Potassium Aluminum Sulfate is probably the most commonly used mordant by Natural dyers of both protein and cellulose fibers. I is used to improve both the lightfastness and washfastness as well as creating bright clear colors. 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.
Use at 12-20% WOF