Imagine rinsing a piece of clothing only for most of its color to wash away. When we put on clothes or pick out a new fabric, we rarely think about the unseen chemical processes that shape how a fabric's color behaves over time. Mordants work to enhance the durability of the dyed fabric by establishing a chemical bond between a fabric and its dye.

This French manuscript illumination from De Proprietatibus rerum ("On the Properties of Things") by Bartholomew of England depicts the dyeing of fabric in the 15th century. Mordants would have been used in the dye bath to make the dye adhere more permanently to the fabric.

This 17th-century Dutch print depicts a dyer pulling up cloth from a furnace designed to dye textiles. In the background, workers rinse out the cloth in a river or canal.

The verse below the print explains that the "pale woolen cloth" is uniformly soaked through with dye to be "reborn", analogous to how Christ's blood soaks people's souls "to please the Eternal Eye."

Mordants enabled dyers to impart uniform color on textiles. They were also used to hand-paint long-lasting, intricate designs, such as this elaborate Indian export cloth (made around 1675-1725 for the Japanese market) depicting figures, plants, and architecture.

We selected three natural fabrics to mordant and dye for our experiment. From left to right, you see untreated regular weave cotton fabric and two types of raw silk: a silk crepe produced by cultivated silk worms and so-called tussah silk produced by wild silk worms.

The mordants in our experiment fell into one of two categories: metallic or plant-derived. Because of the differences in their chemical composition, these mordants form different molecular bonds with dyes and fabrics. Metallic mordants form coordination complexes, while plant-derived mordants form hydrogen bond networks.

The strength of these bonds also depends on the fabric, such as whether it is plant-derived (like cotton) or animal-derived (like silk).

Aluminous rock is first mined, and then heated and purified to form alum, which can be used as a metallic mordant.

1. We began by heating and preparing our mordant baths.

2. Then we cut our textiles into small samples.

3. Next, we soaked the samples in our mordant baths for two hours.

4. We laid the pieces of mordanted textiles out to dry.

6. The plant material was filtered out to create a dye solution.

8. Once the textiles were dry, we compared the undyed mordanted samples to the mordanted samples that we had dyed.

Colors of plant-derived mordants

The plant-derived substances yielded a range of yellows and browns. While the difference in the end color was quite dramatic between the different metallic mordants, the color nuances between the cloths treated with plant-based mordants were more subtle.