Fiber reactive dyes attach permanently to cellulose fibers using a covalent (electron-sharing) bond. These molecules carry a "chromophore" which absorb varying spectrums of the light, allowing only certain spectrums to reflect.
Covalent bonding is the one of the strongest types of chemical bonds. This reaction happens gradually over time depending on temperature and/or the Ph level of the surrounding environment.
Pre-soaking in Soda Ash fixer solution is what allows the fiber reactive dyes to work at room temperature. The Soda Ash works as a catalyst and raises the pH level of the water to approximately 10-11. Raising the pH level of the solution removes hydrogen from the cellulose structure and raises the level of positive hydrogen ions in the dyeing environment. This frees up sites on the cellulose for the dye molecules to bond too.
The reaction can also be aided with heat, so putting your items near a warm location will speed the reaction. Some tie-dyers have had success with using baking soda and microwaving their dyed articles. Since baking soda is a weaker alkali than Soda Ash, it must be accompanied by heat. Some people who are "chemically sensitive" choose to use this method.
The dye is allowed to react in a desirable host environment for up to 24 hours. After this time, the bonding sites on the cellulose should be saturated with dye molecules. Excess dye molecules that have not bonded permanently are washed away using warm water rinse and a dye-carrying detergent like Synthrapol.
For more information see our lesson plan on Color and Chemistry.