What's the difference between mordants and other chemical assistants used in dyeing?

Sodium carbonate is not a mordant! Neither are vinegar or salt. A mordant is a metal ion which attaches to the fiber, usually by being boiled together for a length of time. A dye which has no natural attraction to the fiber can then attach to the metal ion. Most but not all natural dyes are mordant dyes, which require the metal ion to be in the fiber in order for them to have any attraction to the fiber. There are also some synthetic dyes that are mordant dyes.

Mordants include alum, chromium, copper, iron, and tin. Some of these metals are quite toxic and hazardous, in addition to be environmentally damaging. Chromium is the most hazardous of mordants. The hexavalent form of chromium, in potassium dichromate, which called for in some recipes, is a known human carcinogen. Alum is the least toxic of the mordants, though it can be irritating and should be used with care.

Tannic acid is another important mordant, a large, complex, metal-free molecule, used mainly in dyeing cellulose fibers, such as cotton. Tannic acid can be obtained from many plant sources, such as the galls on oak trees. A typical recipe for using tannic acid as a mordant calls for boiling cotton first with alum, then tannic acid, then alum again, each in a separate boiling water bath, followed by boiling with the mordant-requiring dye. I think that the alum attaches to the cotton, the tannic acid attaches to the alum, and the dye attaches to either the tannic acid or to another alum ion attached to the tannic acid. This is much more complex, and much less resistant to washing out, then the bonds formed by reacting a modern synthetic fiber reactive dye directly with the fiber.

Most synthetic dyes have no requirement for mordants. Fiber reactive dyes, such as Procion MX dye, have no need whatsoever for a mordant, because they form chemical bonds directly to the fiber. The dyeing process for these other dyes does make use of other chemicals, but not mordants. In some cases a mordant will improve the washfastness of an acid dye on wool, but there is no need to use a mordant with most synthetic acid dyes.

A low pH is useful for dyeing protein fibers such as wool, and a high pH is useful for dyeing cellulose fibers with reactive dyes. In neither case is this achieved by using a mordant. Instead, pH-adjusting chemicals (which are NOT mordants) are used, such as sodium carbonate or vinegar, though they have opposite effects from each other and are used on different fibers.

An acid such as vinegar or citric acid is used as an auxiliary chemical to reduce the pH of the dyebath for dyeing with acid dyes, but acid is not in itself a mordant. It is a coincidence that some chemicals that are used as mordants, such as tannic acid, have the word 'acid' in the names; it reflects an unrelated aspect of their chemical structures. Vinegar and citric acid, like sodium carbonate, are not mordants, though people who do not understand dyeing sometimes refer to them as such. Acids work as dye auxiliary chemicals by reducing the pH of the dyebath to allow the protein fibers to ionize, which aids in allowing them to form hydrogen bonds to the dyes.

Sodium carbonate is a chemical that is used to increase the pH of the dye bath. A high pH causes the cellulose molecule to be deprotonated in order to form a cellulosate anion, which then can attack the fiber reactive dye, after which a permanent covalent chemical bond is formed to attach the dye to the fiber. Deprotonating cellulose with a high pH does not work for dyeing cellulose with other classes of dye. There is little or no advantage in using soda ash for another class of dye. Soda ash will not improve the attachment of, for example, an all-purpose dye to cotton.

You can see a drawing of the chemical reaction between a dichlorotriazine dye, Procion Red MX-5B, and a cellulose molecule, on my dyeing Q&A blog entry for May 19, 2005 (click on this link).