Making Casein Paint from Milk and Lime — The Ancient Protein-Based Wall Paint

Warm 1 litre of skimmed milk to about 40°C (do not boil). Add 60 ml of white vinegar and stir gently. The milk will curdle within minutes — the casein protein precipitates as white curds, leaving a yellowish-green liquid (whey). Let the mixture stand for 15-30 minutes to complete the curdling. Use skimmed milk, not whole — the fat in whole milk interferes with the binding and makes the paint less durable.

Pour the curdled milk through a fine mesh strainer or muslin cloth to separate the curds from the whey. Press the curds gently to squeeze out as much whey as possible — excess moisture weakens the paint. The curds should be a firm, crumbly white mass. Discard the whey (or use it in the garden — it is a mild fertiliser). From 1 litre of skimmed milk, expect approximately 100-150 g of wet curds. The curds must be used fresh — within 24 hours.

Place the pressed curds in a mixing bowl. Add 15-20 g of slaked lime (calcium hydroxide) and mix thoroughly. The lime dissolves the casein protein and converts it to calcium caseinate — a smooth, creamy paste with powerful adhesive properties. Stir vigorously for 5 minutes. The mixture should become smooth and slightly glossy. If it is too thick, add a small amount of water. The lime smell is normal and will fade as the paint dries. Wear gloves — slaked lime is alkaline and can irritate skin.

Mix earth pigment powder with a little water to form a smooth paste, then stir into the casein base. Any mineral pigment works: yellow ochre, red ochre, umber, sienna, lampblack, or titanium white. Add pigment until the desired colour is reached — casein paint dries slightly lighter than when wet, so make the mix slightly darker than the target. Add water to thin the paint to a brushable consistency — it should flow smoothly from a brush but have good body. Use the paint within 24 hours — casein putrefies rapidly.

Apply the casein paint to clean, dry surfaces — wood, plaster, or masonry — using a brush. Apply 2-3 thin coats, allowing each coat to dry for 4-6 hours. The paint dries to a beautiful, flat matte finish similar to distemper but harder and water-resistant. After 24-48 hours of curing, casein paint is no longer water-soluble — the calcium caseinate cross-links into an insoluble matrix. For extra durability on furniture, seal the dried paint with linseed oil or beeswax. Clean brushes with warm water immediately after use.