Collect your materials. Natural dyes are the most common mordant dyes, and many are available in your local grocery store or even your backyard. You can experiment with a wide variety of natural dyes. Beetroot is a good choice, but onion skins, many types of flowers and even tree bark can all be used for a dyeing experiment. Alum is a widely available mordant and can be found with the spices in the grocery store, as can cream of tartar. Wool yarn or wool fabric is ideal for this dyeing experiment.
Prepare your mordant. Combine alum and cream of tartar in an amount proportional to your fabric or yarn. You need 10% of the total weight of your fabric or yarn in alum and 5% in cream of tartar. A small scale may be convenient, but estimations are adequate. Add the mordants and wet yarn or fabric to a dye pot full of water and simmer for ninety minutes. For this dyeing experiment, you should mordant half your fabric or yarn samples, but leave half without the mordant.
Chop or grind your dye material. Plant materials, like rhubarb or beetroot, should be finely chopped. Coffee or tea can simply be brewed at three or four times its usual strength. Add water to cover your dye material in your pot, or pour in your coffee or tea. Bring your dye stock to a boil and allow to simmer for around half an hour before using it. Longer simmer times may work better for some natural dyes, but you are looking for concentrated color in the dye stock. Add water if needed, but do allow evaporation to occur to concentrate the dye.
Dip at least two pieces of previously untreated but clean fabric or yarn in your dye. Allow this to rest in the dye for fifteen minutes. Rinse and hang to dry one piece of your sample material. Hang the other to dry without rinsing. Now, repeat this process with the samples you previously treated with the alum/tartaric acid mordant.
Experiment with longer soak times, other natural materials or mixing dyes to produce different color effects. You can even try dripping on or hand-painting yarn or fabric as a dyeing experiment to create new effects and colors.
Once your samples have dried, try washing them with a gentle soap and cool water. You should quickly see the changes and differences in the samples that you treated with a mordant before dyeing. Mordants set natural dyes, allowing them to stand up to both water and washing. Without mordants, the dye will continue to run, fading and staining anything with which it comes in contact.