Students often ask for clarification on alum. What is it and what are the different products being sold as alum. I have always used and sold Alum which is Potassium Aluminum Sulfate and is sometimes just called Aluminum Sulfate. But in fact there is a product sold as Aluminum Sulfate that does not contain the Potassium.
So what is the difference? Potassium Aluminum Sulfate is refined in such a way as to remove iron from the final product. It appears to go through several processes of refinement. Yet Aluminum Sulfate can be a low iron grade and can thus be used for mordanting. The problem with iron is it will dull and darken the color response on the fiber thus resulting in disappointing color.
Looking through suppliers I find both products. I've called to verify that a product listed as Aluminum Sulfate is in fact just that. So the question is what is the difference? I'm not sure but the next step is to order some, do some mordanting experiments and compare results.
For a further discussion of alum see About Mordants by Michele Wipplinger at Earthues. This is an excellent article addressing the differences.
In the meantime I am ordering some Aluminum Sulfate to compare with my Potassium Aluminum Sulfate. Stay tuned. The dyepots will be simmering.