Colour Fastness, Testing and Quality Control
Fastness is the resistance of a dyed material to an agency such as washing, light, rubbing or perspiration. The exam tests the scales, the standard test conditions and the diagnosis of a failure in the dyehouse.
Scales and standards
- Grey scale for change in shade - grades 1 to 5, where 5 means no change.
- Grey scale for staining - grades 1 to 5, assessed against adjacent undyed fabric.
- Blue wool scale - grades 1 to 8 for light fastness; 8 is best. A result is written as, for example, change 4–5, staining 4.
- Tests follow ISO 105 or AATCC methods; wash fastness uses a specified temperature, time, detergent and steel balls, with a multifibre adjacent strip.
Common tests
Washing, rubbing (crocking, dry and wet, ten cycles on a crockmeter), light (xenon arc), perspiration (acid and alkaline), water, sea water, bleaching, hot pressing and sublimation for disperse dyes. Quality control also covers shade consistency, pH of the dyed goods, residual unfixed dye, and strength loss.
Worked example
A reactive black on cotton scores wet crocking grade 3 and stains the adjacent fabric. Diagnosis: unfixed hydrolysed dye is sitting on the fibre surface. The cause is inadequate soaping after dyeing and possibly insufficient soda ash. Remedy: hot soaping at 90 °C with detergent until the wash liquor runs clear, correct alkali dose, and re-test. The change-in-shade grade may already be 4–5, which shows that staining and shade change measure different things.
Traps
- Swapping the change and staining scales.
- Thinking blue wool grade 1 is best - grade 8 is best.
- Assuming a good wash fastness guarantees good light fastness; different mechanisms are involved.
- Quoting a fastness result without the test method or condition.