copperas

Iron(II) sulfate.

It were superfluous to describe the Process of making the Aqua fortis; it shall suffice to let you know, that our common Coperas makes this Aquafortis well enough for our purpose[…]

[…] what a change has introduced itself everywhere into human affairs! How human affairs shall now circulate everywhere not healthy life-blood in them, but, as it were, a detestable copperas banker’s ink;

An obsolete sulphate compound with one of the three metals, zinc, copper or iron.