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Copperas

or Iron Sulfate

Ferrous Sulfate crystals

(Green) Copperas and Green Vitriol are the medieval names for Iron(II) Sulfate (FeSO4 x H2O) used in making Iron Gall Ink.

In the middle ages, copper sulfate was known as 'blue copperas' while zinc sulfate was known as 'white copperas.'

Kremer Pigments carries it, although I'm sure you can also purchase it from numerous suppliers as it has many modern uses. https://shop.kremerpigments.com/en/search?sSearch=ferrous+sulfate

Iron sulfate is what turns ink black when it reacts with tannic acid (extracted from oak galls) and oxygen from the air. Iron sulfate is fairly acidic though, and if you add too much to your ink, it can become corrosive and eat right through your paper or vellum over time.

An example of ink eating through writing paper after many years.

It's also a very good idea to rinse your metal pen nibs after using them with this ink to prevent them rusting & corroding too! I suppose you could buy some Ph test strips - to see how acidic the ink is once it's finished. I'll have to see if I have any test strips & give that a try.

Once you write with this ink, it will turn darker on the page as it's exposed to oxygen in the atmosphere while it dries.