Making Verdigris — The Copper Acetate Green of Medieval Painters

Start with a piece of copper sheet approximately 10×15 cm and 0.5-1 mm thick. Clean the surface thoroughly by sanding with fine sandpaper to remove any oxide layer, then wipe with vinegar to degrease. The copper surface must be bright, bare metal — any grease, lacquer, or heavy oxide will slow the reaction. Multiple smaller pieces give faster results than one large piece because more surface area is exposed to the vapour.

Pour 200-300 ml of strong white vinegar (at least 5% acetic acid; stronger is better) into the bottom of a wide-mouth glass jar, ceramic crock, or plastic container with a lid. Create a support to suspend the copper above the vinegar without touching the liquid — a small wooden rack, crumpled wire mesh, or a shelf of wooden sticks across the jar. The copper must be exposed to the vinegar vapour, not submerged in the liquid. Seal the container with its lid.

Place the sealed container in a warm location (20-30°C). Warmth accelerates the reaction. Check progress every few days. Within the first week, a pale green film will appear on the copper surface. Over 2-4 weeks, this builds into a thick, crusty green layer of basic copper acetate. The crust may be bright green, blue-green, or turquoise depending on temperature and vinegar strength. Do not disturb the copper during this period — movement can dislodge the forming crystals.

Remove the copper sheet from the jar. Scrape the green crust off carefully with a palette knife or stiff blade, collecting the powder on a clean surface. The scraped copper can be returned to the jar for another cycle — each round produces slightly less verdigris as the copper thins, but a single sheet can be reused many times. Expect 5-15 g of verdigris from a 10×15 cm sheet over one cycle. Wash hands after handling — copper acetate is mildly toxic if ingested, though not dangerous to touch.

Grind the scraped verdigris in a mortar and pestle to a fine, uniform powder. The finished pigment is a vivid blue-green — one of the most intense greens available from any natural pigment source. Store in a sealed glass jar away from moisture. To use: mix with egg yolk for tempera (most stable), gum arabic for watercolour, or linseed oil for oil paint (note: verdigris darkens over time in oil). Verdigris is also used in manuscript illumination, where it was often glazed over gold leaf for a luminous green effect.