Aluminum Sulfate is commonly used as a mordant by Natural dyers of both protein and cellulose fibers. It is used to improve both the lightfastness and washfastness as well as creating bright clear colors. Aluminum Sulfate is technically not an alum although it is commonly referred to as one.
It is safe when handled properly and relatively inexpensive.
Aluminum sulfate results from the refining of Bauxite which is raw aluminum ore. Sulfuric acid is used during the process to remove most of the impurities such as iron and silica.
Potassium Alum is created by further purification with potassium. This results in fewer impurities, especially iron and results in brighter colors. Potassium alum is more expensive but preferred by many natural dyers.