Making Cochineal Carmine Lake Pigment — Precipitating the Insect Red onto an Alum Substrate for Painting

Weigh out 50 g of dried cochineal insects. Grind them to a fine powder in a mortar — the powder should be a deep purple-red. Transfer to a heat-resistant glass beaker and add 500 ml of water. Heat gently to a simmer (80-90°C) and stir for 20-30 minutes. The water turns a deep crimson as the carminic acid dissolves. Do not boil vigorously — excessive heat can degrade the dye. After extraction, strain the hot liquid through fine cheesecloth or muslin into a clean beaker, squeezing the cloth to extract as much colour as possible. Discard the insect residue. The extract should be a deep, rich crimson liquid.

Dissolve 5 g of potassium carbonate (potash, K₂CO₃) in 50 ml of warm water and add it slowly to the hot cochineal extract, stirring continuously. The potash shifts the pH to alkaline (roughly pH 9-10), which deepens the colour from crimson to a rich, bluish-red — the exact shade depends on pH. The alkaline environment is necessary for the next step: at neutral or acid pH, the aluminum ions do not form a stable precipitate with carminic acid. Add the potash gradually and observe the colour change — stop when the liquid is a deep, bluish crimson.

Dissolve 25 g of alum (potassium aluminum sulfate, KAl(SO₄)₂·12H₂O) in 200 ml of warm water in a separate beaker. Slowly pour the alum solution into the hot alkaline cochineal extract, stirring constantly. The aluminum ions react with the carminic acid and the alkaline conditions to form an insoluble aluminum-carminic acid complex — the lake pigment — which precipitates as a vivid red, fluffy mass. The liquid above the precipitate should become noticeably paler as the dye transfers out of solution and into the solid precipitate. Add the alum solution gradually — too much alum too fast can produce a muddy, dull precipitate. Let the mixture stand for 1-2 hours while the precipitate settles completely.

Pour the settled mixture through filter paper set in a funnel, collecting the vivid red precipitate. The filtrate (liquid passing through) should be only lightly coloured — if it is still deeply red, the precipitation was incomplete and more alum may be needed. Wash the collected precipitate by pouring several portions of clean water through the filter — at least 500 ml total — to remove residual alum, potash, and soluble impurities. Unwashed lake pigment tends to be hygroscopic and can change colour over time. The washed precipitate is a brilliant red, gel-like mass.

Spread the washed precipitate on a clean glass surface and let it dry completely in a well-ventilated area away from direct sunlight (carmine is moderately light-sensitive while wet). Drying takes 1-2 days depending on humidity and thickness. The dried carmine lake forms a hard, brilliant red cake. Grind on a glass muller to a fine, smooth powder. The finished pigment is an intensely coloured, transparent crimson — one of the most beautiful red pigments for glazing techniques. Store in sealed dark glass jars away from strong light. Carmine lake is compatible with oil, egg tempera, watercolour, and gum arabic binders. In oil under varnish, it is reasonably durable; in watercolour exposed to light, it will gradually fade over years.