Interestingly, while googling this, I ran across a modern lab reference: "iron alum" is apparently still a somewhat-current term in histology. It's used in the converse of a Van Dyke process: silver is deposited on iron-treated fibers that don't readily take silver stain. It amounts, chemically, to the same thing, but it's intriguing to me that there's utility in the procedure diametrically-opposite to what we would use.