I’m probably a little to picky about this but I’ve seen alum described as potassium aluminum sulphate, ammonium aluminum sulphate, di-aluminum sulphate, aluminum sulphate, aluminum sulphate hydrate, dehydrated aluminum sulphate. All of these have different amounts of aluminum ions per pound of material. Most web sites or blog posts ignore these differences and just refer to pounds of alum. Most of the rest use these terms interchangeably. Is the chemistry all that forgiving that it doesn’t really matter which of these you use?