copperas

noun

1. (uncountable, usually)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;”

2. (uncountable, usually)An obsolete sulphate compound with one of the three metals, zinc, copper or iron.