How to create a Mordant Bath (for absolute beginners)

In this blog I am going to answer the most basic question I get a lot: how do I mordant fibers.

What’s a Mordant, Anyway?

In simple terms, a mordant is a substance that helps dye bond with your fiber. Think of it like a matchmaker, introducing the color molecules to the fabric and saying, “Now hold hands and don’t let go.” Without it, many natural dyes just sit awkwardly on the surface, looking good for a moment but disappearing faster than you can say “cold rinse.”

The word “mordant” actually comes from the Latin mordere, meaning “to bite.” A little dramatic, but accurate: mordants help color bite into the fiber so it sticks.

The Basics: Alum and Friends

The most common mordant (and the most beginner-friendly) we will discuss today is alum: potassium aluminum sulfate. It’s safe, easy to source, and reliable, making it the go-to for silk, wool, and plant fibers alike. Alum gives bright, clear colors and doesn’t usually change the shade too much. Alum is not an expensive mordant, and together with all the benefits mentioned previously, it is an absolute must-have if you are starting to do some natural dyeing.

You’ll also meet alum’s sidekick:

• Cream of Tartar: not just for your baking cupboard. It helps alum work more evenly and can soften wool and keep it from becoming slimy (which alum tends to do after a while in an aqueous solution).

There are other mordants you should get to know (copper, titanium oxalate, tin), but let’s not run before we walk.

Important: This mordant bath is great for protein fibers like wool and silk., for cellulose there are better mordant methods available.

Important: Not every dye needs a mordant. Tannins and indigo are an example of dye stuffs that will work without the extra step of mordanting.

How to Make a Basic Mordant Bath

Here’s the simple, no-fuss method to get started with alum:

1. Weigh Your Fiber (this is called WOF): Work out how much your dry fiber weighs. This is your base number.
2. Measure Alum: Use about 10–15% of your fiber weight in alum. (So if your yarn weighs 100 g, you’ll use 10–15 g of alum.)
3. Optional Cream of Tartar: Add 5–6% WOF for extra softness and leveling.
4. Dissolve: Dissolve your alum (and cream of tartar, if using) in a cup of hot water. No clumps, please. This will take at least two minutes of consistent stirring.
5. Fill Your Non Reactive, Clean, Dye Pot: Add enough warm water to comfortably cover your fiber and stir in the dissolved mordant.
6. Add Fiber: Wet out your fabric or yarn (pre-soak it so it’s thoroughly damp), then add it to the pot.
7. Heat Gently: Bring the pot up to around 70°C (not boiling, we’re not making soup) and hold it there for about an hour and a half. Stir now and then so the fiber doesn’t get patchy.
8. Cool Down: Let it sit until the water is room temperature. You can even leave it overnight, no one’s judging, BUT make sure nothing is sticking out from your mordant bath or it will get uneven.
9. Rinse Thoroughly, really, don’t hold back. Any leftover mordant clinging to the fabric (instead of bonding properly) will just jump on the dye in the pot, and you’ll end up with blotchy, uneven color.”

After the Bath

Once your fibers are mordanted and rinsed, they’re like a blank canvas that’s ready to hold onto dye. You can use them right away or dry and store them for later. Some dyers even keep a stash of “pre-mordanted” fibers ready to go, which is kind of like having cookie dough in the freezer for emergencies. Just make sure you mark these fabrics well, so you do not end up mordanting twice.