Is Mimosa Hostilis Dye Colorfast?

Colorfast is two questions, not one. Washfastness is whether the color survives water, soap and rubbing. Lightfastness is whether it survives daylight. A dye can do well on one and poorly on the other, and Mimosa hostilis root bark behaves differently on each.

Why tannin-rich bark holds its color

Mimosa root bark is heavy with tannins, the same family of compounds that turns hide into leather. Tannins bond to the protein in wool and silk on their own, which is why this bark can dye protein fiber without a separate mordant, and why the color it leaves does not simply rinse out. What tannins do in a dye bath goes into the chemistry.

Washfastness: good on wool and silk

Once the fiber has cooled in the bath and been rinsed until the water runs clear, the color left behind is well attached. Most dyers find it survives ordinary hand washing without noticeable loss. What does take color out is heat and alkali together: a hot wash with a strongly alkaline detergent will pull the purples toward brown and lift some depth. Wash in cool or lukewarm water with a pH-neutral wool wash, and do not soak for hours.

Lightfastness: it depends on the shade

The rose, pink and lilac tones are the most delicate part of the range. Hung in a sunny window they soften and warm over months, drifting toward tan. The deeper plums and burgundies last longer simply because there is more dye in the fiber. The slate, grey and charcoal shades that come from an iron afterbath are the most lightfast of all, because the iron and tannin form a dark compound that fades very slowly.

How to make the color last longer

• Mordant with alum first. Alum at 10 to 15 percent of the dry fiber weight brightens the color and measurably improves how well it holds in light. Mordanting wool before a bark bath gives the method.
• Dye deep, not pale. More dye in the fiber means more to lose before you notice a change.
• Let the fiber cool in the bath. Taking wool out hot gives the color less time to bond.
• Rinse until clear. Loose dye left on the surface is what rubs off onto hands and other clothes.
• Use iron for the pieces that will see sun. If a shawl will hang in daylight, the grey end of the range will outlast the pink end.

Testing your own results

Water, fiber and mordant all change the answer, so the only colorfastness figure that matters is the one from your own pot. For light, cut a dyed swatch in two, keep one half in a drawer and tape the other to a sunny window for two to four weeks, then compare them side by side. For washing, stitch a strip of undyed white cotton to a dyed swatch, wash them together as you would the finished piece, and check the white for staining. Testing a dye before you commit a fleece covers the rest of the sampling routine.

Cotton and linen

Plant fibers carry no protein, so the tannins have less to hold on to and the color is paler and less durable unless the fiber is mordanted with tannin and then alum before dyeing. With that preparation the results come much closer to wool. Wool, silk, cotton and linen in the same bath shows the difference on one set of swatches.