Verdigris

A long time ago I decided to try and make verdigris.

Verdigris is the common and historical name to for a number of salts of copper and acetic acid, or more simply copper acetate¹.

Salts which could be characterised as copper acetate have a number of different variations and verdigris of this nature could contain up to four slightly different compositions of copper acetates and copper hydroxides with slightly different colours:

• [Cu(CH₃COO)₂]₂.Cu(OH)₂.5H₂0  (blue)
• Cu(CH₃COO)₂.Cu(OH)₂.5H₂0 (blue)
• Cu(CH₃COO)₂.[Cu(OH)₂] (blue)
• Cu(CH₃COO)₂.[Cu(OH)₂]₃.2H₂0 (green)²

It occurs naturally on exposed copper and brass – it’s the green you see on roofs, statues and the grotty pipe behind the toilet. Historically this has been made by a number of different processes, based around exposing copper to acidic vapours (wine, vinegar, piss), sometimes with the addition of salt (which would likely include copper chloride in the final product) and honey (ostensibly to stick the salt to the copper, but also likely to be some action from reducing sugars in the honey). This was left in the son, or buried in a dung pile to keep it warm.

This produces a (normally) turquoise blue crystalline efloursescence wich is scraped off.

This was the basis for my first attempt at making verdigris, using strips of copper made from anti-slug tape stood in a jat with a little white vinegar in the bottom³.

This produced what initially looked quite promising, but was quite hard to scrape off. I was however, intrigued by the blue vinegar and, having been paying about with aluminium lakes, the thought that I could precipitate other copper compounds out of this, specifically copper carbonate. Turns out I could, and I will tell you about that soon.

Another method of producing verdigris given in historical literature, in this case the so-called neutral verdigris – Cu(CH₃COO)₂.H₂0 – involves dissolving the above basic copper acetates in acetic acid. Or as I found here, simply reacting copper with vinegar. This I found when re-discovering the jar with the slug tape and a smalll bit of old pipe which I had ignored for some months. After a resurgance in interest due to my obtaining malachite and verditer from Corneliessens and thoughts of building an “early” pallete I thought I would try some more experiements with this.

This reaction does require oxygen as copper is not reactive enough to substitute for the hydrogen in the acetic acid and form copper acetate – but I have found that even when fully covered in vinegar with a bit of shaking this works fine – presumably oxygen dissolved in the solution is sufficient.

The chemistry is as follows:

• First, copper reacts with oxygen in the air to form copper(II) oxide:
  • 2Cu(s) + O₂(g) → 2CuO(s)
• Then, the copper(II) oxide reacts with acetic acid to form copper(II) acetate and water:
  • CuO(s) + 2CH₃COOH(aq) → Cu(CH₃COO)₂(aq) + H₂O(l)

Basically at this point, I simply evaporated the remaining vinegar by heating in a pan and scraping out the result.

A couple of things to note – I am sure there is also likely to be some copper chloride in here from the tap water. And, even this “neutral verdigris” can also vary in colour – the anhydrous form is dark green, whereas the monohydrate is bluer. Interestingly, my first tentative eavporation was more blue, whereas the majority end product was darker green. I suspect the hot process was driving the water out and leading to a greater proportion of the anhydrous form.

I will experiment a little more. Both in the initial reaction (i.e. more oxygen) and the evaporation.

For some very slack reason I did nor weigh the result. From an estimate of about 200ml of solution, produced by leaving the copper in the vinegar for about 6 months I obtained enough for two full half pans when mixed with 2ml of Schmincke binder.

And here’s the result:

Of course this is quite a huge faff for making a small amount of paint – of a coloour that can be easily reproduced, and that will likely dissolve the paper and change colour over time. The question is of course is it worth it… You can of course buy Verdigris (though it is not common) and while once upon a time it was probably easy to find odds and sods of scrap coppper for the process (such as with my foraged pigments) where the production of the materials becomes a part of the art this does not seem to be easy now with rising prices.

I find myself, however, very engaged with the processes here. I am interested in the variations that I suspect different attempts at the process will give me, and I do have an odd attachment to the idea of Art as emphmeral – images that will change and fade and perhaps even the support become brittle. I need to think how to represent that in a peice, using this paint.

1. See https://en.wikipedia.org/wiki/Verdigris for a general overview ↩︎
2. Kühn, H. (1986) “Verdigris and Copper Resinate” in Artists’ Pigments: A Handbook of Their History and Characteristics, Vol. 2. Cambridge , Cambridge University Press, in: https://travelingscriptorium.com/2013/01/17/verdigris/ ↩︎
3. See The Travelling Scriptorium for some more advice on this ↩︎