This is pure lye, also known as caustic soda or sodium hydroxide. It is an extremely strong alkali used in the cooking of tough fibers such as hemp and flax and other plant fibers that don't break down when cooked in milder alkali such as soda ash.
We suggest using 3 ounces of lye per pound of dry fiber. Lye can cause serious burns if not handled properly. First add lye to a small amount of cold water and then pour the solution into a pot of cold cooking water. Never add lye to hot water. Always wear eye protection and gloves.