Making Verdigris :: Paintings :: Grosse is a geek

Making Pigments - Verdigris
Sometimes, you need to have something that you can't get anywhere.

Before we get started...

SAFETY FIRST.

Many pigments are toxic to some degree - either in low doses over a long time or in higher doses in a single instance (cronic and acute poisoning respectively) - so, it is important for your own safety and that of those around you to adopt certain practices that will preclude such events.

To contaminate yourself with pigments (or other painting-related compounds), there are a number of routes into your body. The most obvious of these is your mouth. Others include your lungs, skin, nose, eyes and so on. Avoiding activities that allow things to come into contact with these routes is the most effective way of avoiding contamination. So, when painting or preparing pigments, paint, solvent:

Do not smoke. Apart from the obvious fire hazard with organic solvents, you will be putting things in your mouth that you have handled with your fingers;

Do not eat or drink for the same reasons;

Keep your painting related activities in an area that is away from your normal living area (you will be less vigilant when not painting so more likely to become contaminated);

Keep away from pets, partners and children, whether the materials are in current use or not;

Pigments are powders that you can breathe in so keep the dust level down by prevention - when mixing pigments, do things slowly and using as little force as possible thereby reducing the energy that can throw powder into the air.

Do not paint when intoxicated - you will not be as aware of the potential dangers;

Wear protective clothing so that your everyday clothes do not become contaminated - an apron will do, just something that is going to stop whatever it is that you are working with coming into contact with you or your clothes;

If you become aware that you might be breathing in more solvent than is healthy, increase the level of ventilation (if you are gilding, don't do it where there are solvents anyway as you cannot have drafts with such an activity.

Finally, clean up after you have finished. You don't want pigments mixing together but more importantly, you don't want to pick up any contamination from stuff that has not been cleaned away which you can then ingest by accident.

The above is not an exhaistive list. Feel free to be even safer in areas that are not mentioned above as well as those that are.

Verdigris

What is Verdigris? analogous to lead white, verdigris is the product of atmospheric action on copper. Malachite is what happens to copper when you leave it out in the rain, like lead white is to lead. However, verdigris is more complex than that depending upon its circumstances of production and your point of view.

'Natural' verdigris can have in it copper carbonate, hydroxide, chloride, acetate and anything else that happens to be knocking around in the environment. It is the turquoise salt that you see on copper pipe fittings in the gents and in older houses because, like its lead analogue, it takes a long time to form.

'Synthetic' verdigris has been around as long as Dutch Stack Lead White and is made in the same way where copper sheet is suspended above acetic acid and allowed to rot through, producing the turquoise crystals or powder. However, this does not build up in layers in the same way that lead white does and you can make it by other methods and it is the same product both chemically and physically - in other words, unlike Dutch Stack Lead White, there is no advantage to doing it the slow way - any process will do and the result will be the same. The formula is this...

Cu(OOCCH₃)₂H₂O

So, as you can see, there is one water molecule of crystallisation for each copper atom. You can make this by having a source of copper salt and displacing the anion with acetate from acetic acid.

One variant - just to add to the complication - is a version with a molar amount of copper hydroxide that is three times that of the copper acetate like so...

Cu(CH₃COO)₂.[Cu(OH)₂]₃.2H₂O

and you could make this by making your copper acetate then adding a measured quantity of sodium hydroxide to it to take out some of the acetate (sodium acetate is very soluble in water) to give you your mixture. In fact, you could add anything you like - acetic acid is a weak acid and will be displaced by any stronger acid so you can have some fun.

How Available is Verdigris? You can get verdigris from some places but it will cost you and - as you can see from the variants you can make yourself, you can't be certain of what you will get. You can buy the ready made stuff for £5 for 10g but you can make it for yourself for a tenth of that.

What About Making Your Own? You can make your own from copper sheet and acetic acid but that takes a long time and the results will not be any better than if you just synthesise it from other materials. Oxidising the copper without forming copper oxide is a potential problem so finding a source of copper salt that we can trust is the main issue but there is an excellent solution to it: Malachite is used a copper ore and a finely divided malachite powder is what is used by the pyrotechnic industry to get a green flame. The green flame is the emission spectrum of copper and if it is contaminate with something else, it will produce a different colour. So, if you look around the internet for 'malachite powder', you will find some at a good price. The other ingredient is acetic acid which you can pick up as glacial acetic acid - see Dutch Lead White above.

The Reaction is as follows...

4CH₃COOH + CuCO₃.Cu(OH)₂ => 2[Cu(OOCCH₃)₂H₂O] + H₂O + CO₂

Molecular weights say: 240g acetic acid + 221g malachite => 399g copper acetate + 18g water + 44g of carbon dioxide

So, if we want to make 100g of verdigris, we need: around 60g of glacial acetic acid and 55g of malachite and in the process, we can expect it to give off 6.6litres of CO₂.

So, what do we do?

weigh out the malachite powder and put it in a container that should be fairly big because of the amount of gas that is going to be given off.

Next, weigh in the glacial acetic acid and pour that into the container with the malachite in it. Nothing should happen. The reason for this is that the acetic acid hasn't got any water to grab to make acid - as far as the malachite is concerned, it is just an organic liquid that it doesn't dissolve in. Stir it around so that the malachite and the acid are mixed well.

Then, add literally a few drops of water. You should see some foaming going on as the water activates the acid and that then drives off the CO₂ so stir it around to make it complete the reaction and then, it should stop. Add a little more water, and do the same again.

As you add more and more water, two things should happen: the mixture gets hot as the reaction progresses; and, the foam goes further up the sides of the container each time.

The reason we have foam is that the acetic acid is like a short-chained detergent. It has an end that likes water and an end that dislikes water (hydrophylic and hydrophobic respectively). These line themselves up, side by side, to form membranes, one from of which is the bubble. As it takes acetic acid to do this, they are going to occur more as the concentration of acid increases so the more water you add, the more it foams. You can break down the bubbles and only add small quantities of water to get around this problem.

One thing you shouldn't do is add too much water. Copper acetate is soluble in water so the more water you add, the more pigment you lose.

Finally, you will stop getting bubble formation when you add water and you should now have a mass of turquoise powder in your container.

As with the lead, filter this off but there is no need to wash it and re-filter - any impurities that you don't manage to lose through the filter such as water and acetic acid are volatile and you will lose them in the drying process.

Next, spread it out and leave it somewhere to dry. If you want to speed it up, you can put it in a fan oven at no more than around 60C otherwise you are starting to get close to the temperature that it decomposes at.

Use the spatula to chop it up and turn it over as it dries and you will end up with a nice, crystaline powder like that on the right which was photographed whilst it was still a little damp.

Once you have a dry powder, bottle it up label it and store it in a dry place away from children and animals.

The final product will have a very slight odour of acetic acid but it s volatile and if you use this pigment in watercolour, that will disappear quickly.

Finally, whilst verdigris has a beautiful turquoise colour that remains in watercolour, something really quite odd happens when you use the pigment in oils.

If you make up some verdigris paint and paint a sample, over a period of a few days, it will become more green as it dries. Jan van Eyck used this pigment and it is stable - evidenced by the fact that the verdigris that he used still is green - it is just that before it becomes stable, it changes hue.

You can see this hue change in the sample I did on the right. The numbers for hue are those that you would get in an image processor going from 0° to 359° so you can see numerically that the change is significant.

All images and original artwork Copyright ©2017 Paul Alan Grosse.