How to Pre-Mordant Wool Yarn for Natural Dyeing

Materials & tools you'll need

Step 1 — Scour your wool yarn

Scour your wool yarn to remove any extra dirt or residue from the fibre. Rinse the scoured yarn thoroughly under warm water. This gives the alum and cream of tartar a clean fibre to bond to in the next steps, which makes for a more even mordant.

Step 2 — Dissolve the alum and cream of tartar

In your pot, add warm water along with 1 tablespoon of alum powder and approximately ½ tablespoon of cream of tartar. Stir until both substances are fully dissolved. Alum acts as the pre-mordant, bonding a metal salt to the fibre, while the cream of tartar softens the yarn after the metal salt has slightly roughened it.

Step 3 — Add the yarn and top up the water

Add your scoured skein of wool yarn to the pot, then fill the pot almost to the top with warm water — you want enough water that the yarn can move around freely rather than being crowded or bunched.

Step 4 — Simmer the pre-mordant solution

Place the pot on medium heat and allow it to simmer gently without boiling. Stir occasionally to make sure any remaining alum dissolves fully rather than settling at the bottom. Let the solution simmer for about an hour, keeping an eye on the heat so it never reaches a full boil.

Step 5 — Remove and rinse the yarn

After an hour, remove the yarn from the pot and rinse it thoroughly in warm water. Rinse well enough that no loose alum or cream of tartar remains on the yarn — you only want what has bonded to the fibres to stay behind. You can reuse the leftover pre-mordant solution for future mordanting projects, topping up the alum as needed.

Step 6 — Dry and store for dyeing

Let your pre-mordanted yarn dry fully. If you're not ready to dye it right away, it stores well for later use — simply keep it somewhere clean and dry until you're ready to move on to your dye bath, whether that's today, tomorrow, or further down the line.

Frequently asked questions

What is pre-mordanting and why is it important for natural dyeing?
Pre-mordanting is a crucial step in natural dyeing that involves depositing a metal salt, such as alum, onto the fibre before dyeing. This enhances the bond between the yarn and the dye, ensuring colourfastness over time.

How do I rinse pre-mordanted wool yarn thoroughly?
After removing the pre-mordanted yarn from the pot, rinse it thoroughly in warm water to make sure no loose alum or cream of tartar remains — you only want what has bonded to the fibres to stay behind. Rinsing well is key to achieving good colourfastness and vibrant results once you move on to dyeing.

Enjoyed this tutorial?

Find 60+ free tutorials on YouTube. Grab a pattern from the Etsy shop, or go deeper with a community membership.

Free tutorials on YouTube →    Shop on Etsy →    Join the community →