copperas

noun2

1. •
   (historical) Any of various metallic sulphates formed from the reaction of sulfur with iron, zinc, or copper, especially (alchemy) solutions of such compounds misunderstood as a mediator or catalyst for the transmutation of iron into copper &c.
2. •
   (no longer used) Synonym of 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[…] — 1707