Indigo, and Dyeing One Color over Another

Every dye so far dissolves in water and soaks into mordanted wool. Indigo doesn’t, and that makes it one of the most interesting things you can do in a dye pot.

How a vat works

Indigo is one of the oldest dyes known, probably first used in India and Egypt. When it reached Europe in the 1500s it met much opposition from growers of a similar native dye, woad. On wool and cotton alike it gives a dark blue, very fast to light and washing, and it needs no mordant ¹.

The trick is that indigo won’t dissolve in water. It’s a vat dye: it has to be chemically reduced and dissolved in an alkali before fiber can take it up. Yarn dipped in a working vat comes out greenish yellow, and turns blue in the air. You build the color by dipping and airing over and over until it’s dark enough ¹.

The fermentation vat

The fermentation vat, or blue-pot, is the oldest method. Bacteria growing in the vat do the reducing. It takes patience, but the USDA found no difficulty getting a fast dark blue on wool or cotton from a well-made one ¹.

For each pound of wool or cotton ¹:

• 8 ounces finely powdered indigo
• 4 ounces wheat bran
• 4 ounces madder
• 1½ pounds washing soda (sodium carbonate)
• 4 gallons water

Mix them and keep the pot in a warm place, about 85°F, for 5 to 10 days, stirring it well each morning. It’s ready when it develops a disagreeable odor, a bluish, coppery scum on top, and green streaks through it ¹.

To dye, wet the yarn thoroughly and dip it in the lukewarm vat, lifting it into the air at intervals. Keep dipping and airing for 30 minutes, squeeze out the dye, and let it air half an hour. Repeat, leaving the yarn in longer each time, until the blue is dark enough; fewer rounds give medium and light blues. Then rinse in lukewarm water and dry ¹.

Two vat rules. Don’t disturb the sediment at the bottom, or the yarn streaks. And don’t stir too much, or the vat turns blue and won’t dye. If that happens, add more indigo, bran, madder, and soda; in a day or two it’s ready again. Kept up this way, a blue-pot can be used many times ¹.

The hydrosulphite vat

The USDA called the hydrosulphite vat the easiest to regulate, and in 1935 it was the one in wide commercial use. It uses a chemical reducer, sodium hydrosulphite, and lye (sodium hydroxide) instead of bacteria and soda. The stock solution is 4½ ounces of powdered indigo mixed with 3 ounces of lye dissolved in water, made up to a gallon and heated to 120°F, with the hydrosulphite stirred in slowly; after half an hour it should be clear and yellow ¹. Lye is caustic. If you make this vat, wear gloves and eye protection.

The USDA’s dipping routine for this vat: wet the cloth, keep it under the surface for 30 minutes, hang it in the air for 30 minutes without rinsing, and repeat until the color is right. Then rinse, wash in soapsuds, and rinse again ¹.

Top-dyeing

Top-dyeing, dyeing one color over another, is how natural dyers get greens, purples, and blacks. The USDA’s rule: only clear colors make clear colors. For a good green, start with a bright, clear yellow, not a muddy yellow-tan, and top it with a good blue. Long boiling dulls yellows, so don’t overdo them ¹. The bulletin’s color wheel explains the rest: yellow and blue make green, blue and red make violet, red and yellow make orange, and a little of a color’s opposite on the wheel softens it ¹.

Three combinations that need nothing but alum, all from the bulletin ¹:

• Green. Mordant wool with alum and dye it with goldenrod. Rinse well and dip it in either indigo vat until the green is dark enough: a dark yellow-green. You can also dye the indigo first and the yellow second; if so, mordant with alum before or after the indigo.
• Black or gray. Mordant with alum and dye a medium blue in indigo. Rinse, then dye with black walnut hulls following the dark brown recipe. The result is black or gray, depending on how deep each color went.
• Terra cotta. Mordant with alum and dye with goldenrod. Rinse, then put the yarn in a bath of 4 ounces of madder, soaked overnight, heat gradually to 140–160°F, and hold it there 30 minutes.

The USDA’s red-purple, indigo topped with cochineal, used its tin-based cochineal recipe ¹, so it’s not listed here.

Try it

Take one of the goldenrod skeins from the yellows lesson and divide it. Leave one half yellow; top-dye the other in madder for terra cotta. If you set up an indigo vat, dip a third piece for green. Hang the three together; that’s the whole idea of top-dyeing.