Using ammonium sulfate for dyeing

Ammonium sulfate, (NH₄)₂SO₄, is an acid-forming salt, used as an acid in dyeing wool. It can be substituted for vinegar or citric acid. Its great advantage is that it forms acid slowly, starting from a neutral ph when first dissolved in room temperature water, and releasing more acid as the dyebath is heated to around 150°F. The gradual release of acid promotes more level colors than citric acid produces; that is, smoother, more perfectly solid colors, as opposed to mottled colors.

Unlike the similar-sounding but very different chemical aluminum sulfate, which is a mordant (also known as alum), ammonium sulfate doesn't harm the feel of the wool.

How much ammonium sulfate to use

When dyeing with Washfast Acid Dyes, use one teaspoon of ammonium sulfate per gallon of dyebath (5 ml per 4 liters).

A recipe posted in the DyersLIST mailing list in 1998 by Don Weiner, formerly of PRO Chemical & Dye, calls for 13.6 grams of ammonium sulfate per pound of yarn in 2.5 gallons of water.

ProChem's WashFast Acid Dyes immersion dyeing recipe calls for the use of one tablespoon, or 15 grams, of either citric acid OR ammonium sulfate, or 11 tablespoons (165 ml) of distilled white vinegar, to dye one pound of wool, silk, angora, or mohair in 2.5 gallons of water.

Dharma Trading company advises the use of one tablespoon of ammonium sulfate per pound of fiber when dyeing with their milling dyes (which are like the WashFast Acid Dyes) and premetallized dyes (similar to some dyes in the Lanaset series of dyes), especially for pale colors. (Dharma suggests also adding some citric acid or vinegar toward the end of dyeing in order to completely exhaust the dye.)

Where to buy ammonium sulfate

Both PRO Chemical & Dye and Dharma Trading Company sell ammonium sulfate powder for use as a dyeing auxiliary. Other specialty dye suppliers are likely to, as well.

Ammonium sulfate is also commonly sold as a fertilizer, with the numbers 21•0•0. I don't know whether fertilizer-grade ammonium sulfate is more likely to be contaminated with iron, as the mordant aluminum sulfate may be; if it is, that will affect your dye colors, making them duller and sadder.

Safety

Ammonium sulfate is safe to use with normal precautions. Ingestion of large amounts can be hazardous but seems unlikely to occur. In small amounts, ammonium sulfate is used as a food additive, but only food-grade materials should be used in foods, in case of unwanted contaminants. Wear gloves to prevent irritation of the skin, wear safety glasses if needed to prevent eye exposure, and wear a dust mask if a significant amount of the powder appears to be dusting into the air. For disposal, can be used as a fertilizer, keeping in mind that overuse of fertilizer can burn plants, so it's better added to a compost pile than dumped directly on the lawn.