Colourfast dyeing usually requires a mordant. Mordants are metallic salts that facilitate the bonding of the dyestuff to the fibre. Cellulose fibres also require a tannin in order to bond well. Tannins are not technically mordants (they are not metallic salts) but they are often included when speaking about the mordant process for cellulose fibres - as in, “mordanted with alum at 15% WOF and and myrobalan at 5% WOF.”

Some natural dye recipes still call for the use of heavy metal mordants such as chrome. Historically these were introduced during the industrial revolution and we do not recommend them. Heavy metal mordants can be toxic, presenting real challenges for safe use and disposal. Moreover most colours obtained through the use of heavy metals may be obtained through overdyeing or variations in the dye procedure. For those who wish to obtain a mordant from plants, Symplocos is a natural bio-accumulator of alum.

Mordant procedures for protein and cellulose fibres are not interchangeable.

Alum

Potassium Aluminum Sulfate

Potassium aluminum sulfate is the mordant most frequently used by dyers for protein (animal) and cellulose (plant) fibres and fabrics. It improves light and washfastness of all natural dyes and keeps colours clear. It is inexpensive and safe to use. This form of alum is refined from bauxite, the raw state of aluminum ore, and is free from the impurities (such as iron) some other alums may contain. Use at 15% WOF.


Aluminum Acetate
Aluminum acetate is often the preferred alum mordant for cellulose fibres and fabrics. It is refined from bauxite with acetic acid as a purifying agent. Some dyes develop to a richer shade on cellulose when mordanted with aluminum acetate. Aluminum acetate is the recommended mordant when printing with natural dyes. It is more expensive and sometimes hard to find. Use at 5-8% WOF.


Homemade Aluminum Acetate
Aluminum acetate can be made from sodium acetate and potassium aluminum sulfate. Depending on the availability of these materials in your area, this can be cost effective.

To make enough aluminum acetate to mordant 1 kilo of fabric, combine in 3 litres of hot tap water:

  • 150 g sodium acetate or calcium acetate

  • 150 g potassium aluminum sulfate

This can be added to your mordant bath (see the how-to section).

For Mordanting Instructions see: