Mordanting Fabric (Alum Brightener)

In this episode of Plants to Dye For - Season 2, we're using a combination of alum and soda ash to mordant our fabric. In previous episodes we scoured the fabric [see Scouring your Fabric (Soaking Method)] and mordanted the fabric using oak gallnut [see Mordanting Fabric (Oak Gallnut)]. You may wonder why we are re-mordanting our fabric. Dyers frequently mordant fabric multiple times to intensify the color or improve colorfastness. For example, after mordanting with an oak gall tannin, a dyer might mordant with an alum bath of 15% WOF, followed by a second fresh alum bath of 10% WOF.

I'm using alum and soda ash (also called washing soda), both of which I purchased online. Together the alum and soda ash act as brighteners. Alum (or Potassium Aluminum Sulfate) also prepares the fiber to bond with the dye. Soda ash (or Sodium Carbonate) changes the pH of the water, and can be added for dyes that respond to a higher pH level.





The basic process, based on Maiwa's online instructions (How to Mordant Cellulose Fibres):

1. Presoak the scoured and mordanted fabric. 
2. Mix alum and soda ash in hot water. 
3. Add fabric and soak. 
4. Rinse and air dry the fabric 

Stainless steel pot

Bucket for soaking and optional thermometer

For equipment I'm using a large plastic bucket for presoaking, a stainless steel pot for soaking, and optionally I can use a candy thermometer to check water temperature. Maiwa recommends using hot water between 120-and-140 degrees Fahrenheit (or 48-and-60 degrees Celsius). For this technique, you do not need to “cook” the fiber. You can start with hot tap water, and let it cool naturally. It is not necessary to reheat the bath to maintain temperature. For safety, wear a face mask when measuring powders, so you don't inhale small particles.

Alum and soda ash for second mordant

According to Maiwa, the Weight of Fabric % for the brighteners is:

• 15% WOF for Alum
• 2% WOF for Soda Ash

We have just over a pound of fabric, or 448 grams, so we'll use:

• 1/4 cup of Alum (67.2 grams)
• 1 3/4 teaspoons Soda Ash (8.96 grams)
• 2-4 gallons of water (I used about 3 gallons)

First, soak the pre-washed and scoured fabric in a bucket of hot water for at least and hour (or overnight). This helps ensure that the fiber soaks up the alum brightener evenly.

Soak the scoured and mordanted fabric

After pre-soaking the scoured fabric:

1. Add the fabric to the stainless-steel pot. 
2. Dissolve alum in hot water and add to the pot. 
3. Dissolve washing soda in hot water and add to the pot. 
4. Fill the pot with enough hot water to cover the fiber. (If you are curious, you can check the temperature with the thermometer). 
5. Stir. 
6. Let soak for 1-2 hours. You can cover the pot so that it stays warm. 
7. At this point you can: (a) Rinse and re-mordant, for example, at 10% alum. (b) Rinse and dye. (c) Steep an additional 8-24 hours. (d) Air dry, to dye later. 
8. I continued to steep the fabric overnight, stirring periodically. 
9. Squeeze out excess liquid. Soda ash is caustic so wear rubber gloves. 
10. Wash and rinse thoroughly in the washing machine, and hang to dry.

Air dry fabric

Now you can dye your fabric, or store it until you are ready to dye. According to one source, the alum can weaken the fiber, so it is best to use the fabric within a month to prevent this. For this video, we are labeling the fabric for storage, with plans to dye in a couple of weeks.

Label and store the fabric