How To Mordant: Alum

I created a video on how I mordant my wool before dyeing, and decided written instructions would make a nice companion. You can watch the video here, keep reading, or do both for the best of both worlds.

What is mordanting? A real simple explanation: your wool takes a metallic salt bath before a dye bath. You soak your fiber in a substance, such as a metallic salt, that binds to the fiber. Once that fiber is in the dye pot, the dye molecules bind to the substance-coated fiber.   The dye molecules have an easier time attaching to the substance than they do to a “naked” fiber. Therefore, more dye molecules attach. More dye molecules mean more color, which helps solve naturally dyed fiber’s biggest issue: fading. Mordanting your wool will increase light and water fastness.

Do you have to mordant? No. There are some natural dyes that don’t need it; the dye will absorb nicely into the fiber without that extra coating. Onion dye is an example. However, notice the difference between the wool with and without an alum mordant. That’s the same wool, added to the same dye pot at the exact same time.  So, while you don’t have to mordant, there is a color benefit to mordanting.

Onion dyed wool. Left: premordant with alum; Right: no mordant

I will do anything to keep my color as bright and saturated for as long as possible. Which is why I mordant my wool 99% of the time.

There are many types of mordants used by dyers. My experience is primarily with alum and iron; alum brightens color, iron darkens (saddens) color. Iron is also used to shift colors, yellow to green for example.

The focus of this post is alum (aluminum sulfate). You can find alum online or at your grocery store in the spice aisle.

Is it safe? When used properly and in the recommended small quantities, it is considered nontoxic. However, the dyer should be careful. The crystals may irritate your skin, and the vapor once in solution can irritate the lungs or mucus membranes. It’s smart to protect yourself with gloves, mask and proper ventilation.  I highly recommend you do your own research into this metallic salt. That is the best method of making an informed decision. While I’ve used it for years without issue, I am not a scientist nor a medical expert. I’m a dyer. My expertise is in creating color.

Fiber: I work with animal fibers, not plant fibers. Animal fibers (wool) have more dye receptors on their fiber than plant fibers. It’s an easier fiber to use if your goal is rich color. Tip: soak the fiber in warm water before the mordant bath. This “opens up” all those fiber receptors making them ready to absorb the alum evenly.

Mordant supplies: Your mordant supplies are NOT your kitchen supplies. Keep your cookware for cooking and check out your favorite thrift store for pots and spoons.

  • Stainless steel pot with lid
  • Measuring spoons
  • Small stainless steel or plastic cup
  • Stir spoon
  • Scale for measuring weight of dry fiber
  • Mask, gloves, and apron are a good idea. I don’t use toxic mordants, but alum or iron may still irritate you.
  • Cream of Tartar – this is an additive that helps keep the wool soft and can help brighten the color. I use 1 teaspoon per 100g of wool.

How much? This is based on the weight of dry fiber. Different sources will give you different ratios of alum to fiber, from 10-20%.  If you have 100 g of wool, you use 10-20 grams of alum. In other words, between 1-2 tablespoons. If you go towards 20% you’ll likely have alum leftover in the water and you can try reusing it. Any more than that, you risk your wool feeling sticky. I’ve always used a high ratio of alum and it’s a habit I haven’t been able to let go of, yet. This is another important point that I find helpful as a dyer: there is no one right way to do it. There’s a general framework to follow and lots of wiggle room for experimenting and deciding on what works best for you.

The mordanting process:

1. Fill your pot ¾ full of water and bring to a simmer. The pot needs to be large enough to allow your skeins to move freely in the water.

2. Take a scoop of the hot water with the measuring cup, add the mordant and cream of tartar, stir until it completely dissolves into a solution.

3. Add the solution back into the pot of water and stir again until the solution is combined. The goal is for the yarn to absorb the mordant as evenly as possible. You don’t want clumps of mordant on a few strands, and no mordant on the other strands.

4. Add your wet yarn into the pot and gently stir. Keep it at a simmer for an hour, gently stirring a few times to keep the alum solution evenly dispersed in the water. Don’t let it boil. After an hour, turn off the heat and let the yarn cool in the bath.

That’s it! Your skeins are now ready for the dye pot.

If possible, mordant several skeins at once so that you have a ready-to-dye supply of fiber. You never know when that dye pot will insist you use it.


  1. What a great video, Myra!(and Martin!)… Very clearly and simply explained mordanting process!!!

    And I really like the mini video demonstrating how the mordant and dye molecules attach to the fiber.. .


  2. Thank you sooo much 😍😍😍
    This is really a great and clear explanation. I am extremely grateful for this amazing information.

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