the developed colour system out of the fibre and observe its characteristic response.
7. Confirming Pigments
Pigments behave differently from dyes because they are not usually absorbed into the fibre in the same way. They are often held on the fibre surface by a binder. So the confirmatory route first attacks the binder system. For vat pigments, the specimen is treated with methyl pyrrolidone, which plasticizes the resin binder. After that, the usual vat dye confirmation route is followed.
For azoic pigments, a specimen of about 200 mg is treated with 1 ml methyl pyrrolidone for about 30 seconds and cooled. Then 5 percent sodium hydroxide solution and 25 to 50 mg sodium hydrosulphite are added. The mixture is boiled until the sample becomes white, light yellow, or orange. The solution is filtered, and 25 mg sodium chloride plus a few pieces of cotton are added. After boiling for about 1 minute and cooling, the white cotton is removed and dried. Yellowing or browning of the cotton helps distinguish pigment type.
The logic is very important. A pigment does not reveal itself like a normal dye because it may be trapped in a binder film. So the binder has to be disturbed first. Only then can the colour system be tested.
8. Confirming Oxidation Black
If the preliminary route points towards oxidation black, the confirmation checks for reactions typical of aniline black type colouration. One test digests the specimen with concentrated sulphuric acid in the cold. On dilution with water, a green colour is obtained. Another test treats the specimen with sodium hypochlorite solution for about 1 minute; the specimen turns brown. A further route ashes about 5 g of specimen and tests the ash for iron or copper; a positive result supports this class.
The logic is that oxidation black is not just a black dye sitting on cotton. It is a colour developed through oxidation chemistry. Therefore, the confirmation is not about simple dye transfer; it is about the special reactions associated with that black colour system.
9. Confirming Ingrain Dyes Other Than Azoics
If the preliminary route suggests an ingrain dye other than azoic, Part B gives specific confirmation routes for particular dye types such as Phthalogen Green, Phthalogen Blue, and Alcian Blue. These tests use methyl pyrrolidone, heating, cooling to around 70°C, 10 percent sodium hydroxide, and 20 to 40 mg sodium hydrosulphite. The interpretation depends on the shade change and whether the colour reduces or remains stable.
For Phthalogen Green, the colour reduces to dark violet, and when the specimen is placed in 20 percent acetic acid, the violet colour remains. For Phthalogen Blue, the colour does not reduce under the same reduction treatment, while spotting with concentrated nitric acid changes it to violet and spotting with concentrated sulphuric acid changes it to bright green. For Alcian Blue, the colour changes to violet under reduction, then changes to green in 20 percent acetic acid; acid spotting reactions also give characteristic colour changes.
The logic here is that not all ingrain colours behave alike. Once the broad class is suspected, the confirmation becomes shade-system specific. We are no longer asking only, “Is it an ingrain dye?” We are asking, “Which ingrain dye behaviour does it match?”
The Whole Confirmatory Sequence in One Flow
The second diagnostic journey begins only after the first journey has created a suspicion. If direct dye is suspected, the confirmation checks whether extracted colour can dye mercerized or white cotton and remain resistant to mild ammonium hydroxide stripping. If formaldehyde after-treatment is suspected, a colour reaction with chromotropic acid confirms the after-treatment angle.
If basic dye is suspected, the extract is challenged through wool uptake, tannin precipitation, and ferric chloride reaction. If sulphur dye is suspected, the test looks for sulphur-related staining on lead acetate paper, stripping with ethylenediamine, and bleaching behaviour with hypochlorite. If vat dye is suspected, the confirmation checks whether reduction changes the colour and oxidation restores it.
If azoic dye is suspected, the confirmation uses extraction, permanent decolourization, turbidity, pigment flake formation, reduction to colourless or yellow compounds, and transfer/staining behaviour. If pigment is suspected, the binder is first plasticized before the colour system is tested. If oxidation black is suspected, the confirmation checks acid digestion, hypochlorite browning, and metallic evidence in ash. If ingrain dye is suspected, specific shade reactions are used to distinguish different ingrain systems.
Simple Practical Table
| Suspected Dye Class | Confirmatory Logic | Positive Direction |
|---|---|---|
| Direct dye | Extract and re-dye cotton; check resistance to mild ammonium hydroxide stripping | Cotton stains evenly and stain remains |
| Formaldehyde after-treated direct dye | Acid extraction followed by chromotropic acid reaction | Reddish violet colour |
| Basic dye | Extract, then test wool uptake, tannin reaction, and ferric chloride reaction | Wool uptake / coloured precipitate / black precipitate |
| Sulphur dye | Boil with stannous chloride and detect stain on lead acetate paper | Brown stain on paper |
| Vat dye | Reduce colour, wash, then restore by oxidation/developer | Original colour returns |
| Azoic dye | Extract, reduce, dilute, and observe pigment behaviour | Permanent decolourization or pigment flakes |
| Pigment | Plasticize binder first, then test dye system | Binder disturbance reveals vat or azoic pigment behaviour |
| Oxidation black | Acid digestion, hypochlorite reaction, and ash test | Green dilution / brown hypochlorite response / metal evidence |
| Ingrain dye | Specific reduction and acid spotting reactions | Characteristic violet, green, or non-reduction behaviour |
Why Part B Matters
Part A is like asking the fabric, “What do you generally do?” Part B is like asking, “Can you prove it?” This is why both parts belong together. The first part narrows the field; the second part strengthens the identification.
For a merchandiser, this distinction is useful because it explains why two similar-looking fabrics may behave differently in washing, rubbing, stripping, bleaching, or reprocessing. For a lab technician, it provides a structured confirmation route. For a textile student, it shows that dye identification is not memorization of shade names, but interpretation of chemical behaviour.
The deeper lesson is this: a dye class is not defined only by colour. It is defined by how the colour is attached, how it can be removed, how it can be transferred, how it reacts with acids and alkalis, and whether it can be reduced, oxidized, restored, precipitated, or developed.
Final Thought
Part A gives the suspicion. Part B gives the confirmation. Together, they form a complete diagnostic journey. The tester begins with broad behaviour and then moves to sharper proof. A direct dye must show cotton affinity. A basic dye must show its characteristic extract reactions. A sulphur dye must reveal sulphur behaviour. A vat dye must show reversible reduction and oxidation. An azoic dye must reveal its developed pigment character. A pigment must first be freed from its binder logic. An oxidation black must show the chemistry of oxidation black. An ingrain dye must reveal its own special colour reactions.
In the simplest words: first observe the behaviour, then confirm the identity. That is the discipline of dye-class identification.
Safety note: The tests discussed in this article may involve hazardous chemicals such as strong acids, strong alkalis, reducing agents, oxidizing agents, organic solvents, phenol, methyl pyrrolidone, ethylenediamine, stannous chloride, lead acetate, sodium hypochlorite, and other laboratory reagents. These should be handled only by trained persons in a properly equipped laboratory.
Acknowledgement: This article is based on the confirmatory identification logic given in Annex B of IS 4472 Part 1:2021.
Goyal, P. Part B: Confirming the Dye Class on Cotton — The Second Diagnostic Journey. My Textile Notes. Available at: https://mytextilenotes.blogspot.com/2026/05/part-b-confirming-dye-class-on-cotton.html
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