There is something quietly satisfying about turning a bowl of onion skins destined for the bin into a pot of simmering gold. Natural dyeing is slow, forgiving, and remarkably cheap, and the two ingredients I recommend to anyone starting out are the ones you already have: onion skins and used tea leaves. Between them they cover the warm end of the spectrum beautifully — honey, amber, copper, russet and soft grey-brown — and they behave predictably on both wool and linen.

Start With Clean Fibre

Dye will not take evenly on dirty or greasy fibre, and linen in particular arrives with natural waxes and pectins that resist colour. Scour your materials first, and treat the two fibres differently.

• Wool: wash gently in hand-hot water (around 35–40°C) with a teaspoon of soap flakes or mild washing-up liquid per litre. Leave for 30 minutes without agitating — wool felts if you fidget with it. Rinse in water of the same temperature.
• Linen: simmer for an hour with a teaspoon of washing soda per litre of water, then rinse thoroughly. It can take rougher handling than wool and needs it.

Weigh your dry fibre before you begin. All the quantities below are based on that weight, which dyers call WOF — weight of fibre.

Mordants: The Difference Between Pale and Permanent

Without a mordant, most plant dyes rinse away within a few washes. The mordant binds the colour to the fibre. Alum is the gentlest and most reliable choice for both wool and linen.

• Dissolve 10–15% alum and 5% cream of tartar (both by weight of fibre) in hot water, then top up the pot so your fibre can move freely.
• Add the fibre once the mordant has fully dissolved, bring to around 80–85°C for wool or a bare simmer for linen, and hold for an hour.
• Let it cool in the pot overnight. This slow cooling improves uptake noticeably.

Keep a dedicated stainless steel or enamel pan for dyeing and never use it for food afterwards. Work in a ventilated room, and wear gloves if you are handling iron.

Onion Skins for Golds and Coppers

Save the papery outer layers from brown and yellow onions in a paper bag — they keep indefinitely. You want roughly the same weight of skins as fibre for a strong result, so about 100g of skins per 100g of wool. That sounds a lot, but a busy kitchen accumulates it in a few weeks.

Simmer the skins in water for 30–45 minutes, then strain them out. Add your pre-mordanted, wet fibre to the clear liquor and hold it at 80–85°C for wool, or a gentle simmer for linen, for 45–60 minutes. Let it cool in the pot, ideally overnight. You will get a warm honey-gold on alum-mordanted wool, and a slightly deeper amber on linen.

Red onion skins behave differently: on alum they push towards olive and soft green, which is a lovely surprise if you were expecting pink.

Tea Leaves for Browns and Soft Greys

Used tea leaves, or cheap loose black tea, give tans and russets. Use 20–30g of dry tea per 100g of fibre. Pour boiling water over the leaves, leave to steep for an hour, strain, then dye exactly as with onion skins. Expect biscuit, tan and warm mid-brown depending on strength and fibre.

Tea is also where a modifier earns its keep. After dyeing, rinse the fibre and then dip it briefly into a weak iron bath — a handful of rusty nails left in a jar of water and vinegar for a fortnight will do, or ferrous sulphate at 2% WOF. Watch closely: iron darkens colour fast, and thirty seconds can turn a tan into a deep charcoal-brown or a smoky green-grey. Remove, rinse immediately, and wash well.

Layering, Modifying and Aftercare

Once you are comfortable, the fun is in the variations. A few things worth trying:

• Exhaust baths: dye a second batch of fibre in the used liquor for paler, softer tones — oatmeal and blush rather than amber.
• pH shifts: a splash of vinegar deepens yellows on wool, while a pinch of washing soda in the dyebath pushes onion-skin golds towards orange.
• Overdyeing: dye first with tea for a base brown, then with onion skins to warm it into copper.
• Even results: stir gently and often, and make sure the fibre is never crowded. Cramped fibre dyes blotchy.

Rinse your finished fibre in cool water until it runs clear, then wash with a pH-neutral soap and dry in the shade. Sun is the enemy of plant dyes, so store finished pieces away from bright light. Colours will soften over years, which is part of their charm — but a well-mordanted onion-skin gold on wool will hold for a very long time, and you made it from the bin.