I’m fairly sure that the idea of using alum (potassium aluminium
sulphate) solution originated in the watch and clockmaking trades.
There, the problem would be hardened carbon steel bits breaking off in
brass workpieces. In the early days workers often used hand made
drills, and a variety of drilling methods, none of which aligned the
drill accurately the way a drilling machine (drill press) does.
Anyway, the point is, that a boiling alum solution will dissolve the
steel without attacking the brass, whereas sulphuric acid and
bisulphate solutions (including Sparex) would go for brass as well as