Milk paint is a nontoxic, milk-based paint bound with casein. It can be made from milk and lime, generally with pigments added for color.^[1][2][3] In other recipes, borax is mixed with milk's casein protein in order to activate the casein and as a preservative.^[4][5] The paint adheres best to porous substrates such as wood, its common traditional use, but will adhere also and with greater sheen to non-porous ones, especially if mixed with appropriate bonding agents.^[6][7][8] It has been in use for thousands of years. Traditionally, and usually still, made wholly of natural ingredients, it is extremely durable, often lasting for centuries if protected from the elements.

The ancient history of milk in paint is attested by a primitive combination of wild bovid milk and ochre used on a stone tool found in a South African cave and dated to 49,000 years ago, tens of thousands of years before the Neolithic domestication of cattle.^[9] Goats’ milk provided the binder for lacquer paint used on the Ancient Egyptian tomb of King Tutankhamun.^[10]

Before the invention of acrylics, most house paint binders not made from oil instead used casein. Abundant historical production documents outline the manufacture of milk paint on an industrial scale.^[11] However, because oil-based and acrylic-based paint resins have come to be made in vast quantities with common oils, such as linseed, and petrochemicals, their price can be much lower than that of milk paint, which commercially is now made only on a small scale.

Borax-casein milk paint combines the advantages of an all-natural paint with the convenience of a ready-made paint, as it keeps for six months or more if sealed very tight to retain the moisture, although in time the casein binder will break down. Lime-casein milk paint does not come pre-mixed but is sold as a powder;^[12] once water is added, the lime activates the casein and yields a durable paint that is caustic till it dries but safe and odorless thereafter. Once lime-casein milk paint has been mixed, it must be used within a day, or a few days if refrigerated.^[13]

Today, milk paint is used not just on antique furniture and accurate replicas of it but on pieces of modern style.^[14] The resulting finish has depth of color and a mottled appearance that may be used to give furnishings and rooms a rustic, shabby chic character.^[15][16]

• Casein paint
• Painting and the environment