What a perspiration test on fabric involves
Short answer: A sweat test shows whether the colour in a fabric holds when the fabric gets damp from perspiration. The sample is placed against white fabrics in a liquid that resembles sweat, pressed together and kept warm. Afterwards you can see how much colour has moved onto the white fabric.
Why the test exists
Underwear and other clothes that sit close to the skin get damp with sweat during an ordinary day. If the dye is not fixed well in the fibres, it can transfer to skin or to other clothes in the wash. That is why a standard method tests this in a laboratory before the clothes reach the shops.
The liquid imitates sweat
The test uses artificial sweat, a liquid made to resemble what the body produces. Its composition is fixed in advance, so every laboratory works with the same solution. That makes it possible to compare results from different samples and different suppliers, instead of each place having its own idea of what sweat is.
Why two different liquids
Both a slightly acidic and a slightly alkaline liquid are used, because sweat is not the same for everyone. Some people have more acidic skin, others more alkaline, and it can change with the day, the heat and what you are doing. Some dyes cope well with one liquid and worse with the other.
The sample meets white fabrics
The fabric sample is stitched or placed together with a white companion fabric that is meant to catch any colour that comes loose. Both are soaked in the liquid. The white fabric acts as a witness: if it stays white, the dye holds well, and if it picks up a tint, some colour has moved.
Pressure and warmth speed things up
Next the sample is pressed between two plates and kept warm for several hours. Pressure and warmth imitate clothes sitting snugly against a warm body, and they make the process faster, so nobody waits for months. The method stands in for long wear in a short time. For underwear, see Boody.
The result is two things
After storage the sample is dried and two things are judged separately. One is whether the sample's own colour has changed. The other is how much colour has passed onto the white companion fabric. A fabric can keep its colour and still stain others, or the reverse. See also JBS of Denmark and Decoy.
How the result is read
The judgement is usually made by comparing against a fixed grey scale, looking at how large the difference is between before and after. The smaller the difference, the better the sweat fastness. It is an assessment by eye under the same light, so it matters as much as the liquid and heat, because conditions must match every time.
What poor sweat fastness means
Poor sweat fastness does not necessarily mean the colour fades on the garment itself. Most often it means the dye can rub off when the fabric is wet and warm. You notice it mostly on light items lying close to dark ones, such as a white vest under a dark shirt on a warm day.
Frequently asked questions
Why is a sweat test important for underwear
Because underwear lies directly against the skin and gets damp with sweat all day. If the dye is not fast to sweat, it can transfer to skin or to the clothes worn over it, which is something you want to avoid.
Does poor sweat fastness mean the colour fades
Not necessarily. It means the colour can transfer to other clothes when the fabric is damp and warm. Whether the colour on the garment itself fades is measured by other tests, such as light fastness and wash fastness.
Can you see it yourself
Yes, if a light shirt gets a faint tint of colour under the arm after a warm day, it may come from poor sweat fastness in the clothes worn beneath. It is not a measurement, only a sign.
You can find the full range at Bamboo-Clothing.com.
