The only type of vat dye that’s used to color Oriental rugs is indigo, which is derived from a plant but has been made synthetically since around 1890. Indigo, which is a deep purple blue, is an insoluble pigment that’s dissolved in an alkaline vat. The fiber is soaked in the indigo then aired. This causes the dye to turn blue through re-oxidation. Indigo is colorfast and doesn’t run.
To apply a mordant dye, the fiber is first soaked in a mordant or a type of metal hydroxide. The mordant is usually derived from iron salts or an alum solution. These colors are reds that are obtained from the roots of plants or from insects like lac or cochineal. They're also shades of yellow derived from plants like onion skins or vine leaves. The mordant can change the colors of these dyes. For example, madder is red when exposed to an aluminum mordant but turns blue violet with an iron mordant.
The challenge with mordant dyes is that they can be degraded by acids, even the weak acids found in wine. If the carpet maker neutralizes the acid with bicarbonate of soda, the color can be restored. Simply washing the fiber will remove both the dye and the mordant. Light can also cause mordant dyes to fade, so it's best to keep the Oriental rug in a room that isn’t subject to direct sunlight. Some people keep their mordant dyed rug in a shadowy room to protect it.
Most of these dyes are synthetic and only work with animal fibers like wool or silk. To dye these fibers, they’re simply dipped into the bath. Excess dye is washed out. Aniline direct dyes are applied to the fibers in an alkaline solution. They tend to be brilliant magentas, violets, blues and greens. The problem with them is that they also fade in strong light or even after being washed with soap. There are also acidic direct dyes, the most popular of which is the azo-dye. These dyes are yellow, orange, red or violet red, and at least one type of red can also be used on cotton and linen. They tend to resist fading in the light. They also resist alkalis and acids, though some of the colors run. Chrome dyes are types of azo-dyes that use a mordant called chromic oxide. They also resist light, alkalis and acids. However, the colors of chrome dyes are less intense than other direct dyes.