Mordanting Wool with Alum — The Foundation of Natural Dyeing

Place the dry wool skein on a kitchen scale and record its weight in grams. This is the WOF (weight of fibre) — every mordanting recipe is calculated as a percentage of this number. For a first mordant bath, 100 g of wool yarn is a practical quantity: enough to see results clearly, small enough to handle in a kitchen pot. The wool must be dry for an accurate weight — damp wool can hold 30% of its weight in water, which throws off all calculations.

For protein fibres (wool, silk, alpaca), use 10% WOF alum and 6% WOF cream of tartar. For 100 g of wool: 10 g of alum and 6 g of cream of tartar. These ratios are the standard used by professional natural dyers worldwide — less alum gives poor colour fastness, more alum makes the fibre sticky and harsh. Write down the exact amounts before measuring. If working with a larger quantity of wool, scale proportionally: 200 g wool needs 20 g alum and 12 g cream of tartar.

Fill a basin with warm water (about 40°C — hand-warm, not hot) and submerge the wool skein. Gently press the air out — dry wool is hydrophobic and traps air bubbles that prevent the mordant from reaching the fibre evenly. Let the wool soak for at least 30 minutes, or up to overnight. The fibres swell as they absorb water, opening the protein structure to accept the mordant. Never agitate wool suddenly between hot and cold water — the thermal shock causes felting. Always change temperatures gradually.

Measure 10 g of alum and place it in a heatproof cup or jar. Add about 250 ml of hot water (just off the boil) and stir until every crystal has dissolved completely. Undissolved alum granules that touch the wool can cause uneven mordanting — bright spots and pale spots in the final dyed fabric. The solution should be perfectly clear with no grit at the bottom. Alum dissolves readily in hot water but poorly in cold, so use genuinely hot water.

Measure 6 g of cream of tartar and dissolve it in a separate cup with 250 ml of hot water. Cream of tartar is less soluble than alum — stir vigorously until no white powder remains at the bottom. The cream of tartar serves three purposes: it helps the alum penetrate the fibre evenly, it keeps the wool soft (alum alone can make wool feel harsh), and it slightly brightens the eventual dye colour by creating a mildly acidic bath. Keep this solution separate from the alum until you add both to the pot.

Fill a large stainless steel or enamel pot with enough water to cover the wool freely — about 4 litres per 100 g of wool. The wool must be able to move in the water without being compressed or folded tightly. Use a non-reactive pot: stainless steel, enamel-coated, or glass. Never use aluminium, copper, or iron pots — these metals act as unintended mordants and shift dye colours unpredictably. An aluminium pot adds extra aluminium; an iron pot saddens (darkens) colours. Dedicated dye pots should never be used for cooking food afterward.

Pour the dissolved alum solution into the pot of water. Then pour in the dissolved cream of tartar. Stir gently with a wooden spoon or stirring stick to distribute both chemicals evenly through the water. The solution should be clear and colourless — if you see any cloudiness or sediment, stir until it clears. The mordant bath is now ready to receive the wool.

Lift the wool from its soaking water and gently squeeze out the excess (do not wring — wringing distorts the yarn). Lower the wool into the mordant bath, pressing it below the surface. Arrange the skein so it lies loosely in the pot without tight folds or bunching — any area where mordant cannot circulate freely will mordant unevenly and dye unevenly later. The water should cover the wool by at least 2-3 cm.

Place the pot on a medium-low heat source. The goal is to raise the temperature gradually — about 2°C per minute — until the bath reaches 85°C. Use a thermometer to monitor. Rapid heating causes the outer fibres to mordant before the inner fibres, creating uneven results. It also risks felting: wool fibres have microscopic scales that lock together under heat and agitation, turning yarn into a solid matt. Slow, gentle heating avoids both problems. If steam begins to rise vigorously, reduce the heat — you are approaching boiling, which is too hot.

Maintain the temperature between 80-90°C for 45 minutes. Gently turn the wool with a wooden spoon every 10-15 minutes to ensure even exposure — lift and turn, do not stir vigorously. The alum molecules are bonding to the keratin protein in the wool fibres during this time. At 85°C the protein structure is relaxed enough to accept the metal ions but not so hot that the fibres are damaged. Never let the bath boil — boiling wool at this stage felts it irreversibly and wastes the yarn.

After 45 minutes at temperature, turn off the heat and leave the wool in the mordant bath to cool naturally. Do not remove the wool while hot — the mordant continues bonding as the temperature drops slowly, and removing hot wool into cold air risks felting from thermal shock. Overnight cooling (8-12 hours) gives the best mordanting results. The slow cooling allows maximum uptake of alum into the fibre. Cover the pot with a lid to retain heat longer.

The next day, lift the wool from the cooled mordant bath. Gently squeeze out the excess liquid — do not wring or twist. The wool will look unchanged — mordanting is invisible. The alum is now bonded inside the fibre structure, waiting to grab dye molecules. The mordanted wool can be dyed immediately (damp) or stored in a sealed plastic bag in the refrigerator for up to a week. If storing longer, let it dry completely and re-wet before dyeing. Mordanted wool that dries out fully is still mordanted — the alum does not wash out in plain water.

The spent alum mordant bath can be poured down the drain — potassium alum in these small quantities is non-toxic and used in water treatment plants at much higher concentrations. Some dyers save the bath and reuse it for a second batch (called an exhaust bath) — it contains less alum, so the second batch will take up slightly less dye and produce paler colours. If reusing, add 5% WOF fresh alum to top up the exhausted bath. The cream of tartar is consumed during mordanting and cannot be reclaimed.

To verify the mordant took properly, dye a small sample alongside an unmordanted control. Cut a 5 g piece from the mordanted skein and a 5 g piece of unmordanted wool. Prepare a simple dye bath — yellow onion skins work well: simmer a handful of dry onion skins in water for 20 minutes, strain, then add both wool samples. After 45 minutes at 85°C, rinse both. The mordanted sample will be a rich, deep gold. The unmordanted sample will be a pale, washy yellow that fades quickly with washing. The difference is dramatic and proves the mordant is working.