Making Copper Resinate Green — The Transparent Glaze of Medieval and Renaissance Painters

Dissolve dammar resin in turpentine spirits to make a thick varnish. Place 50 g of crushed dammar resin in a glass jar and add 100 ml of turpentine spirits. Seal and let dissolve over 24-48 hours, shaking occasionally, until the dammar has fully dissolved into a clear, thick, honey-coloured varnish. Alternatively, use Venice turpentine (larch balsam) directly — it is already a thick, honey-like resin that dissolves verdigris readily. Venice turpentine was the preferred medium in 15th-century Venetian painting for this recipe.

Grind verdigris (copper acetate) to a fine powder on a glass muller. Verdigris is a bright blue-green crystalline powder — commercial copper acetate from a chemical supplier works perfectly. You need approximately 20-30 g for this batch. Wear gloves when handling — verdigris is moderately toxic and irritating to skin. The finer you grind it, the more completely it will dissolve in the resin, producing a more transparent and uniform glaze.

Warm the dammar-turpentine varnish (or Venice turpentine) gently in a double boiler or water bath to approximately 60-80°C — NEVER over direct flame, as turpentine vapour is highly flammable. Slowly add the ground verdigris to the warm resin, stirring continuously with a glass rod. The verdigris gradually dissolves, and the resin transforms from honey-gold to a deep, brilliant, transparent green. Continue heating and stirring for 15-30 minutes until all the verdigris has dissolved and the mixture is a uniform, intensely green, transparent liquid. The chemical reaction converts copper acetate into copper resinate — a copper salt of the resin acids.

While still warm and fluid, strain the green resin through fine cheesecloth into a clean glass jar to remove any undissolved particles. The finished copper resinate should be a deep, brilliantly transparent green — when spread thinly on white paper or glass, light passes through it like a stained-glass window. Store in a sealed glass jar. The resin solidifies as it cools into a hard, translucent green mass. For use in oil painting, warm gently or dissolve a portion in turpentine spirits to make a fluid glaze. Apply over a white or light-coloured underpainting for maximum luminosity.

Copper resinate produces the most vivid, transparent greens in oil painting — no other historical green pigment matches its depth and luminosity when used as a thin glaze. However, it has a well-known flaw: over centuries, copper resinate glazes turn brown. This darkening (oxidation of the copper compound) has affected many Renaissance paintings — passages that were once vivid green foliage now appear dark brown. Van Eyck's 'Ghent Altarpiece' and Raphael's landscapes both show this deterioration. Despite this, copper resinate remained in use for centuries because no other green could match its transparency when fresh. For modern reproduction painting, apply thin and protect with good varnish to slow the inevitable browning.