Verdigris is a blue-green pigment used in manuscript illumination, hand-coloring of maps, and prints, from at least the medieval period to the 19^th Century. In illuminated manuscripts it could be used alone or in mixtures with other pigments such as saffron, sap green or iris green to alter its original hue.

Verdigris is very simple to make. Many recipes can be found in Medieval and Renaissance painting and illumination treatises. One of the simplest methods is to expose pieces of copper or brass to organic acid vapors (vinegar, wine lees, curdled milk, urine, etc.) in a closed container. The reaction between the metal and the vapors form basic copper acetates on the surface of the metal.

According to Kühn (2) there are two types of verdigris: basic verdigris and neutral verdigris. Basic verdigris is the type formed by the action of acetic acid vapor, water vapor and air on copper or copper alloys. It can contain up to four different forms of copper acetates, three of which are blue and one green.

• [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)

Neutral verdigris Cu(CH₃COO)₂.H₂0 (also called “distilled”, “purified” or “crystallized” verdigris) is formed when basic copper acetates are dissolved in acetic acid and then recrystallized by evaporation or when verdigris is ground with strong acetic acid (this is suggested in many medieval painting treatises including Cennini in Il Libro dell’ Arte).

Verdigris has a tendency to change from blue green to green during the first month after it’s been applied to a substrate. This shift towards green will depend on whether it is basic or neutral and on the binder used.

Early painters and illuminators were aware of the corrosive nature of verdigris (see recipe below), but mostly as it related to its supposed effect on other pigments. In many treatises of the time it was recommended not to use verdigris with orpiment or white lead as they were said to be incompatible. However more recent research suggests that it is not as harmful to other pigments as previously believed, except for sulfur-containing pigments.

“300. To make corrosive green, without substance or body. To make a green transparent in its nature, and without body, that is, having no substance, such, for example, as is the colour of saffron, i.e. of crocus, which does not cover up other colours so as to conceal them, on account of its thinness, transparency, and rarity, owing to which other colours appear through it, wherefore this colour as well as the said green colour is overpowered, and shows little or not at all, nor can it be much seen over other colours. But this green colour is not mild like saffron, on the contrary it is, by nature, acrid and corrosive, so that it destroys and corrodes other colours if it is put over them, or they over it, and this on account of the verdigris which is in it; and such is its nature, and it is used upon parchment and paper. Take verdigris and a little of the dried lees of wine, which in Latin is called tartarus, and in French gravelle and pulverize it and grind both the ingredients together upon a hard and smooth stone with vinegar. Afterwards draw all those things which you wish, both in parchment and paper, and the empty spaces which are between the lines of black ; afterwards fill in with the green colour made in the above manner, and colour according to your taste, the things which you have so drawn as aforesaid. And note, that no other colour can be laid over this green colour, as has been already observed, nor can it be laid over others; nor can it be used otherwise than by itself and upon white paper and parchment, because this green colour, made as above, is corrosive and acrid, and, by reason of its acrid nature, it destroys other colours, as has been already mentioned above.”
(3) “Manuscripts of Jehan Le Bègue”, p. 284

Today, most conservators and collections custodians are keenly aware of verdigris’s possible corrosive action on the substrates it is applied to. Like iron gall ink, it can cause discoloration and embrittlement of paper and parchment. In some cases, the pigment will also turn brown. In recent years many cultural heritage institutions around the world have started researching copper pigments to better understand what deterioration mechanism(s) are at play to hopefully develop safe preservation and treatment methods.

Other common names for verdigris are Spanish green, copper green, copper rust, basic copper acetate, vert-de-gris (French), Grünspan (German), verderame (Italian), cardenillo or verdete (Spanish), aerugo or viride aeris (Latin).

How to make verdigris

“If you want to make verdigris, take a new pot, and put sheets of the purest copper into it; and so fill that pot with very strong’ vinegar; and cover it thus, and seal it. And place that pot in some warm place, or in the ground, and put it away thus for six months; and then open that pot, and put what you find in it on to a wooden panel, and set it in the sun to dry.”
(5) Endnote 26, p. 29.

 “106. The recipe for verdigris
Take very clean copper leaf and hang it over very sharp vinegar. Leave it undisturbed in the sun for 14 days. Open it up, take away the leaf and collect the efflorescence; and you will make the cleanest verdigris.”
(4) p. 42.

“157. Also, How to make verdigris for writing.- Whoever wishes to make a green colour for writing, let him pour into a copper or brass vessel equal quantities by weight of honey well mixed with vinegar, and then bury the vessel in horse-dung, in the hottest part of the heap. After 12 days are passed, he may take the colour out of the vase, scraping it out ; then dry it in the sun, and keep it for use.”
(3) “Manuscripts of Jehan Le Bègue”, p. 126.

To try your hand at making verdigris, all you need is copper pieces, a jar with a lid, vinegar, sand paper and a little patience. Below is a description of the materials I used and steps I followed to make verdigris.

Materials

• 2 inch copper tube sections
  (about $2/piece at your local hardware store)
• Glass jar with tight lid
• 6% acetic acid (you can also use plain white distilled vinegar)
• Sand paper or a Dremmel
• Plastic egg crate (or string and awl/drill)

When you buy copper, it will most likely already have a thin layer of corrosion on its surface. You will need to remove it using sand paper.

Once your copper pieces are nice and clean, you can put them in a jar in which you have poured a small quantity of vinegar. You need to keep the copper from touching the vinegar. You can do this in different ways. You can use string to suspend the copper tubes in the jar (you will need to punch holes in the lid to thread the string through and something to seal the holes) or place plastic egg crate at the bottom of the jar to hold your copper above the vinegar.

Now wait and watch as the corrosion layer gets thicker and thicker every day from the build-up of crystals. The surface will at first take on a green color, then more bluish as time goes by.

When you feel you have enough pigment, open your jar and let the crystals dry and scrape them off into a container.

References

(1)   Estaugh, Nicholas. Pigment Compendium: A Dictionary of Historical Pigments. Oxford: New York: Butterworth-Heinemann, 2004.

(2)   Kühn, Hermann. “Verdigris and Copper Resinate” in Artists’ Pigments: A Handbook of Their History and Characteristics, Vol. 2. Washington, DC, Cambridge Cambridgeshire: National Gallery of Art ; Cambridge University Press, 1986.

(3)   Merrifield, Mary P. Original Treatises, Dating from the XIIth to XVIIIth Centuries on the Arts of Painting, in Oil, Miniature, Mosaic, and on Glass; of Gilding, Dyeing, and the Preparation of Colours and Artificial Gems; Preceded by a General Introduction; with Translations, Prefaces, and Notes. 2 vols. London: J. Murray, 1849.

(4)   Smith, Cyril Stanley, and John G. Hawthorne. Mappae Clavicula: a Little Key to the World of Medieval Techniques, Transactions of the American Philosophical Society. Philadelphia: American Philosophical Society, 1974.

(5)   Thompson, Daniel Varney, George Heard Hamilton. An Anonymous Fourteenth-Century Treatise: De Arte Illuminandi, the Technique of Manuscript Illumination; Translated from the Latin of Naples Ms. Xii. E. 27. New Haven, London: Yale University Press; H. Milford, Oxford University Press, 1933.