Why naturally dyed fabric deserves a proper test

Natural dyes behave nothing like their synthetic cousins. Madder, weld, indigo, onion skin and avocado all bond to fibre through mordants rather than the chemical lock that industrial dyes rely on, which means colour can shift, soften or bleed over time. That isn't a fault — it's the character of the craft. But if you've spent an afternoon dyeing a length of linen, or you're about to sew wool into a jumper you expect to wear for years, you want to know how that colour will hold up before you commit to the scissors.

Two simple tests — light and wash — will tell you most of what you need to know. Neither requires specialist equipment, and both can be done in a kitchen or on a windowsill.

Begin with a control swatch

Before you test anything, make yourself a benchmark. From every dye bath, keep a small offcut — roughly 15cm square — and label it with masking tape or a fabric pen: date, fibre, mordant, dyestuff and any afterbath. Cut that swatch into three pieces. One goes into a paper envelope in a dark drawer as your reference. The other two are your test subjects.

Store the reference somewhere genuinely dark. A shoebox in a wardrobe is ideal. Light damage is cumulative and invisible until you compare, so that untouched swatch is the only honest measure you'll have.

The light test: how to spot fading

Pin one test swatch to a south-facing windowsill, or peg it outside on a dry, bright day. The trick is to expose only half of it. Cut a piece of card or kitchen foil slightly wider than the swatch and pin it over the top half, so the lower half takes the sunlight and the upper half stays shaded. This gives you a built-in comparison without needing a colour chart.

Check the swatch at 24 hours, then at three days, then again after two weeks. Around 100 hours of direct British summer sun is a reasonable stand-in for a full season of indirect daylight. Look for three things: overall dulling, a change in hue (indigo and woad often drift grey; yellows from weld can turn slightly brown), and any difference between the covered and uncovered halves.

The wash test: checking bleed and shift

Fill a bowl with lukewarm water — around 30°C to 40°C — and add a small amount of pH-neutral soap flakes or a gentle liquid detergent. Drop in your second test swatch along with a piece of undyed white cotton of similar size. That white cloth is your witness: if dye transfers to it, you have a bleeding problem.

Agitate gently for five minutes, then leave to soak for thirty. Rinse in water of the same temperature — sudden temperature changes can shock wool especially — and dry flat in the shade. Repeat the whole process three times, noting the colour of the water each round. Three washes mimics a few wears and a couple of gentle laundry cycles, which is a fair test for anything worn against the skin.

Reading your results

Always compare your test swatch against the reference in natural daylight, never under a bulb — artificial light flatters colour and hides shifts. A rough grading is enough:

• No change after light exposure and three washes: excellent. Suitable for garments, upholstery and anything washed regularly.
• Slight change: good. Fine for clothing washed gently and infrequently.
• Noticeable change: better suited to cushions, bags, wall hangings or quilted pieces that rarely meet water.
• Major change: keep it away from daily use. Consider re-mordanting or overdyeing the whole batch, or reserve it for decorative work.

Do a rub test too, while you're at it. Take a dry white cloth and rub the dyed fabric firmly for thirty seconds. Colour on the cloth means the dye is sitting on the surface rather than bonded into the fibre — common with linen, which has a smooth, waxy surface that resists deep penetration.

Fibre matters: linen, wool and cotton behave differently

Linen takes dye beautifully with an alum mordant, but its smooth cellulose surface means more dye sits near the surface, so it can crock and fade with friction. It tolerates warmer washing than wool, but alkaline soap and hot water will shift madder and cochineal quickly.

Wool is a protein fibre and bonds readily with mordants, which often makes it the most lightfast of the three. It also felts with heat and agitation, so wash it cool and still, and never wring it.

Cotton is cellulose like linen but denser and thirstier. It often benefits from a tannin pre-treatment — gallnut or sumac — before mordanting, otherwise wash fastness can be disappointing. Expect cotton to need the gentlest handling and the most patience.

Improving fastness next time

If your results weren't what you hoped, the fix usually lies upstream. Use enough mordant — 10 to 20 per cent of the fibre weight for alum is a sensible starting point — and give the dyed fabric three or four days to cure before it meets water. Rinse gradually, cooling the water down in stages rather than plunging hot fabric into cold. A pH-neutral soap preserves colour far better than anything alkaline, and drying in the shade rather than full sun will slow fading dramatically.

Slow, shifting colour is part of what makes naturally dyed cloth feel alive. Testing simply means you choose where that change happens, rather than discovering it on a favourite cushion two summers from now.