Rubbing fastness is a ten-second test. A weighted finger travels back and forth over the specimen, a white cloth picks up whatever transfers, and the staining is graded against a grey scale.
It is also the test that generates the most "your lab says 4, ours says 2-3" emails. Almost never because of the crockmeter.
1. The rubbing cloth is not conditioned
The white rubbing cloth is a specified, standardised cotton fabric. It is also a textile, and it absorbs moisture from the room like any other.
A dry cloth and a conditioned cloth do not pick up dye the same way. Condition the rubbing cloths in the standard atmosphere along with the specimens — 20 ± 2 °C, 65 ± 4 % RH — and store them there, not in a drawer next to the instrument.
This single point resolves a surprising share of disputes.
2. Wet pickup is guessed instead of weighed
For the wet test, the rubbing cloth must carry a defined amount of water. ISO 105-X12 specifies a pickup of 95–100 % of the cloth's dry mass.
"Damp" is not a specification. Weigh the dry cloth, wet it, squeeze or blot, weigh again, and adjust until you are in range. An operator who wets by eye will run at anywhere from 60 % to 150 %, and the staining grade moves with it — more water generally means more transfer.
Keep a small balance at the crockmeter bench. If the cloth has to walk to a balance across the lab, it will not be weighed.
3. The cloth is reused
Each rub uses a fresh piece of cloth. It is tempting to turn a cloth over, or to use a clean corner, when a batch is large and the cloths are on order.
Do not. Dye already on the cloth changes both the friction and the visual assessment. A reused cloth reads better than it should, in the direction that hides a problem.
4. The specimen direction is not recorded
Rubbing fastness is direction-dependent. Warp and weft give different results on the same fabric, and on a printed fabric the result depends on which part of the pattern sits under the finger.
Test both directions. On prints, test the darkest area — that is where a complaint will come from — and say on the report which area you chose.
5. Assessment lighting is casual
Staining is graded against the grey scale for staining, ISO 105-A03, by eye. The eye needs a controlled light source.
Grade in a light cabinet under a specified illuminant, not at a bench under whatever tubes the ceiling has. Fluorescent office lighting shifts perceived contrast, and half a grade is easy to lose or gain — which is exactly the margin that decides a pass.
Two graders in the same lab should also cross-check periodically. Grey scale assessment drifts with the person.
Linear or rotary?
Two instrument types exist and they are not interchangeable.
| Linear crockmeter | Rotary crockmeter | |
|---|---|---|
| Standard | ISO 105-X12, AATCC 8 | ISO 105-X16, AATCC 116 |
| Motion | Straight back-and-forth | Rotating finger |
| Typical use | Woven and knitted fabrics | Prints and small-pattern fabrics |
The rotary version exists because a straight rub across a small printed motif does not represent the whole pattern. If your specification names X16, a linear instrument will not produce a valid result no matter how carefully it is run.
The checklist
- Were rubbing cloths conditioned with the specimens?
- Was wet pickup weighed into the 95–100 % range?
- Was a fresh cloth used for every rub?
- Are warp and weft — or the correct print area — recorded?
- Was grading done in a light cabinet against ISO 105-A03?
A laboratory that can answer all five reproduces its own results week to week. That is the real test of a fastness bench.