Alum mordanting gives wool a prepared surface for many natural dyes. It can help colour develop more evenly and remain better anchored through ordinary handling and washing, although it is not a guarantee of lightfastness or permanence. The process is modest in equipment: a scale, a non-reactive pot, clean water, alum, and gentle heat.
The important parts are accurate weighing, thorough scouring, and patient temperature control. Wool does not need vigorous stirring or boiling. Sudden changes in temperature, friction, and rough handling are more likely to cause felting than a quiet, steady bath.
What alum does
A mordant is a substance used to help a dye bond with a fibre. Alum, commonly sold as potassium aluminium sulfate, is widely used for protein fibres such as wool and silk. It is generally considered a relatively mild mordant, but it should still be handled carefully and kept away from food preparation surfaces and utensils used for eating.
Alum is useful for many plant dyes, but not every dye requires it, and some colours may change depending on the mordant and the dye bath. If a dye recipe gives its own mordanting instructions, follow those in preference to a general method. A small test skein is worthwhile when working with an unfamiliar plant material.
How much alum to use
Measure alum as a percentage of the dry weight of the wool, often written as WOF, or weight of fibre. For a first batch, 10 percent WOF is a practical starting point: 10 grams of alum for every 100 grams of dry wool. Some recipes use a higher proportion, but more is not automatically better. Excess alum can leave unnecessary residue and does not turn a weakly lightfast dye into a durable one.
Weigh the wool before scouring if it is already reasonably clean. If the wool is greasy, heavily soiled, or contains vegetable matter, weigh it after drying and scouring so the calculation reflects the fibre rather than dirt and lanolin. Record both the dry wool weight and the alum weight in a notebook.
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Materials and preparation
Gather:
- dry, washed wool yarn or fleece
- alum measured at 10 percent of the dry wool weight
- a stainless-steel, enamelled, or other non-reactive pot
- a kitchen scale
- pH-neutral soap for scouring
- a wooden spoon or other gentle stirring tool
- clean water
- heatproof gloves or tongs for handling the pot and wet wool
Avoid iron, copper, or aluminium pots unless you know how the metal will affect the bath. Natural dyeing vessels can alter colour, and a mordant bath is best kept as neutral and predictable as possible. Do not use a pot for cooking after it has been used for mordants or dyes.
If working with fleece, open it into loose locks and remove visible straw, burrs, and heavily soiled tips. Do not pack the fleece tightly into the pot. Yarn should be wound loosely, with a few ties of scrap cotton or other secure fibre to prevent tangling. Ties should be firm but not tight enough to mark the yarn.
Step 1: Scour the wool
Scouring removes grease, dust, and spinning oils that may keep the mordant and dye from reaching the fibre evenly. Fill the pot with enough warm water to allow the wool to move freely. Add a small amount of pH-neutral soap and let the soap disperse before adding the wool.
Lower the wool into the warm bath. Press it under the water gently rather than stirring. Bring the temperature up gradually to roughly 60–70°C, below a simmer, and hold it there for about 20–30 minutes. The exact time depends on how dirty the wool is. Very clean yarn may need less; greasy fleece may need a second bath rather than harsher treatment.
Turn off the heat and allow the wool to cool in the water. Lift it out with both hands or drain it through a colander, supporting the wet fibre so it does not stretch. Rinse in water of a similar temperature until the water is clear and no soap remains. Do not wring, rub, or run cold water directly over hot wool.
If the fleece still feels greasy after rinsing, scour it again. A clean fibre is more valuable than a hurried mordant bath.
Step 2: Prepare the alum solution
Fill the clean dye pot with enough water to cover the wool generously. The fibre should be able to move without being compressed. Warm the water, but do not bring it to a boil.
Measure the alum according to the dry weight of the wool. For example, 50 grams of dry wool requires 5 grams of alum at 10 percent WOF. Dissolve the alum first in a separate container of hot water, stirring until the crystals have disappeared. This makes it less likely that concentrated alum will settle directly onto the wool.
Pour the dissolved alum into the pot and stir the bath gently. There is no need to adjust the pH for a basic alum mordant bath. If your local water is unusually hard or alkaline, the results may vary, so keep notes and test a small quantity before committing a large skein.
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Step 3: Mordant the wool
Place the scoured, wet wool into the alum bath. It should enter while both the wool and the bath are warm, not at sharply different temperatures. Press the fibre below the surface with a wooden spoon, using slow movements to release trapped air.
Raise the bath gradually to approximately 80–90°C, keeping it below a boil. Hold it at this temperature for about 45–60 minutes. A gentle wisp of steam is sufficient; vigorous bubbling is not. Turn the wool occasionally only if needed to keep it evenly submerged. Excessive stirring and lifting can encourage felting, especially with loose fleece.
Keep the lid partly on if evaporation is a concern, but do not leave the pot unattended. Use ventilation, avoid splashes, and follow the handling instructions on the alum package. Wear gloves when moving the hot pot or wet wool, and wash your hands and work surface when finished.
After the heating period, switch off the heat and let the wool cool in the mordant bath. Cooling in place gives the treatment time to settle and avoids a sudden temperature change. Several hours is sufficient; leaving the wool in the cooled bath overnight is also commonly practical if the bath remains clean and covered.
Step 4: Rinse and prepare for dyeing
Lift the wool from the cooled bath and allow the liquid to drain without wringing. Rinse it in water close to the bath temperature. Continue until the water is mostly clear and the wool no longer feels slippery or chalky. A slight change in handle is normal, but a heavy residue suggests that the alum was not fully dissolved or that the bath was overly concentrated.
The wool may now go directly into a prepared dye bath. Keep it wet while transferring it, and make sure the dye bath is not dramatically hotter or colder than the mordanted wool. Equal temperatures reduce the risk of shock and felting.
If you are not ready to dye, squeeze out excess water gently and dry the wool away from strong direct sun. Once fully dry, store it in a breathable cloth bag or paper sack in a clean, dry place. Label it with the fibre weight, alum percentage, date, and any notes about the bath. Do not store damp wool, since it can develop an unpleasant smell or mildew.
Photo by Anastasia Zhenina on Unsplash.
Useful mistakes to avoid
Using the wet weight
Wet wool can weigh several times more than dry wool. Calculating alum from that figure produces an uncertain and usually excessive amount. Use the dry weight of the fibre.
Adding dry alum directly to the wool
Undissolved crystals may settle in one place and treat the fibre unevenly. Dissolve the alum separately in hot water before adding it to the pot.
Boiling or stirring hard
Wool felts through heat, moisture, friction, and sudden movement working together. Keep the bath just below a simmer, move the fibre as little as possible, and avoid temperature shocks.
Skipping the scour
A mordant cannot compensate for wool that is still coated in grease or spinning oil. If water beads on the fibre or the fleece remains slick, scour it again before mordanting.
Assuming alum ensures permanence
Alum may improve the way some dyes take to wool, but final durability depends on the dye plant, extraction, fibre, bath conditions, washing, and exposure to light. Keep dyed wool out of prolonged strong sunlight, wash it gently, and make a small sample before dyeing a treasured garment or blanket.
A quiet, repeatable method
For most small batches, the reliable sequence is simple: weigh the dry wool, scour it gently, dissolve alum at 10 percent WOF, heat the bath without boiling, cool the wool in place, and rinse at a similar temperature. Write down the quantities and times. Natural dyeing has many variables, but careful notes make the next batch less mysterious and easier to repeat.