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 most basic and strongest types of chemical reactions. This reaction happens gradually over time depending on temperature and/or the Ph level of the surrounding environment.
The Soda Ash pre-soak raises the pH level of the garment or fabric to approximately 10.5. Raising the pH level of the solution that the fabric or garment is soaked in raises the level of negative hydrogen ions in the dyeing environment. The chemical bonding process uses these ions in the reaction. Pre-soaking in Soda Ash fixer solution is what allows the fiber reactive dyes to work at room temperature.
The reaction can also be aided with heat. 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.