How to Create an Alum Mordant Bath

Alum mordant baths are a must for almost all natural dye. I break down how I create my alum mordant baths for you here so you can start dyeing too.

Here is easy natural dyeing with black beans! See this post for for my favorite shibori folds. And while you’re dyeing, don’t worry about making a mess because I teach you how to clean a stainless steel sink here.

Alum mordant baths are used to create a richer color with natural dyes as well as prolong the color fastness. Colorfastness is basically how long the color will last between washes. I mostly use these baths to get more beautiful colors. For example with logwood extract, without an alum bath, the color is light violet. After the alum bath, the color is a dark, dark purple (which I of course prefer!)

What is a mordant

Mordant, in the situation of natural dyeing, is a way of bonding the color to the fabric. Soaking your fabric in a mordant bath provides color fastness and richer colors. There are lots of ways to mordant fabric but today we are looking at alum baths.

Alum baths are a must for almost all natural dye. I break down how I create my alum baths for you here so you can start dyeing too.

I don’t think it will surprise you to know that the key ingredient, and most important thing you will need for an alum bath is… alum. But it’s true! The better quality of alum the better quality of bath you will get. I’ve tried the cheaper stuff and you can definitely notice a difference in richness of color.

What you need for your alum mordant bath

  • Alum
  • 5 Gallon bucket
  • Pot for boiling water
  • Cotton or linen fabric (wool and silk will felt if you do it this way!)

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First you will want to boil some water. Alum will dissolve best in hot water and it will create a better mordant if the fabric is hot.

Once the water is boiling, put in about 1/4 cup of alum per 400 grams of fabric. I know this seem like a weird way to talk about fabric but you’ve got to know the weight of fabric to get the right colors. As much as I hate to admit that. Because I truly hate weighing dry fabric!!

boiling water with alum

Make sure to pour in the alum slowly because it will bubble up in the boiling water.

Once all of your alum is dissolved, place your cloth in the 5 gallon bucket and pour in the boiling water.

cloth in alum bath

If you used a 5 gallon stainless steel pot, you can cover the bucket really well! That’s what I do. If you have a lid on your bucket that will work well too. Let it sit like this over night at least for the best results!

alum bath

Now you are ready to get dyeing! I have all sorts of dyeing tutorials from pinecones to cochineal and you will want to know how to make an alum bath!

Is Alum safe to use?

If you are sensitive, I suggest wearing a mask. But truthfully with this method, you are exposed to the alum very briefly and then you cover it. Don’t let you child get into the bag or play with your mordant water, and everyone will be just fine!

boiling water with alum

How to Create an Alum Mordant Bath

Active Time: 1 hour
Total Time: 1 hour


  • Alum
  • Cotton or linen fabric


  • 5 Gallon bucket
  • Pot for boiling water


  1. Bring water to a boil
  2. Add alum to boiling water
  3. Put cloth in the bucket
  4. Cover the cloth with the boiling water after alum has dissolved

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