Key Fabric Tests Explained: GSM, Strength, Colour Fastness, Shrinkage, Pilling and More

Why tests exist, and how to read one

Two fabrics can look and feel the same on the day you see them and behave very differently after fifty washes. A test is a controlled way of asking the cloth a question, such as "how much does it shrink?", and getting an answer as a number or a grade instead of an opinion.

Every test follows a written method, called a standard, that fixes the sample size, the machine, the temperature and how the result is scored. Standards come in families: ISO is international, AATCC and ASTM come from the United States, and IS is the Indian family from the Bureau of Indian Standards. A result only means something next to the method it was run to, so a good test report always names it. Many results are given as grades from 1 to 5, where 5 is best, judged against a grey scale (a card of grey pairs showing set amounts of colour change).

This lesson explains what each common test measures. For the checks we run on our own batches, read how we test our fabric. At Benny Cotts we weave in-house at Village Atoon in Bhilwara; dyeing, processing, finishing and testing happen through partnered Bhilwara process houses and the labs they work with.

GSM: how heavy the cloth is

GSM means grams per square metre. A lab cuts a sample of known area, often a circle cut with a fixed-size cutter, weighs it on a precise balance and scales the figure up to one square metre. ISO 3801 is a common method for woven fabric weight.

A uniform buyer cares because GSM shows how thick, warm and opaque the cloth feels, and because it is the easiest way to spot a cheaper substitute. It does not on its own prove strength or quality. The lessons on what GSM means for uniform fabric and summer vs winter weights go further.

Tensile and tear strength: how hard it is to break

Tensile strength is the pulling force a strip of fabric takes before it breaks. A machine grips both ends of the strip and pulls them apart slowly, recording the peak force, usually in newtons. It is tested separately along the warp (the lengthwise yarns) and the weft (the crosswise yarns), because the two are rarely equal. ISO 13934 covers this for woven fabric, and ASTM D5034 is a common grab method in American specs.

Tear strength is different. It measures the force needed to keep a small cut or snag growing, which is how real garments usually fail: a pocket corner catches on a door handle and rips. ISO 13937 is the ISO family for tear tests. Buyers of workwear, police and security uniforms, and school trousers care most, because these garments get snagged and pulled. A finish that softens the cloth can lower tear strength, so it is worth checking after finishing, not just on the grey cloth.

Colour fastness: washing, rubbing, light and sweat

Colour fastness asks whether the shade stays put. Each test gives two kinds of grade: change in the colour of the fabric itself, and staining, meaning how much colour moved onto a white test cloth placed next to it. The ISO 105 series is the main international family, with AATCC methods common for American buyers.

• Washing (for example ISO 105-C06): the sample is washed with steel balls and detergent at a set temperature, then graded for fading and staining. This is the one institutional laundries care about most.
• Rubbing, also called crocking (ISO 105-X12): a white cloth is rubbed on the fabric, dry and then wet, to see how much colour comes off. Dark navy and black trousers that mark a white shirt or a car seat fail here.
• Light (ISO 105-B02): the sample sits under a lamp that imitates daylight, next to blue wool references that fade at known rates, and is graded on a 1 to 8 scale. Guards, ground staff and anyone in the sun need this.
• Perspiration (ISO 105-E04): the sample is soaked in artificial sweat, pressed and dried, then graded. Collars, underarms and backs of shirts are where this shows.

For why shades fade in real use and how fastness ties into dye choice, read why uniform fabrics fade.

Dimensional stability: will it shrink?

Dimensional stability is the formal name for shrinkage, or sometimes growth, after washing or steam. The lab marks a square of known size on the fabric, washes and dries it by a set procedure, measures again and reports the change as a percentage, separately for length and width. ISO 6330 sets the washing and drying procedures, and AATCC 135 is the common American method.

A uniform buyer cares because a garment cut to size has to stay that size. A few percent of shrinkage in length decides whether trousers fit or end above the shoe. The full story of why cloth shrinks, and how processing controls it, is in fabric shrinkage explained.

Pilling and abrasion: how it wears

Pilling is the small balls of tangled fiber that form on a fabric surface with rubbing. In a pilling test, samples are rubbed or tumbled in a machine for a set number of cycles and then graded from 1 to 5 against photographs. ISO 12945 is the ISO family, with the Martindale and pill box methods the most common. We cover why pills form, and how to write a pilling line in your spec, in fabric pilling explained.

Abrasion is wear by rubbing until the cloth thins or a hole appears. The Martindale tester rubs a sample against a standard wool cloth in a moving figure pattern, and the result is the number of rubs before breakdown. ISO 12947 is the usual method. Knees, seats, cuffs and pocket edges are where a uniform wears through, so this test matters most for workwear and school wear.

Seam slippage: will the seam pull open?

Seam slippage happens when the stitches hold but the yarns of the fabric slide along each other next to the seam, leaving a gap or a row of open holes. It shows up on loose or very smooth weaves, and on cloth with slippery filament yarns, especially at back seams of trousers and armholes of shirts.

The lab sews a standard seam into a strip of fabric, pulls it in a strength tester and measures the force needed to open the seam by a set gap. ISO 13936 is the ISO family for woven fabric. A buyer cares because a seam that gapes looks like a sewing fault, but the cause is in the fabric, so it is worth testing before a large garment order.

What this means when you buy

• Ask for results with the method named next to each figure, not just "passed".
• Pick the tests that match the use: light fastness for outdoor staff, wash fastness and shrinkage for laundered uniforms, tear and abrasion for workwear.
• Agree the minimum grade or limit in writing before bulk, as part of the spec. The lesson on reading a fabric spec sheet shows where these lines sit.
• Test the finished cloth, not the grey cloth, because dyeing and finishing change most of these results.
• Use the same tests to check what arrives; inspecting uniform fabric on delivery explains how.

When you want finished fabric in hand to compare, Request Catalog from our quote page.

FAQ

Frequently asked questions

What does a colour fastness grade of 4 mean?

Most fastness results are graded from 1 to 5 against a grey scale, where 5 means no visible change and 1 means a large change. Light fastness is the exception and uses a 1 to 8 scale. A 4 means only a slight change, which many uniform specs treat as a sound level for washing.

Is GSM a measure of fabric strength?

No. GSM is weight per square metre. Heavier cloth is often stronger, but fiber, yarn and weave matter as much, so strength is checked with its own tensile and tear tests.

Are ISO and AATCC results the same?

Not always. They test the same properties but with different procedures, temperatures and sample sizes, so a result is only comparable to another run to the same method. Agree on one family in your spec.

Does Benny Cotts have its own testing lab?

No. We weave in-house at Village Atoon in Bhilwara. Dyeing, processing, finishing and testing are done through partnered Bhilwara process houses. To see finished fabric, Request Catalog at /quote.