Dyes, Mordants, and Fibers

Before synthetic dyes, color came from plants, bark, roots, insects, and minerals. In 1935 the U.S. Department of Agriculture’s Bureau of Home Economics tested about 65 natural dye materials on wool and cotton and published recipes only for those that held up to light and washing ¹. That bulletin is the backbone of this course.

Wool is the easy fiber

The USDA’s first lesson: of all textile fibers, wool dyes most easily and its colors change least. It combines with almost every dye. Cotton doesn’t combine with dyes nearly as well, and fast colors on it take complicated processes ¹. A modern dye supplier gives the same advice: start with wool. Cotton and linen work, but the results may be less pleasing or less permanent ². This course uses wool yarn throughout and notes cotton where the method differs.

Why dyes need a mordant

A few natural dyes, such as black walnut hulls and some lichens, bond to wool on their own. Dyers call these substantive dyes. Most are adjective dyes. They need a mordant, a metal salt that goes deep into the fiber first. When the dye goes in, it combines with the mordant to make a color, and because the mordant is embedded, so is the color ². Without one, many natural dyes “fade and bleed badly” ¹.

The mordant also changes the color. On wool, dahlia flowers give orange with chrome and light yellow with alum, and cochineal gives red with alum and purple with chrome ¹. That’s why one dye pot can make a whole family of shades.

Which mordants

The 1935 bulletin used four: alum, chrome (potassium dichromate), copperas (iron sulfate), and tannin ¹. Older books add tin and copper. Today you should use three of them:

• Alum (potassium aluminum sulfate), usually with cream of tartar. It’s the most common mordant and barely changes the color. When in doubt, mordant with alum ².
• Iron (ferrous sulfate, or copperas) “saddens” colors, darkening them and bringing out greens. Too much makes wool harsh ².
• Tannin, from oak galls, sumac leaves, or tannic acid. Cotton takes it much better than metal salts, so it’s used to prepare cotton ¹.

Leave out the others, even when an old recipe calls for them:

• Chrome. Potassium dichromate is a hexavalent chromium compound, and OSHA states that Cr(VI) is known to cause cancer and damages the lungs, kidneys, liver, skin, and eyes ³. Many of the USDA’s best yellows used it. This course substitutes alum and tells you what that costs in color.
• Tin (stannous chloride) is corrosive to skin and eyes ⁴ and makes wool brittle ².
• Copper sulfate (blue vitriol) is poisonous ², causes eye damage, and is hazardous to aquatic life ⁴.

Even iron sulfate is harmful if swallowed and irritates skin and eyes ⁴. Keep every mordant stored away from children and animals, and don’t breathe the steam ².

Equipment

The USDA’s list is short: scales that weigh fractions of an ounce, a kettle big enough to cover the yarn, pails for rinsing, measures, cheesecloth for straining, smooth sticks for stirring, a thermometer, rubber gloves, a clothesline in the shade, and plenty of soft water such as filtered rainwater ¹.

Two cautions on the pot. Use unchipped enamel or stainless steel. Iron kettles darken colors and tin ones make them harsh ¹, and brass, copper, or iron pots act as mordants themselves ². And once a pot has held mordant, it’s a dye pot for good: never use the same pots for cooking and dyeing ².

Try it

Find a large enamel or stainless pot at a thrift store and mark it for dyeing. Buy a pound of undyed, light-colored wool yarn, a pound of potash alum, and a few ounces of cream of tartar. That’s everything the next lesson needs.