Making Azurite Blue Pigment — Grinding the Copper Mineral into the Blue of Medieval Painting

Obtain azurite mineral specimens from a geological supplier or mineral dealer. Good pigment-grade azurite should be a deep, vivid blue — the best specimens are an intense royal blue. Avoid specimens that are heavily weathered to green (indicating conversion to malachite) or mixed with large amounts of brown matrix rock. Small amounts of green malachite inclusions are normal and unavoidable, but the specimen should be predominantly blue. The purer and more intensely blue the starting material, the more vivid the final pigment.

Break the azurite into small fragments in a stone mortar using firm, controlled blows with the pestle. Crush to roughly rice-grain size — do NOT grind to fine powder at this stage. This is the critical point: azurite pigment must be kept at a relatively coarse particle size to retain its blue colour. Over-grinding produces a pale, grey-green powder that is nearly useless as a blue pigment. Wear a dust mask during crushing. Separate out any obvious pieces of brown rock matrix or green malachite by hand — these dilute the blue colour.

Place the crushed azurite in a glass jar and add water. Stir gently, then let settle for just 10-15 seconds. The coarsest, deepest blue particles sink immediately — this is your best pigment grade (the 'first blue'). Pour off the still-cloudy water into a second jar. Let the second jar settle for 30-60 seconds — this produces the 'second blue', slightly finer and slightly lighter. Repeat once more to collect a 'third blue' — the finest, palest grade. Each successive grade is finer and paler. The first blue is the most vivid and valuable. The second and third grades are useful for lighter passages. Keep each grade separate.

Pour off the clear water from each settled grade. Spread each wet pigment paste on a separate clean surface and let dry completely (overnight to 1 day). Do not mix the grades — label them clearly. The first grade (coarsest, from the fastest settling) will be the deepest, most vivid blue. The third grade (finest) will be noticeably paler and more grey-blue. This grading system is identical to that described in Cennino Cennini's 'Il Libro dell'Arte' (c. 1390), the great medieval painter's manual.

Store each grade of dried azurite pigment in separate sealed glass jars, clearly labelled by grade. Azurite pigment is lightfast, non-toxic, and compatible with egg tempera, oil, fresco, encaustic, and watercolour binders. For oil painting, grind with linseed oil — note that azurite becomes slightly greenish in oil over time as the oil yellows, so it works best in tempera and fresco. Apply over a dark underpainting (indigo, smalt, or dark grey) to maximise the blue intensity. In humid conditions over very long periods (centuries), azurite can slowly weather to green malachite — the same geological transformation that occurs in nature. This is why some medieval paintings that were originally blue now appear green.