Lets say you decided to dye some fabrics, cotton of course, with onion skins this weekend and failed to pre-mordant your fabrics, or prescour for that matter. Your attempts at dyeing with onion skins is not all loss.
You can actually take your fabric and post mordant it using alum and tannin. When dyeing cotton fabric with onion I use alum, onions have a good amount of tannin.
Assuming you have about a pounds worth of cotton fabric (The weight varies depending on the weave and width of the fabric) wet it out really well and then place it into your dyepot with 1 ounce of alum and enough water to adequetly cover your fabric.
Bring the fabric to just under a boil and let it simmer for two hours. This should be suffecient to retain color. You will probably loose some of the color from your initial attempts, or it may become even darker.
If you want you can add more onion skins at this point. NOTE: Your onion skins will give the best color if you heat the dye bath up, cover them and let them soak overnight.
You can check out my website for information on scouring cellulosic fibers, which is a must when dyeing with natural dyes. http://www.prairiefibers.com/Information.htm
NOTE: I don't ever use chrome or tin, I either live w/o those colors or I use other dye methods, that involve tons of work, to achieve the colors yeilded by using chrome and tin.
NOTE: When dyeing any fibers use only dedicated pots and utensils. Never re-use this equipment for food preparation.