Mordant means to bite or attach. We often use mordants in natural dyeing to encourage the dye colour to affix to the fibre. Most mordants are minerals, and some of them can be harmful to humans and environment in large quantities.
In this thread, let's talk about mordant safety: how to use and dispose of mordants without harming ourselves or the environment around us.
A mordant is a chemical that becomes part of the molecular bond between the fiber and the dye. Primarily these are metal salts. (They are salts in the chemical sense of the word – the hydrogen atom of an acid is replaced with a metal ion. They are NOT edible salts.) You can think of a mordant as a molecular glue. In general, dyes and fibers have a weak affinity for each other. If you tried to dye yarn without a mordant, the color would be very dull, and it would wash out promptly and fade easily. A mordant sticks to fiber well, and it also sticks to dye well.
Alum (aka Potash Alum)
The most common mordant used. It’s considered a neutral mordant, in that it does not result in a color that is appreciably different than that of the dye bath. It is considered to have good color fast properties, though other mordants result in even more color fast shades.
This is also a chemical used to strengthen magnolias so when I dye, I put the cool alum bath near the tree roots.
Iron (aka Copperas or Green Vitriol)
Ferrous Sulfate or Ferrous Sulfate Heptahydrate
Iron can be used as a mordant on its own, but it’s generally used as an afterbath, to modify color dyed on fiber that was initially mordanted with alum. It “saddens” color, making it more greenish-brown. Yellows become olive, and pinks become plummy purples. Protein fibers like wool are very sensitive to iron, and too high of iron concentrations or exposure for too much time can damage the fiber and/or make it have a harsh feel. This is why iron is generally used as an after-bath step, to modify color that has already been dyed using alum as a mordant. Exposure can be easily controlled, and the color is already fixed to the fiber with the alum mordant.
I've been making an iron solution out of rusty nails, vinegar and water. I put a tiny splash in a big pot and now I'm afraid to dispose of the used water. I keep using it over and over again, and the strength of the iron bath shows no sign of diminishing. Unfortunately, the article doesn't tell me how to dispose of this safely but I do know that potatoes love iron rich soil. Perhaps I can dilute it and put it there.
Copper (aka Blue Vitriol or Bluestone)
Copper is used to “sadden” colors, as it tends to turn them more blue-green. Yellows become greens, and pinks become purples. It can be used as both an after-bath to adjust an alum-mordanted color, or it can be used as a pre-mordant on its own. In the pre-mordant case, it’s typically used with citric or acetic acid to aid solubility, create a favorable environment for protein fibers, and increase copper uptake. Unlike iron, copper does not harshen protein fibers. The colors dyed with copper are generally more colorfast than those dyed with alum.
I haven't tried this yet, but I suspect it's the same chemical I feed to my goats. I wonder if we can make a solution like we do with iron. A bit of copper pipe, some vinegar and water... possibly?
Anyway, I'm still learning about natural dyeing. It can be amazing, but it also takes a lot of knowledge to use it safely.