Sweater Lab Testing: Colour Fastness, Shrinkage, Pilling and More

By YZR Sweaters · 2026-09

Lab testing is how a knitwear order stops being a matter of opinion. Instead of arguing about whether a sweater pilled too much or shifted in the wash, you point to a test report produced to a named method, with a result and a pass or fail against a limit you agreed in advance. The tests themselves are usually straightforward — the skill is knowing which ones to ask for, when to run them, and what to write into the contract. Acceptable limits vary by country and customer, so always confirm against your destination regulation and your own spec.

Colour fastness

Colour fastness measures how well a dyed yarn or garment resists transferring colour or fading. A sample is subjected to a specific condition alongside a multi-fibre strip, then any staining on the strip and any change in the sample's own shade are assessed against grey scales.

• Fastness to washing — shade change after laundering plus staining of adjacent fabric.
• Fastness to rubbing (crocking) — dry and wet rub tests, the latter catching deep or saturated shades.
• Fastness to perspiration — relevant for anything worn against the skin, often requested alongside wash fastness.

Knits are a particular challenge because the yarn sits on the surface and moves with the body. Deep navies, blacks and reds tend to attract the most scrutiny, especially where a garment may be worn against white.

Dimensional stability and shrinkage

Shrinkage testing measures how much a garment or swatch changes in length and width after a defined washing and drying procedure. Results are reported as a percentage change, with limits normally set separately for length and width.

Knits can also grow rather than shrink, particularly under their own weight in heavy wool or cotton. A garment that measures flat on the table can measure differently after a wash and a day of rest, so relaxation time belongs in the schedule.

Pilling and snagging

Pilling is the formation of small fibre balls on the surface, caused by loose fibres migrating and tangling during wear and abrasion. It is tested on a pilling tester that rubs samples against each other, or by a Martindale abrasion method, with results expressed on a grading scale rather than a percentage. Because fibre length, yarn twist, ply and knit density all influence pilling, the effective fix is upstream — combed or longer-staple yarn, higher twist, tighter structure.

Seam strength, slippage and stretch recovery

These tests target the places knitwear fails first: shoulder seams, neckbands, cuffs and hems. Seam strength measures the force needed to pull a seam apart; seam slippage checks whether yarns shift out of the seam under load; stretch and recovery tests assess how far a rib stretches and how completely it returns.

Fibre composition analysis

Composition testing identifies what is actually in the yarn, usually through microscopic examination combined with chemical dissolution. It matters because labels have legal weight: a mislabelled blend is a compliance problem in most destination markets, not just a commercial one. Testing the actual yarn lot before bulk production is cheaper than correcting labels or recalling stock later.

Restricted substances: formaldehyde and azo dyes

Restricted substance testing looks for chemicals that are limited or banned in textiles, with formaldehyde and certain azo colourants among the most commonly cited. European and North American buyers routinely request it, and compliance is generally documented through Oeko-Tex or equivalent certification rather than per-order testing. If you buy certified yarn and trims, the chemical risk shifts to dyes, washes and any finishing applied after knitting.

How testing is actually run

Testing is usually carried out by an independent lab rather than the factory itself: SGS, Bureau Veritas and Intertek are the names most buyers work with, and an accredited lab issues the report that carries weight in a dispute.

• Timing: test bulk yarn or pre-production samples, so failures can still be corrected before knitting starts.
• Sampling: test the colourways and materials most at risk rather than every style equally.
• Ownership: decide who pays and who holds the report — buyers usually want it addressed to them.

Writing testing into the contract

A test only protects you if the standard and the limit are agreed before production. Name the test, the method, the acceptable limit, who commissions and pays for it, and what happens on failure — rework, replacement or a claim. Vague wording such as "good colour fastness" tends to be unenforceable when it matters.

Common failures and how to prevent them

Most failures are cheap to prevent and expensive to fix. Wash fastness problems usually trace back to insufficient rinsing or dye selection, and correcting the recipe early costs far less than re-dyeing garments. Shrinkage over limit often follows from rushing relaxation after washing. Pilling and composition complaints belong at yarn selection and approval, not at final inspection.

Testing standards vary between countries, retailers and product categories, so treat this as a starting framework and confirm the exact requirements with your customer before bulk production.