Shop towel dyes are made using a direct dye. Congo Red was the first direct dye synthesized by chemist Paul Böttiger in 1883 and patented in 1884, which proved to be the forerunner of a large class of synthetic dyes that are specifically for dyeing cellulose.
Direct dyes get their name from the process that is used for dyeing, wherein they are applied directly to the cellulose without the use of any mordant. Mordants used to be necessary in the dyeing process when trying to dye any cellulose with virtually any natural colorant, prior to the discovery of direct dyes.
Direct dyes are water soluble and can be applied to fabrics such as cotton, rayon, linen, hemp, jute, and ramie. Most of them have limited wet fastness in medium to full shades unless they undergo after-treatment.
A typical dye bath usually consists of dye, salt, and an anionic wetting agent. Water acts as the application medium while the dye dissolves in water, forming an ionized solution. The dye anions are then fixed in the fiber through hydrogen bonding. Adding salt, which is the electrolyte, is necessary for exhaustion of the dye as the cotton fibers take on a negative charge. The addition of the salt allows for the neutralization of the negative charge around the fiber, allowing the direct dye anion to deposit on the surface of the fiber and diffuse into it. Wetting agents may also be added to facilitate better wet-out of the fiber.