Name: Vicki

Message: I am new to your website and very appreciative of your generosity in answering questions. I am learning to use natural dyes (madder, logwood,etc.) I want to order alum for mordanting cotton cloth primarily, but also possibly for linen, tencel, and bamboo. I would like to know which alum you would recommend (sulfate or acetate) based on both color fastness and safety?

Different aluminum salts that have been sold as "alum" for mordanting fabric for use with natural dyes include potassium aluminum sulfate, aluminum sulfate, and aluminum acetate.

Potassium aluminum sulfate is the most traditional alum mordant. Aluminum sulfate is not as good as potassium aluminum sulfate because it is more likely to be contaminated with iron, which will result in duller colors. If aluminum sulfate is certified as iron-free, it is suitable for use as an alum mordant.

However, aluminum acetate is the best alum mordant for cotton and linen, according to Jim Liles in his book, The Art and Craft of Natural Dyeing: Traditional Recipes for Modern Use. I strongly recommend his book for its wealth of well-tested recipes for many different natural dyes on a range of different fibers. He includes detailed recipes on how to make your own aluminum acetate if you cannot purchase it; I'd far rather purchase it, now that it is available from natural dyes suppliers.

He adds to be sure to treat alum-mordanted material with fixing solution, OR rinse thoroughly before dyeing. He describes the fixing solution as follows: 

Liles does not mention bamboo or Tencel, since they are only recently available. Most bamboo that is commercially available is in the form of a rayon; both Tencel (lyocell) and rayon would be mordanted like cotton. Bamboo fiber that is prepared enzymatically rather than by making ayon from it is more similar to linen, which has a higher content of lignen in addition to cellulose. In most cases, Liles groups linen together with cotton in the same recipes.

I would strongly recommend that you obtain a copy of Liles' book, since it has the best recipes for dyeing with natural dyes. Too many books on natural dyes almost completely neglect the fact that mordanting and dyeing on plant fibers requires very different mordants and recipes than natural dyeing of animal fibers. Don't follow any recipes that are not specifically labeled as being for a similar fiber to the one you are dyeing.

As far as safety is concerned, all forms of alum are safer than other metallic mordants (only the non-metal mordants such as tannin are safer, but they often cannot be substituted). The only serious safety concern is to keep them out of reach of children. For an adult, the fatal dose of alum is about one ounce, or 30 grams, an amount which seems unlikely to be ingested accidentally. The fact that children are smaller in size makes them much more vulnerable, since their fatal dose is correspondingly far smaller, and of course they are less likely to understand about health hazards. Be very careful to keep alum, and especially dissolved alum, away from young children. Always wear waterproof gloves when working with small quantities of alum; take additional precautions as indicated in the MSDS for larger quantities. 

Here are links to Materials Safety Data Sheets for the three forms of alum mordant, along with their Saf-T-Data ratings:
Aluminum Acetate, Basic: health 1, flammability 0,  reactivity 1, contact 2
Aluminum Sulfate: health 2, flammability 0,  reactivity 1, contact 2
Aluminum Potassium Sulfate: health 2, flammability 0, reactivity 1, contact 2
As you can see, aluminum acetate is considered slightly safer, with respect to health, than the other two form of alum mordant. It's convenient that the safest form of alum is also the most effective as a mordant.


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