Dyeing Eighteen Shirts Navy in Forty-Eight Hours

The pattern of the fault tells you where to look: it is a penetration and exhaustion problem, not simply a weak dye recipe.

• Double-layer seams, collars and cuffs are thicker and denser. Liquor circulates less freely through them, so the dye solution reaches the inside later, and the alkali may reach it unevenly. The result is a lighter seam line and patchy thick areas.
• Softener finish on some shirts is hydrophobic. It resists wetting, so the dye cannot enter those panels evenly. A pre-scour is essential.
• Hard water at 640 mg/L ties up dye and alkali and can cause precipitation, reducing effective concentration and levelness.
• Chemical addition too fast is a classic cause of patchiness. If salt or soda ash is dumped in, the dye fixes before it has migrated.
• Loading and circulation: 18 shirts weigh 6.3 kg, which is within the 10 kg limit, but bulky cotton can pack the drum and reduce flow.

Preventive measures: pre-wash or scour all shirts to remove softener; wet out uniformly; add a leveling agent; use a sequestrant for hardness; add salt in two portions; add soda ash in three slow portions over 15–20 minutes; keep the temperature controlled and the drum turning; run 10–15 minutes before sampling. Document each addition time.

Calculations, assuming 3% owf dye, 60 g/L salt, 15 g/L soda ash and 1 g/L leveling agent:

• Total weight: 18 × 0.35 kg = 6.3 kg.
• Liquor at 1:10: 6.3 × 10 = 63 L.
• Dye: 3% of 6.3 kg = 189 g.
• Salt: 60 g/L × 63 L = 3,780 g.
• Soda ash: 15 g/L × 63 L = 945 g.
• Leveling agent: 1 g/L × 63 L = 63 g.
• Sequestrant for hardness: follow the supplier dose, typically 1–2 g/L.

Procedure:

1. Fill at 30 °C, add sequestrant and leveling agent, then add predissolved dye. Run 10 minutes.
2. Add half the salt, run 10 minutes; add the rest, run 10 minutes.
3. Add one-third of the soda ash, run 10 minutes; repeat twice at 10-minute intervals.
4. Raise to 40 °C if the cold-brand recommendation allows, and fix for 45–60 minutes.
5. Drain; rinse; soap at 60–80 °C with a non-ionic detergent to remove unfixed dye; rinse; sour; dry.

Assumptions and warnings: the dye strength and shade must be confirmed on a swatch; the plastic buttons may be alkali-sensitive, so check them or remove them; polyester thread will stay white because reactive dye does not bond to it; hard water may require extra sequestrant and a slightly longer fixation. Do not exceed the drum limit, and do not shorten the fixation time to meet the deadline.

Match on a swatch, disclose the thread contrast, and correct a single light shirt by re-dyeing it, not by spot-painting.

• Shade matching: cut a 20 g swatch from a spare shirt or an inside facing and dye it with the same recipe and water. Dry it before comparing, because wet fabric looks darker. Compare with the approved sample under the light box in daylight conditions, not under a coloured tube light. Adjust the dye percentage in small steps and record the final recipe. Do not judge the batch from the first wet shirt.
• White polyester thread and buttons: reactive dye will not colour polyester thread, so the seams will show a white stitch line. This is a fibre limitation, not a fault in the dyeing, and the school should be told in writing before the job starts. Buttons should be checked for alkali damage or removed and resewn. Offer a matching thread as an optional extra rather than promising an invisible seam.
• One shirt lighter: first check whether it is lighter because of poor wetting, softener or a different fabric. Re-dye that shirt alone in a fresh bath at a lower dye concentration, typically 0.5–1% owf, with leveling agent and slow alkali addition. Do not overdye it at full strength, which will make it darker than the batch. If the difference is minor, keep it as a spare rather than risking a mismatch.
• Record everything: weights, dye lot, water hardness, temperatures, addition times and the approved swatch. With only a light box and no spectrophotometer, the written recipe is your only quality control.