Why Onion Skins Are Worth Saving

Every kitchen in Britain throws away a free dyestuff several times a week. The papery brown skins you peel from a bag of onions are packed with quercetin, a flavonoid that bonds readily to protein fibres and produces some of the warmest, most reliable golds and browns you can get without buying a single thing. It is the perfect first dye for anyone keeping a few sheep, a few Angoras, or simply a basket of fleece waiting to be spun.

What makes onion skins so forgiving is that they need no exotic mordant, no precise weighing, and no expensive equipment. A dedicated stainless steel or enamelled pot from a car boot sale, a wooden spoon, and a bit of patience will do the whole job. The colours sit beautifully alongside the natural greys and moorit browns of native breeds, and they deepen rather than fade with a second dip.

Saving and Storing Skins Through the Year

Start a paper bag in the airing cupboard and drop the dry outer skins into it as you cook. Only the crisp, papery layers are worth keeping — the damp fleshy pieces underneath will go mouldy and turn your dye bath sour. Give the skins a week or two in the warm cupboard to dry out completely before tipping them into a Kilner jar or a cotton pillowcase, and they will keep for years.

Yellow and brown onions give the classic golden brown. Red onion skins push towards a warmer, coppery tan, so keep the two sorts in separate jars if you want to control the outcome. As a rough guide, allow 100g of dry skins for every 100g of dry wool. For a really deep, rich brown, double that. It sounds a lot until you realise a busy household generates a carrier bag full in a month or two.

Scouring and Mordanting the Wool

Dye will not take evenly on greasy fleece. Scour first: soak the wool in hand-hot water with a squirt of washing-up liquid or a tablespoon of soda crystals per 5 litres, then rinse gently in water of the same temperature. Sudden changes in temperature, or any rubbing while the wool is hot, will felt it.

Next, mordant. Alum is the standard choice and gives the clearest gold. Use 8% alum (potassium aluminium sulphate) and 7% cream of tartar by weight of dry fibre — so 8g of alum and 7g of cream of tartar for 100g of wool. Dissolve both in a little hot water, top up the pot with enough lukewarm water to let the wool move freely, add the wool, and bring it slowly to around 80°C. Hold it there for an hour without boiling, then let it cool in the pot, ideally overnight. Soft water or rainwater from a butt gives noticeably brighter results than hard tap water.

Making the Dye Bath and Dyeing Gently

Put your onion skins in the pot with enough water to cover them generously — roughly 10 to 15 times their weight. Bring to a simmer and hold it there for an hour. The liquid should turn a deep amber, like strong tea left too long. Strain the skins out and compost them.

Now add your wetted, mordanted wool. It should be able to move without crowding. Raise the heat very slowly to 80–85°C, just below a simmer, and hold for 45 to 60 minutes, stirring only occasionally and with the back of a spoon rather than a jabbing motion. Turn the heat off and leave the wool in the cooling bath overnight. This slow cooling is where the depth of colour really develops, and it costs you nothing but time. A second batch of wool dyed in the exhausted bath will come out a paler, softer gold, which is lovely for contrast in a striped knit.

Modifying the Colour with Vinegar and Iron

This is where a single dye pot becomes three. Split your dyed skeins and dip them in different after-baths for twenty minutes.

• Vinegar — an acid modifier. Add 2 to 3 tablespoons per litre of warm water. It brightens and clears the gold, pushing it towards a sunny ochre.
• Iron — the saddener. Use ferrous sulphate at 1 to 2% by weight of fibre, dissolved in warm water, and dip briefly. It shifts the colour to a deep olive-brown or a soft grey-green. Iron can make wool brittle if overdone, so keep it weak, rinse thoroughly, and use a separate pot you never cook in.
• A rusty nail brew — if you would rather not buy chemicals, drop a handful of old nails into a jar of water and vinegar and leave it for a fortnight. Dilute the rusty liquid until it looks like weak tea before dipping anything.

Even a brief dip produces a visible shift, so pull the skein out and check it against a white cloth before committing.

Rinsing, Drying and Keeping the Colour

Rinse in water of the same temperature as the bath, gradually cooling, until it runs clear. Squeeze — never wring — the water out, roll the skein in a towel, and hang it in the shade to dry. Onion skin dyes are reasonably lightfast, but strong summer sun will mellow the paler golds over a season or two, so store finished wool in a cupboard or a cotton bag away from a south-facing window.

Write the recipe on a luggage tag and tie it to the skein: date, weight of skins, mordant, and modifier. That way, when a particular golden brown turns out to be exactly what you want for a gansey or a pair of mittens, you can make the same colour again next autumn.