Name: Susan
Message: How can I "set" the yellow color in a 100% cotton shirt that has been dyed using yellow-onion skins? 

In general, the best way to set natural dyes on cotton is by pre-mordanting the fabric. Unfortunately, it is much more difficult to get a satisfactorily colorfast result on cotton than it is on wool.

The most effective way to mordant cotton for natural dyeing is to boil it in alum for an hour, let it dry, then boil it in tannin for an hour, let it dry, then boil it in alum for an hour. Only after that process has been completed should you dye the cotton by boiling it in an equal or greater weight of the natural dyestuff. That is, if the shirt weights eight ounces, use one-half to one pound of onion skins to dye it. There are good recipes for mordanting cotton in Dagmar Klos's book, A Dyer's Companion.

You may sometimes read advice to "set" dye colors by washing with vinegar or salt . Neither of these will do anything to make your natural dye more permanent on cotton, however. Soda ash also will do nothing to help set natural dyes.

Post-mordanting is the application of mordants to fabric which has already had the dye added. It is likely to be less effective on cotton than pre-mordanting. I have not seen any recipes for post-mordanting cotton, but you could try following the same procedure as for pre-mordanting. You could simmer your shirt for a couple of hours in a pot with one ounce of alum for every pound of fabric (e.g., use 15 grams of alum if your shirt weighs half a pound). You may need to repeat the onion skin dyeing after this step, if too much of the color leaches out into the water.

Jill Goodwin's book, A Dyer's Manual (second edition), lists onion skins as producing yellows when mordanted with alum, orange when mordanted with (toxic and carcinogenic) chrome, bright orange when mordanted with tin, and browns when mordanted with iron; however, this list is referring to the use of natural dyes on wool. I recommend against the use of chrome, tin, or iron mordant by those who are novices in the art of dyeing, because each of these mordants can be deadly. Chrome mordant causes serious injury and sometimes fatal cancers if inhaled or otherwise absorbed, and the other mordants can be fatal when swallowed, and therefore much more dangerous than the use of synthetic dyes. Even alum, the safest of the metal salt mordants, can be fatal to a small child who eats enough of it; the fatal dose is supposed to be about one ounce for an adult, but could be considerably less for a toddler. It is very important to keep all mordants completely out of the reach of children.

Even after proper mordanting, do not expect your onion-dyed shirt to be as washfast as a shirt that has been dyed with high quality synthetic dyes such as Procion dyes. Wash the shirt as infrequently as is practical, and wash it only in cool water, by hand. Do not machine-wash clothing that has been dyed with natural dyes.

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