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Acidic dye is a type of dye that can be used to color certain fabrics, and is called acidic because of the acetic acid it contains. Bright and long-lasting colors are typical of acidic dye. Acidic dye is most effective on textiles that are made of animal hair fibers, such as mohair, wool, and alpaca. Silk, even though it is not technically hair, also dyes well. Most synthetic fibers do not respond well to acid dyes, with nylon being the notable exception.
When acidic dye is purchased, it comes in a concentrated, powdery form, in glass or plastic containers, or sometimes sealed in plastic, if it is a small amount. Acid dyes are very easy to use, and as such, are well suited to home and amateur use. They produce a uniform color, and are also very potent; half an ounce (14 g) of dye can color up to two pounds (0.9 kg) of material, depending on the exact shade.
The powdery dye dissolves in vinegar to form the acid dye bath. The rate at which the fabric is colored by the dye is controlled though the use of salt in the mixture. The chemical process by which acidic dye bonds to fibers is quite complex, and requires more than a passing knowledge of chemistry to fully understand. However, it is usually sufficient simply to know that the dye's particular chemistry is what controls the degree of colorfastness once the fabric has been dyed.
For fibers other than wool, the process of applying acidic dye takes place in a washing machine. This is done by setting the water temperature knob to the hot wash/ cold rinse position, then by filling up the washer to the depth needed to fit all of the fabric that will be dyed. Once this level has been reached, place the fabric in the water just long enough to get it wet, then set it aside.
At this point, the dye powder is added to the water and allowed to dissolve. When it has dissolved, the fabric is added and agitated for a few minutes, at which point about one cup (240 ml) of vinegar is added. When the machine has run through the entire wash cycle, the dye job should be complete. Once this process has been carried out and the fabric removed, the washer must be run through at least one rinse cycle to eliminate excess dye that might otherwise transfer onto other clothing later.
I have used acidic dye at home for various sewing and other DIY projects. It really is quite easy to use, and you can alter the dye a bit as well if you don't like the color. One thing I would suggest, and they often tell you this on the package, is to try some sort of dye test to see what they shade will be, which is as easy as dipping a small scrap into a bowl full of the dye.