Natural Dyeing- Comparing Mordants

Natural dye tests with alum and iron on silk and wool

Synthetic dyes have only been around for about 150 years, which is relatively short in the timeline of textiles. Dyeing with plants, lichens, and insects has been practiced for at least 4,500 years. Although textiles are notoriously absent from the archaeological record, some conditions have allowed for the preservation of fabric that still retains evidence of being dyed. Early Iron Age bog finds from Northern Europe and woven textile scraps from the Hallstatt salt mines in Austria dating from as early as the Bronze Age show the use of natural dyes. In ancient Egypt, cloth from about 2500 BCE has been found to contain dyes, and additional records of dye recipes support the existence of the practice. It is likely that people had been experimenting with dyeing for much longer than this.

While the use of natural dyes is uncommon as a conservation technique due to their unpredictability, it is important to understand the process and its conservation implications. Dye fading, usually by light and UV, can erase or dramatically alter original colours in both natural and synthetic dyes. Seeing what an original colour may have looked like before time took its toll is informative, although it must be noted that the actual hue produced can be incredibly variable based on the species, origin, and growing conditions of the dye material as well as an almost infinite combination of dye processes and recipes that depend on concentration, impurities in water, temperature, dyeing times, and nearly any factor imaginable. Undertaking your own dye experiments certainly gives an appreciation for the expertise of dyers from the past!

Fabric is generally dyed by one of three processes: direct, mordant, or vat dyeing. Direct dyeing is the most straightforward, requiring only the fabric, water, and the dyestuff. Turmeric is a common example of this, although it should be noted that it has poor light-fastness, which means that it is likely to fade when exposed to any source of light, natural or artificial. Vat dyes, such as indigo and woad, rely on a more complex chemical process that allows a dye compound that is originally insoluble in water to chemically change and become attached to the fibre.

Mordants, from the French mordre, or “to bite,” are compounds that bind to both the fabric and dye compound, creating a bond strong enough to prevent the dye from being washed away and result in fading or loss. While these compounds are usually metal salts such as iron, tin, and alum, tannic acid has also been used.

The choice of mordant affects both the colour achieved and the strength of the fibre itself. Iron is what is called a “saddening” mordant, as it tends to produce colours that are duller, cooler, and darker than alum, even when the same dyestuff is used. It can be utilised to produce the somewhat elusive black. Unfortunately, iron mordants often cause weakness within the fibre. This can sometimes be seen in printed textiles and tapestries when only a certain colour such as black or yellow appears to have deteriorated.

Despite the impressive range of hues achieved in historic textiles, the number of dyes actually utilised is relatively small. For one of our last days of the term, we got the chance to play around with some of these dyes. In addition to dyeing cotton with indigo, we also carried out mordant dyeing with iron and alum on wool and silk. For reds, we tried the oft-used madder root and the insect dye cochineal. We also tried out old fustic for yellow and logwood, which was often used to produce a range of blues, purples, greys, and blacks.

Madder root

Dried madder root, used in dyeing oranges and reds.

The end result was an interesting comparison between mordants and materials. Side-by-side, it was easy to see the “saddening” effect of the iron mordant, which produced a more sombre set of colours than the cheerful red, yellow, and orange created by the alum mordanted fibres. The rich red and sunny orange produced from cochineal and madder on alum turned to a warm brown and blue-grey with iron. The silk, with its smooth and glossy surface lent a lustrous richness to the colours that was dampened by the rough surface of the wool fibres.

It was interesting to be able to sense the weakness introduced by iron mordanting even on new fibres. The wool in particular seemed “fuzzier” than its alum counterpart, and it was easy to see how the fibres would be more prone to breakage. Despite some of the beautiful complex colours created by the iron mordant, this experiment has actually nudged me away from using iron in my personal dyeing, as I have concerns for the longevity of my products.

Although I have done some natural dyeing myself, each time is a learning process. Of the four mordant dyes, I had only ever used madder with alum, and actually achieved a much calmer red as compared to the bright orange that we got in the lab. That alone highlights what is so difficult but interesting about natural dyes. The unpredictable nature is both nerve-wracking and delightful. I am looking forward to using beautiful, rich cochineal and subtle logwood purple in future projects, as well as trying more overdyeing combinations.

For a little more dye history and chemistry:

Natural History Museum – Seeds of Trade

Encyclopaedia Britannica – Dye

Some more on natural dyeing:

Paradise City Homestead- Dyeing with Marigold. Although marigold isn’t an historically important dye, it does produce a beautiful, cheery golden yellow with alum on wool.

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