Definition of "copperas"

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, J[ohn] Mortimer, The Whole Art of Husbandry; or, The Way of Managing and Improving of Land. […], 2nd edition, London: […] J[ohn] H[umphreys] for H[enry] Mortlock […], and J[onathan] Robinson […], published 1708

[…] 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;

1843 April, Thomas Carlyle, “ch. 6, Monk Samson”, in Past and Present, American edition, Boston, Mass.: Charles C[offin] Little and James Brown, published 1843, book II (The Ancient Monk)