While some natural dyes give beautiful, lasting results all by themselves, most benefit from the use of a mordant. Some dye and fiber combinations really need a mordant to get any notable amount of color. Other dyes may not need a mordant to bind to a fiber, but give a much more brightly colored result when used. A few mordants may also significantly change the color that binds to the fiber.
What is a mordant?
A mordant is a substance that helps bind dye to a fiber. The term is believed to derive from the Latin word mordere, meaning “to bite”. Most mordants are acids or mineral salts, including things like tannic acid, oxalic acid, ferrous sulphate, copper sulfate, and potassium dichromate.
While there are many modern manufactured options for mordants, many of these compounds were available prior to the rise of synthetics. Some of them, like tannic acid, are naturally occuring. Others, most notably some of the metallic salts, can be extracted from metal goods or the waste from ore processing.
How are mordants applied?
Mordants can be applied before, during, and after dyeing. The technique most commonly referred to as mordanting is technically a premordant, meaning the fiber is soaked in the mordant before dye is applied. Another mordanting technique involves adding the mordant to the dye bath, or even preparing the dye bath in a reactive metal pot that adds metallics salts to the dye bath as it’s heated. Copper and iron can both be added to a dye bath with relative ease using this approach. Finally, a mordant can also be applied after the fiber is dyed. This method is more commonly called a modifier and typically is used less to bind the dye to the fiber, and more to change the color of the dye. Iron in particular is known for being able to darken and desaturate a dye color when added at any stage, but can be used as a modifier to avoid shifting the color of the entire dye bath.
Mordants, like dyes, are typically applied in a ratio of a certain weight of mordant material to weight of fiber (WOF). Most of the time this is discussed as a percentage. For example, if dye directions say to mordant using alum at 10% WOF and your fiber weighs 100g, you would use 10g of alum in your mordant mixture. Mordants typically use a much smaller ratio of material to fiber than dye baths do. There is not really a correct percentage, but different ratios will yield different results.
Time, much like weight ratio, can also vary. For best results, most fibers should be submerged in a mordant solution for several hours. Overnight generally works well, but longer is fine in most cases.
Where did medieval people get their mordants?
While Amazon, specialty shops, and occasionally even the grocery store may address most of our modern mordant needs, the past was a bit different. Dyeing in a metal pot is probably the easiest method for adding metallic salts to a dye bath if you have access to one, but can’t order a big cannister of ferrous sulphate. Iron smelt, rusty nails, or any other oxidized iron can be submerged in acetic acid (vinegar) to create a very simple iron mordant solution.
Some mordants can be derived from plants, even some metallic salts. Clubmoss is well known as a dynamic accumulator of aluminum, meaning it absorbs in well from the soil and holds it in the structure of the plant. Clubmoss was a likely source of alum mordants before the modern era. Tannic acid is similarly widely accessible from the plant kingdom. Oak, pomegranate peels, chestnut bark, and sumac all contain tannins that can function as mordants.
These sources can’t be controlled in the same way a modern dyer can control their outcome with precise measurements of specific compounds, but they do function as mordants. This is not without its downsides, however, as some historic textiles show signs of deterorioration directly connected to the use of certain mordants. Iron in partiucular is well known as a source of textile rot.