Bleeding Bandhani Silk Scarves
A women's cooperative is making 25 silk scarves in a bandhani design. The scarves are tied with cotton thread, wetted, and dyed with direct dyes at 80°C for 45 minutes with 10% owf sodium chloride. After dyeing they are rinsed in running water and dried in the sun; no aftertreatment is given. The first customer wash at 40°C with household detergent causes the red and green to bleed badly, the white ground to become stained, and the tied dots to lose their sharp edges. The cooperative has acid dyes, reactive dyes, acetic acid, sodium bicarbonate and a steamer available. The buyer requires wash fastness of at least 4 and rubbing fastness of 3–4, and wants the tie marks to remain crisp. The scarves are sold as handloom gift items, so brightness and handle also matter.
1. Select a dye class for the next batch and justify the choice against the buyer's fastness and handle requirements. Explain why the direct dyes failed.
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Choice: reactive dyes, with acid dyes as a fallback
Direct dyes have substantivity for silk but no covalent bond; they sit in the fibre and migrate out in a warm detergent wash. That is why the red and green bled, stained the white ground and gave poor wash fastness. Acid dyes give bright shades and a soft handle, but standard acid dyes also need backtanning (tannic acid/tartar emetic) to reach wash fastness 4; backtanning can alter shade and make the handle harsher.
Reactive dyes form covalent bonds with silk's amino and hydroxyl groups. A mild alkaline fixation (sodium bicarbonate, pH about 8–8.5) at 60–70°C gives good wash fastness without the severe silk damage that strong alkali and high temperature can cause. Reactive dyes also leave the handle relatively soft if washed properly. For a gift scarf that must combine brightness, handle and fastness 4, a low-alkali reactive system is the best first choice; acid dyes with backtanning are the fallback if the reactive shade is not available.
2. Outline the revised bandhani dyeing and aftertreatment process, including pH, temperature, electrolyte and handling precautions to keep the tie marks crisp.
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Revised process
- Keep the tie threads tight and uniform; the tightness controls the crispness of the dots. Wet out the silk in water with a little nonionic wetting agent.
- Dye with reactive dye 2–3% owf, sodium sulphate 30–50 g/L and sodium bicarbonate 5–8 g/L at pH 8.0–8.5. Enter at 40°C, raise to 60–70°C at 1°C/min, add bicarbonate in two portions, and hold for 45–60 minutes.
- Keep agitation gentle; do not stir vigorously, or dye will creep under the ties and blur the dots.
- Rinse in warm water, then soap at 60°C with 1 g/L nonionic detergent for 10–15 minutes to remove unfixed dye; rinse and dry in shade, not strong sun.
- If acid dyes are used instead: pH 4.5 with acetic acid, 5–10% Glauber's salt, 60–80°C, then backtan with tannic acid and tartar emetic, and test the shade before bulk.
Reasoning: mild alkali fixes the reactive dye without damaging silk; gentle handling preserves the bandhani pattern; soaping removes loose dye that would otherwise stain the white ground.
3. State the fastness tests you would use to verify the claim, and explain what a result of 3–4 on the grey scale would mean.
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Verification
- Wash fastness: ISO 105-C06 (or the equivalent IS method) at 40°C, with colour change and staining assessed on the grey scale.
- Rubbing fastness: ISO 105-X12, dry and wet, again on the grey scale.
- Also check staining of the white ground after a simulated home wash, since that is the customer's actual complaint.
A result of 3–4 on the grey scale is 'slight change/staining' - better than the failed direct-dyed scarves but still below the buyer's requirement of 4. It would indicate that fixation or soaping is incomplete, or that the dye dose/alkali needs adjustment, and the batch should not be shipped as passing.