Mordants and Fixatives

Mordants and fixatives help bond dye to fiber, and improve the color-fastness of the dye. Without mordants, most natural plant dyes fade quickly. We touched on mordants and fixatives a bit in Tooling Up to Dye and Equipment for Dyeing with Plants, now I'd like to reinforce some of these ideas.

Techniques for mordanting protein-based and plant-based fibers differ. Protein-based fiber requires metal mordants in order for dye to chemically bond to the fiber. Protein-based fabric includes wool, angora, silk and so forth. Metal mordants include iron, copper, chrome, tin, and alum (often used with cream of tarter as a brightener). Some of the metal mordants are toxic, so must be handled and disposed of carefully.

Metal mordants for protein-based fibers

Plant-based fiber can use mordants such as tannins, soda, salt, vinegar, and ammonia, as well as the metal alum. Tannins are often naturally-occurring in plant dye source, such as in black walnut hulls, sumac, and oak galls. Most of these mordants can be purchased at the grocery store, others, like dried, ground oak galls can be found at specialty stores. So far I have only dyed with plant-based fabric, so I'm focusing on their mordants. They are not toxic like metal-based mordants but some can be caustic so should still be used with care.

Mordants for plant-based fibers

Weigh the Fabric

In either case, you need know the weight of your fabric, so you can calculate the quantity of mordant to use. In my case, I'm dyeing white cotton dish towels, which weigh about 77 grams (2.25 ounces) each. Recipes frequently state the mordant ingredient as a percentage (% WOF, or weight of fabric). For example, if the recipe calls for 100% WOF, and you are processing 100 grams of fabric, you need 100 grams of alum for the mordant. If the recipe calls for 50% WOF, you would need 50 grams of alum to mordant your 100 grams of fabric. 

Weight of Fabric * (% WOF) = Weight of Mordant 

100 grams fabric * (100% of 100g) = 100 grams of alum

100 grams fabric * (50% of 100g) = 50 grams of alum

Wash and Scour the Fabric

Always wash the fabric to prepare it for dyeing. Even sparkling white cotton fabric may have a coating applied, which could prevent dye from bonding with the fiber. Some books recommend that you "scour" the fabric before you wash it to remove any wax, finish, or sizing. Sasha Duerr, in The Handbook of Natural Plant Dyes, recommends using a mixture of 2 tablespoons neutral detergent (like Dawn dish washing detergent) plus 1 tablespoon washing soda (like Borax, soda wash, or soda crystals) to remove these impurities. Simmer the fabric in the mixture for about an hour, then rinse, wash, and air dry.

Wash and scour fabric to remove impurities that prevent the dye from bonding to the fiber

Mordant the Fabric

Once you've scoured and washed your fabric, you're ready to mordant. The basic process for mordanting fiber is: 

1. Add the mordant to boiling water.
2. Add the fabric and simmer for about an hour.
3. Remove the pan from the heat and let the water cool. 
4. Remove the fabric and squeeze gently.
5. Use immediately, or air dry and label for future use.

Fabric simmering in a salt mordant bath

Following are a couple of mordant recipes for plant-based projects, condensed from Duerr's book. You can find alum and ground tannin through suppliers such as Amazon, MAIWA, and Dharma Trading Company.

Ingredients	Steps
Basic Tannin Mordant 113g fiber (4 ounces) 1 teaspoon (1 ounce) powdered oak galls	Soak fabric overnight in cool water. Dissolve oak gall powder in 4-6 gallons of water. Simmer tannin water for 30-60 minutes, then let cool. Move fabric from soaking water to tannin water and let steep for up to 24 hours. Remove fabric, rinse in lukewarm to cool water, then wash in a pH neutral soap, rinse thoroughly, and hang to dry.
Alum Mordant with Tannin-Treated Fiber 113g fiber (4 ounces) 20% WOF alum (4 teaspoons) 6% WOF washing soda (1.5 teaspoons)	Follow basic tannin recipe above to prepare fabric. Dissolve the alum and washing soda in a large pot that is half full of water. Add the tannin-treated fabric and add water to cover. Heat the solution to 82 degrees C. (180 degrees F.), turn off heat, and let fabric steep for 4-8 hours, stirring occasionally so the fiber absorbs mordant evenly. Remove the fabric and squeeze the mordant solution back into the pot (you can reuse it). Wash fabric in a pH-neutral soap, rinse thoroughly, and hang to dry.

Additives

Additives such as vinegar or ammonia can change the pH of the water, and the color of the dye. They can be added to the mordant, or to the dye. I haven't tried adding them to the mordant yet, but have added vinegar and ammonia to separate red cabbage dye batches. I was amazed to see the dye vats turn from from blue to dark blue/purple from the ammonia (base), and from blue to green from the vinegar (acid). It's always fun when art and science converge!

Additives can change the dye results