How colour fastness is tested by rubbing

Short answer: A white standard fabric is rubbed against the sample, dry and wet, at a fixed pressure. Afterwards, the amount of colour transferred to the white is judged. The result is two numbers, one for dry and one for wet rubbing.

A white fabric as the yardstick

The white standard fabric is the yardstick. It is rubbed against the sample, and afterwards you check whether colour has come onto it. Because the white fabric is the same from test to test, colour transfer can be compared between samples and laboratories. What is measured is colour release, not the fabric's own colour.

Dry and wet give two numbers

Colour transfers more easily when the fabric is damp, because water loosens what sits on the surface. So both cases are tested, and both numbers are noted in the report. A fabric can pass the dry test well and still release more colour when wet. Only the two numbers together give a true picture.

The pressure is fixed

The rubbing uses a fixed pressure and a fixed number of strokes, so the result does not depend on who performs it. Rub harder and more colour transfers, and then the number would say more about the pressure than the fabric. With the pressure fixed, the colour in the sample decides the number.

The scale runs from one to five

Five means no colour is seen on the white fabric. One means heavy transfer. The numbers between are steps, judged by comparing the white fabric with a reference scale. See Bamboo Basics, and read the care label so you know how each item is meant to be washed.

Dark colours are hardest

Black, navy and strong red contain the most dye and so carry the greatest risk of transferring onto light clothing. The more colour sits in the fibres, the more can come loose. See JBS and Urban Quest, and wash dark pieces separately, especially the first few times.

Why new clothes bleed

New clothes can bleed because excess colour still sits loosely on the fibre surface after dyeing. That is not necessarily a sign of poor quality, but it is a sign that the piece should be washed separately the first time. Rinse or wash it alone until the water runs clear.

Does it go away

Usually it goes after a few washes, if the dyeing is otherwise done properly. The loose colour rinses out, and what is bound in the fibre stays. If it keeps bleeding after many washes, that suggests the colour is not sitting properly, and then you should contact the shop.

What is an acceptable number

Four or above counts as acceptable for dry, and slightly lower is accepted for wet. Wet is stricter because moisture loosens more colour. If the wet number is a little lower, that is not in itself a fault, but it is a reason to wash the piece separately.

How to avoid bleeding at home

Wash new and dark pieces separately, use a temperature that fits the care label and do not leave wet pieces lying together in a heap. Take the laundry out of the machine promptly and dry it on a rack or hang it up. That gives loose colour less chance of settling on something light.