Copperas is crude ferrous sulfate, and was formerly known as green copperas to distinguish it from white copperas - zinc sulfate, blue copperas and copper sulfate.
Making copperas was a vast industry here in Britain from the 1600's till the late 1800's. It was sited along those areas of the coast - Kent, Essex, the Thames estuary and elsewhere, where there are still abundant supplies of pyrites (or more probably, marcasite) from the Lias and other clays of the costal cliffs. It is not the golden nuggets of mineral collectors and museums, but rather, a dull black pebble, which is remarkably heavier than other pebbles as it contains a lot of iron. Only when smashed with a hammer does its true nature become evident - the grey-green metallic lustre of the fresh surface, and a slight smell of sulfur.
From copperas you can make ink, sulfuric acid, hydrochloric and nitric acids, aqua regia, jeweller's rouge, and Venetian Red, and thus it formed the basis of the earliest chemical industry.
The process for making copperas is the weathering of iron pyrites, as practiced in Nordhausen in Germany as the first stage in the process for making the famous Nordhausen sulfuric acid. The same process was practiced along the Thames estuary, probably brought to England by Flemish settlers. On an industrial scale, large pits, the size of tennis courts, and about fifteen feet deep were lined with timber. A channel at the bottom allowed liquors to run into a cistern. These pits were partly filled with the abundant local pyrites, and simply left in the sun, wind and rain to weather for two or three years. From time to time the liquors were run into the cistern, then boiled down. The concentrated solution would deposit on cooling, green crystals of copperas - ferrous sulfate - FeSO4.7H2O. A greater yield could be obtained by adding scrap iron to the hot solution. The crystals were often gathered on twigs, where they formed like barley sugar. Because big crystals look like green glass, it is also known as green vitriol. Roasting this gives off white fumes of sulfur trioxide, which can be condensed to strong sulfuric acid. This is a heavy oil-like liquid, hence its old name of Oil of Vitriol. But it's not an oil, for it doesn't float on water. Instead it mixes with water vigorously releasing a lot of heat.
My home process consists of putting a layer of iron pyrites nodules into the bottom of a plastic bucket, just covering with RAIN water, and allowing the contents to stand and evaporate in a warm room - or outdoors in the summer heat, for several weeks. My first attempt didn't work quickly - all I got after several weeks was a pale rusty solution, which was slightly acidic, and tasted metallic. I don't know if this is due to an inherent time lag before the process gets started, or else because I'd used chlorinated tap water, or else had put in too much water. In disgust, I transferred my experiment from the attic to my back yard over the summer. Some weeks later I peered inside. To my surprise and delight, the black nodules were all covered with a white and green encrustation - I'd made copperas - accidentally! Since then I've made a lot, from the same nodules. The process depends on the decomposition of pyrite - FeS2, not by atmospheric oxygen as it states in old chemistry books, but by bacteria - Thiobacillus Ferrioxidans and similar. These extremeophile organisms eat pyrites depositing ferrous sulfate as a waste product. The organisms have been living inside the nodules in a latent state for thousands or perhaps millions of years, and presumably need warmth, air and fresh water to waken up from their dormant state.
Though I've used pyrites nodules I gathered myself from Lias clays near Lyme Regis, which are black on the outside, I suspect the process will work with any form of pyrites. My golden nuggets which mostly come from South America are too expensive to ruin in such an experiment, but probably would work just as well. Pyrites is a very common mineral - the gold streaks found in coal are made of it ("coal brasses") and most metalliferous mine tips will provide adequate supplies - look for the rocks which are particularly rusty. Any geologist will be glad to point out local sources, and usually provide you with free samples. Pyrites, the plural, is the name used by chemists, geologists nowadays tend to use the singular, pyrite.
The encrustation on the pebbles is a layer of beautiful white needles up to 5 mm long. Underneath is a moist layer of pale green ferrous sulfate - looking like grains of green sugar. When the mixture dries out completely, the pale green crystals turn white - presumably due to loss of water of crystallization. I don't know the chemical composition of the outer layer of white needles - it may be simply a different crystal form, or a different hydrate, or perhaps a different oxidation state.
Once the pebbles have become active, a fresh layer of copperas will appear within two or three days of removing the original crust - you don't have to wait weeks as I had to do at the start. It is important that the nodules are left poking out of the rain water into the air. Exposure to daylight doesn't seem to matter, but rain water seems to be preferable to tap water.
I haven't tried to get oil of vitriol from my crystals yet, but I have used them to make black ink. Simply add a solution to cold tea - and Hey, Presto ! The dense black can be rendered colourless by adding oxalic acid to the liquid. The black solution was the only black dye permitted by law for dyeing silk in Britain for several centuries. It was the only black ink used from ancient times which was not based on soot. Cold tea contains tannin - as does an extract of oak or other barks, oak galls, pomegranates and many other vegetable sources. Tannin produces a black pigment with iron. Try hammering a nail partly into a piece of oak and leave out in the rain for several weeks. You will get a black streak, not a rusty one as you would expect.
Roasting the copperas will leave a rusty residue of iron oxides, known as Jeweller’s Rouge or Venetian Red, depending on the hue and the method of preparation. This can be used for polishing glass and gold, or as a pigment.
Roasting the copperas with potassium nitrate should yield brown fumes of nitric acid.
My home experiments are continuing - getting sulfur from pyrites, and ways of making phosphorus.
Thank you for your interest, I hope you can put my ideas to some use.