Every fastness test in the ISO 105 series — washing, rubbing, perspiration, light — ends the same way: someone holds a grey scale next to a specimen and writes a number between 1 and 5.
That number is where most disputes start. The test itself was done correctly; the reading was not. The good news is that the reading has rules too, and most of them fit on one page.
Two scales for two questions
| ISO 105-A02 | ISO 105-A03 | |
|---|---|---|
| Question | How much did the specimen change colour? | How much colour did the adjacent fabric pick up? |
| Pairs | Grey on grey; the second chip gets lighter | White on white; the second chip gets darker |
| Typical use | Colour change after washing, light, perspiration | Staining of multifibre or rubbing cloth |
Using the change-in-colour scale to grade staining, or the reverse, is not a small slip. The steps are spaced differently.
What a "grade" actually is
Each step of the scale is a pair of chips with a specified colour difference in CIELAB units. The 9-step versions add the half grades.
| Grade | A02 colour change (CIELAB ΔE) | A03 staining (CIELAB ΔE) |
|---|---|---|
| 5 | 0 | 0 |
| 4–5 | 0.8 | 2.2 |
| 4 | 1.7 | 4.3 |
| 3–4 | 2.5 | 6.0 |
| 3 | 3.4 | 8.5 |
| 2–3 | 4.8 | 12.0 |
| 2 | 6.8 | 16.9 |
| 1–2 | 9.6 | 24.0 |
| 1 | 13.6 | 34.1 |
Values from ISO 105-A02:1993 and ISO 105-A03:2019; each step also carries a tolerance.
Two things follow. First, the steps roughly double — the scale is closer to how the eye perceives difference than a linear scale would be. Second, the staining scale is much wider: a staining grade 4 is a larger colour difference than a colour change grade 3. Comparing "4 for change" with "4 for staining" as if they were the same severity is a misunderstanding of the scales.
The viewing set-up is part of the method
Both standards describe the same viewing arrangement:
- Original and tested pieces side by side, in the same plane, oriented the same way, with the grey scale nearby in that plane.
- A neutral grey surround, roughly midway between grades 1 and 2 of the colour change scale — approximately Munsell N5.
- Light from north sky (south sky in the southern hemisphere) or an equivalent source of 600 lx or more, falling at about 45°, viewed roughly perpendicular to the surface.
- If the backing shows through, two or more layers of the original fabric under both pieces.
In practice, "equivalent source" means a light cabinet with a daylight simulator. A desk under ceiling fluorescents meets none of these points.
Masks: the clause most labs miss
ISO 105-A03:2019 adds a requirement that matters more than it looks: samples must be masked with material identical in colour to the sleeve used on the grey scale. If the scale sits in a neutral grey sleeve, mask the samples in the same grey; if it sits in a black sleeve, use the same black.
The colour around a sample changes how its contrast is perceived. A specimen viewed against a white bench and a grey scale viewed inside a grey sleeve are being judged by two different eyes.
Five-step or nine-step
With a 5-step scale, you give the grade whose contrast equals the specimen's; if the specimen sits nearer the imaginary midpoint between two pairs, you give the half grade. With a 9-step scale, you give the grade whose contrast is nearest.
In both cases, a 5 is given only when there is no perceived difference. "Almost nothing" is 4–5.
Habits that move the grade
- Grading too soon. Specimens graded warm or damp from the dryer look different from conditioned ones.
- Grading against the wrong original. Compare with the untested fabric from the same piece, not with a lab-dip or a swatch from another batch.
- Looking too long. The eye adapts. Make the judgement in a few seconds, look away, and confirm.
- One assessor, no check. Compare all pairs given the same grade at the end of a session. The standards recommend exactly this: pairs that do not look like the rest of their group are re-checked. Two assessors grading a shared set every week catches drift early.
- Old scales. The scales carry a colorimetric specification precisely so that working scales that may have changed can be compared against it. Dirty, scratched or faded scales should be replaced, and kept in their cover when not in use.
When to measure instead
ISO 105-A04 and ISO 105-A05 provide instrumental methods for staining and colour change. They remove assessor variation but bring their own requirements for instrument and procedure. Many labs use them to settle disputes and keep visual assessment for routine work. Either way, the report should say which was used.
Checklist
- Right scale: A02 for colour change, A03 for staining?
- Light cabinet, ≥ 600 lx, about 45° / 0°, grey N5 surround?
- Samples masked in the same colour as the scale's sleeve?
- Specimen compared with the correct original?
- Same-grade pairs cross-checked at the end?
TLI-TexLabInstruments supplies both grey scales and manufactures light cabinets for visual assessment in its Istanbul workshop.