community of dyers

I just saw this yesterday. In the July 2007 issue of the scientific Journal Analytica Chimica Acta is an article about the fading of madder, a natural dye, on wool, entitled "A spectrometric and chromatographic approach to the study of ageing of madder (Rubia tinctorum L.) dyestuff on wool.". I've seen only the abstract (summary) of the article so far, but it says clearly that alum improves the lightfastness of madder.

How could this work? When a dye molecule absorbs a photon of light, the energy has to go somewhere. (My doctoral thesis was about using dyes, in the presence of light and oxygen, to damage DNA and kill cells - not reactive dyes, but some acid and basic dyes.) This energy might go on to drive a chemical reaction that results in the breakdown of the dye itself or the fiber, or it might just warm up the fabric a tiny bit. Just as the metal ion can alter the color of a dye, it can also alter its chemical properties. Perhaps the metal ion might cause a particular dye to react in such a way that it does not produce harmful oxygen radicals that result in dye fading.

Some of the other abstracts I was looking at talked about the mutagenicity or carcinogenicity of madder and related natural dyes. Apparently madder has traditionally been used as a medication for kidney stones. It's probably safe to work with if you wear gloves and take the usual precautions, but it's not safe for use as a food additive or drug, for consuming large quantities, and it makes sense to be careful with this and all other chemicals. It's funny how often people assume that natural dyes are safe and synthetic dyes are unsafe, when in fact there are both safe and unsafe dyes among both groups.

Now the fact that alum may improve the lightfastness of madder says absolutely nothing about whether it does so for onion skins or any other particular dye. It indicates that it is possible, as a concept, but you'd have to actually test this, or find someone who has tested it, in order to have any confidence that it works. It's not a general rule that mordants always aid lightfastness! There are a lot of traditional recipes that are based more on someone's whim than on any actual observations, though you tend to see this more in herbal medicine than in textile dyeing.

Keeping a garment away from light can be done to some extent. If your dye is not as lightfast as you'd like, you should dry garments dyed with it indoors, in a dryer or by hanging in a dark room, rather than outside on a clothes line, and you should store it in a closed drawer or a closet whose door is kept closed, not on an open shelf or hook. You might want to avoid wearing a particularly fragile garment to an outdoor activity such as an all-day picnic.

Paula