In a word, a mordant is a metal salt which creates a bond between natural dyes and cloth. So, you need to use a mordant on most fabrics if you want vibrant lasting color.
There are many different mordants and mordant combinations some of which are suitable for fibers from animals (silk and wool) and some which are used on fibers from plants (cotton, linen, hemp and others). All mordants improve color fastness and bring out the depth and vividness of colors. Without a mordant, natural dyes will fade in sunlight and wash out in the laundry.
In the beginning, if you want to keep things simple, you can use just two mordants for all your natural dyeing projects. Aluminum Acetate is very commonly used for plant fibers and Aluminum Potassium Sulphate (often just called ALUM) is used for animal fibers. The method for mordanting is basically the same for both except that extra care should be taken with wool as it can very easily shrink and felt if not treated gently and if it is subjected to quick changes in temperature, like taking wool yarn from the hot dye pot and plunging it into cold water.
Before mordanting it is important to wash new fabric or clothing thoroughly in hot water to remove processing chemicals from the manufacturing process. This is called scouring the fabric. The first step is to weigh the dry fabric, clothing or yarn and make a note of it. You can use the hot wash setting on your washing machine for plant based fibers using a gentle washing liquid (ph neutral) and no fabric softener. Or you can heat the textile gently in a stainless steel pot of water with a little washing soda (1% of the dry weight of the fiber WOF) and keep it at a simmer (not boiling) for up to an hour, then turn off the heat and allow the textile to cool completely. Once cool you can rinse the fabric or yarn in clean water and throw out the dirty water (if the water is very dirty it may be necessary to repeat the scoring process). It is now ready to mordant. For wool and silk use the pot method and only a very little (1% of dry weight of fiber WOF) ph neutral soap for the scouring. Start with lukewarm tap water, add the gentle soap, then the fabric or yarn and slowly raise the temperature to just under a simmer, looking for steam rising from the surface of the water. Slowly and gently are essential for handling wool. If not, the matted, shrunken results can be very disappointing. Your clean, scoured fibers are now ready for mordanting.
Choose your mordant, depending on the fiber you will dye. For plant fibers you will use Aluminum Acetate at 5% of the WOF and for animal fibers you will use Aluminium Potassium Sulfate at 12% WOF. These are the amounts recommended by Botanical Colors, the natural dye specialists you can find on-line.* Then fill a stainless steel pot which is large enough to allow the textile to move freely in the mordant solution with water. Measure out your mordant and dissolve it in a little hot tap water in another container. Although non-toxic, It is a good idea to wear a mask when weighing out mordant powders and gloves while handling the wet fibers because both can cause irritation to lungs and skin. Add the dissolved mordant to the pot of water and give it a stir then add your wet fabric or yarn. Slowly raise the temperature to just under a simmer and keep that temperature for up to an hour. Gently stir the fabric in the pot every few minutes to encourage even absorption of the mordant. Then turn off the heat and let the fibers cool in the pot. When cool enough to handle, remove them and rinse in clean tap water. Remember wool is sensitive to abrupt changes in temperature so do not take hot wool straight out of the pot and drop it into cold water. Also do not wring or rub wool especially when wet.
You can continue on to dyeing your fabric or yarn or let it dry, label it with the mordant you used and store it away from dirt and dyes or plant material until you are ready to dye. I tend to scour and mordant many pieces of fabric and yarn at a time so that I have a ready supply on hand. Mordant baths can be reused a couple of times, just by topping up with about 25% of the original amount of mordant used. This saves water, mordant and money !
So that's it ! It's a very simple but essential step in the natural dyeing process and will ensure that you get good results with all your dye projects, whether bundle dyeing, Hapazome (flower and leaf pounding), botanical printing or immersion dyeing.
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