According to the OED, the term "allum" can be: "Applied to various native minerals, which are chemically alums proper, as native alum or kalinite; also to others (pseudo-alums), which are compounds of aluminium sulphate with the sulphate of some other base, as magnesia alum (magnesio-aluminic sulphate) or pickeringite; or with the protoxides of iron, manganese, etc., as feather or plume alum (ferroso-aluminic sulphate) or halotrichite, manganese alum or apjohnite, manganoso-magnesian alum or bosjemanite." - Thus, it is likely that "alumen plumeum" is ferroso-aluminic sulphate, or some such psuedo sulphate of aluminum and potassium.