Anyone have good experience in flocking using alum?

I bought a spice container of it at the grocery store and did
an experiment with my son. We filled a glass with tap water
and added a spoonful of dirt and gravel. Stirred to get a very
cloudy mix then added a couple of pinches of crushed alum.
Within 5 minutes the water was clear enough to see through again
and a lot of debris was on the bottom.

I am curious about formulas for amounts etc for using this
before treating with other methods. Anyone?