Dharma Fiber Reactive Dye must be 'fixed' (made permanent) with soda ash on cotton and other cellulose fibers. It's a mild alkali that enables the reaction between the dye and the cellulose fibers at lower temperatures, causing them to "become one", and the reason Fiber Reactive Dye is so permanent. How much you use is normally a function of the amount of water, not the amount of fabric or dye. Roughly 1 oz. per gallon of water is needed when garment dyeing or 1 cup per gallon of water as a pre-soak when tie-dyeing. If you don't use enough, the dye won't yield full strength. A pH of 10.5 is ideal. It can also be used to fix fiber reactive dyes on silk when you don't want to use a lot of heat (as with steaming or the simmering with vinegar method), but BE CAREFUL, as it can reduce the sheen, or even cause silk to fall apart - use half as much, and expose the fabric to it for less time than cotton (presoak right before tie-dyeing and cure for no more than 4 to 6 hours). The thinner the silk, the more likely it will be damaged by Soda Ash. An alternative on silk is the much weaker alkali, baking soda, then curing several days or steaming or microwaving to "set". Wool feels "brittle" after exposure to Soda Ash. Acid Dyes are better for wool, and silk too, but must be cured with lots of heat/steam.
Caution! The soda ash solution is mildly caustic - it can irritate your skin and you definitely do not want to get it in your eyes or breath the dust. Treat it with the same respect as you would treat powdered bleach or other caustic laundry products. Keep away from children and pets!
Wear at least an N95 dust mask when mixing the powder, gloves and eye protection. Rinse off at once to avoid eye and skin irritation. If you get it in your eyes, immediately flush with water for 15 minutes and call you doctor if you have remaining irritation.
Also, protect all surfaces that might be vulnerable to this common caustic household product.