How to Use Metal Mordants

By using alum and iron as metal mordants you can increase the color quality of your home dyeing project.

  • The Handbook of Natural Plant Dyes Book Cover
    “The Handbook of Natural Plant Dyes” by Sasha Duerr is full of helpful tips and recipes for home dyeing enthusiasts.
    Cover courtesy Timber Press
  • Fig leaves in a steel pot
    Fig leaves in a stainless steel pot ready for extraction and iron mordanting.
    Photo courtesy Timber Press

  • The Handbook of Natural Plant Dyes Book Cover
  • Fig leaves in a steel pot

Home dyeing can be a gamble if you are new to the idea, but even if you are experienced in the art, knowing the reaction difference between animal fibers and plant based fibers can be crucial to proper dye absorption. Using The Handbook of Natural Plant Dyes (Timber Press, 2010) Sasha Duerr walks you through using mordants and natural dyes in perfect harmony. This excerpt explains the use and handling of metal mordants.

Metal Mordants

Handle all mordants with care. When working with mordants, always wear gloves and a dust mask. And when working with mordants and heat, keep the mordants and dyes well below the boiling point so fumes aren’t created that you might inhale.

The only metallic mordants we will be working with in this book are alum and iron. These mordants are nontoxic when used in the proper proportions. But they can be irritants and caustic, so work with them carefully to avoid any health hazards. When handled properly, alum and iron are excellent, easy mordants to use, to get longlasting and even dramatic colors.

Using Alum as a Mordant

Alum, or aluminum sulfate, is a substance found in the earth, and has been used as a mordant for thousands of years. Alum will brighten colors and will make many natural plant dyes colorfast. Since some plants love alum, you can often find it at gardening stores in the fertilizer section. And since it is used for pickling, you can usually get it at the grocery store in the spice section. Or try a specialty textile supplier. Alum can help extend the range of colors your plant dye can achieve, often allowing your dye pot to bloom into brighter and more intense color.

Using Iron as a Mordant

When iron (ferrous sulfate) is used as a mordant, it often turns dye colors darker in tone. Iron is an earth-based substance, and we use it in powder form as a mordant material. It can be ordered from dye specialty stores or made yourself; a little goes a long way. Iron can be used as a premordant, but works just as well as an aftermordant. Iron can extend or alter the color from the initial dye bath. Iron usually takes effect very quickly, darkening the color or sometimes changing it completely.

Iron is a natural color modifier. Modifiers are usually applied to the dye bath to alter the color after the initial dyeing has occurred. Some modifiers, like iron, can also be mordants, but most modifiers change the color but do not help bind the color to the fiber unless a mordant has been used first. You can create iron liquid solution ahead of time and store it in clearly labeled in glass jars. One of the first traditional mordants was probably mud with a high level of iron in it. Ancient peoples may have become aware of iron’s mordant abilities when watching leaves fall into iron-rich water and turn black from the chemical change.

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