Why Onion Skins Deserve a Place in Your Dye Pot

Every kitchen in Britain throws away a dye stuff several times a week. Brown onion skins, the papery outer layers you peel off and bin, carry a surprising amount of colour: soft butter yellows, clear golds, warm apricot oranges and, with a little encouragement, deep copper-browns. They are free, endlessly available, and need no specialist equipment beyond a stainless steel or enamel pan you keep for dyeing only.

The colours they give are warm and forgiving. They sit beautifully alongside madder, weld and walnut, and they are an ideal first project if you have never dyed with plants before. Because the dye is already fixed in the dried skin, you can store it for months and use it whenever the fancy takes you. Onion skins also produce reliable results on wool, silk and cotton, provided you match your mordant to the fibre.

Collecting and Storing Your Skins

The colour you get depends on which onions you peel. The golden-brown skins of standard brown onions give yellows, golds and tans. Red onion skins, those deep purple-pink papers, give softer greens and greyish pinks on their own, and produce lovely olive tones with an iron modifier. Shallot and banana shallot skins behave much like brown onion and often carry a slightly deeper orange.

As a rough guide:

• Skins from about 15 brown onions, roughly 40 to 50g dried, will dye 100g of wool a clear, mid-tone gold.
• Equal weights of skins and fibre give a rich, saturated amber or russet.
• Half the weight of skins to fibre gives delicate primrose and pale butter yellow.

Spread the skins somewhere airy for a day or two until bone dry, then store them in a paper bag or cotton sack. Keep them away from damp and from direct sunlight, which fades the pigments. Label the bag with the date and the onion type. If you are dyeing a large project, freeze the skins in a bag until you have enough; they keep perfectly and need no thawing before use.

Preparing the Fibre

Clean fibre takes dye evenly. Scour wool in hot water, around 60°C, with a little pH-neutral washing-up liquid or soap flakes, and rinse until the water runs clear. Never plunge wool from hot to cold water; the fibres will felt. Silk needs a gentler hand: lukewarm water and a mild soap, with no rubbing.

Cotton is the awkward one. Its cellulose fibres hold far less dye than protein fibres, so scour it thoroughly, simmering for an hour with a teaspoon of soda ash per litre if the fabric is sturdy. Give cotton a tannin pre-treatment before mordanting: steep it overnight in a strong brew of oak gall, sumac or even used black tea, then mordant as normal. This one step can double the depth of colour you achieve.

Mordants and What They Do

Onion skins contain flavonoids that need a metal salt to bond with the fibre. Alum, or potassium aluminium sulphate, is the safest and most reliable choice, and it keeps colours bright and clear.

• Wool: 10 to 15 per cent alum and 5 to 8 per cent cream of tartar by weight of dry fibre (WOF). The cream of tartar brightens the yellow and softens the wool.
• Silk: 10 per cent alum, no cream of tartar, and keep the bath below 85°C.
• Cotton: 15 to 20 per cent alum after a tannin pre-treatment.
• Iron: 2 to 3 per cent ferrous sulphate as a modifier, not a main mordant. It saddens golds into olive, bronze and deep brown.
• Copper: 2 to 3 per cent copper sulphate pushes colours towards a richer, greener gold.

Dissolve the mordant in hot water, add the wetted fibre, and hold at 80 to 85°C for an hour. Let it cool in the bath overnight, then squeeze gently. Use separate pans for iron and copper, and never use them for food afterwards.

The Dye Bath

Cover the skins with cold water in your dye pan and bring them slowly to a simmer. Cook gently for 45 minutes to an hour, until the water is a deep amber and the skins look pale and spent. Strain the liquid into your dye pot and compost the skins.

Add the mordanted, wetted fibre. Start cold and raise the temperature slowly. Wool and silk should sit at 80 to 85°C for 45 to 60 minutes, never boiling, stirred only occasionally and very gently. Cotton and linen can take a proper simmer for an hour. Then turn off the heat and leave the fibre in the cooling bath overnight. This slow cooling stage is where much of the depth develops, and it costs you nothing but patience.

Rinse in water of the same temperature as the bath, gradually cooling, until the water runs clear.

Shifting the Colour: Modifiers and pH

One dye bath can give you a whole family of colours. Divide your fibre and try:

• Acid shift: add a tablespoon of white vinegar per litre to the bath for a brighter, cleaner lemon-yellow.
• Alkaline shift: add a pinch of soda ash or a little household ammonia for deeper orange and rust tones.
• Iron afterbath: dip dyed fibre briefly in a weak iron solution for olive, slate and chocolate browns. Keep the dip short and rinse well, as iron can make wool brittle.
• Overdyeing: onion skin gold over a weak madder bath gives terracotta; dipped in indigo it produces soft greens.

Testing, Labelling and Washing

Onion skins are moderately lightfast, better on wool and silk than on cotton, but they will soften over years in strong sunlight. Keep a small sample card of every batch, noting the fibre, mordant, quantities, dates and any modifiers. Attach a matching label to the finished yarn or cloth.

Wash dyed pieces by hand in cool water with a pH-neutral soap, avoiding prolonged soaking and direct sun while drying. Store textiles in the dark, folded with acid-free tissue. Do it once, write it down, and you will have a repeatable golden palette from nothing more than the onions you would otherwise have thrown away.