Casein paint or milk paint is unlike any other natural water-based paint as it dries waterproof. However, it must cure for a specific time, usually about a week to a month. Despite this, it dries to the touch quickly if painted thinly so that many successive layers can be painted in one session. Thicker layers may take about a day to dry.
Cracking can occur for several reasons: The paint may have an excess binder, or the previous layer may not have dried completely. On the other hand, this can create beautiful crackle effects. Paint on top of an incompletely dried casein gesso.
How to Prepare Casein or Milk Paint
The recipe for the casein binder is relatively straightforward:
- Soak 2 1/2 oz. (80 g) casein powder in 9 fl. oz. (250 ml) room temperature water* for at least 10 minutes.
- Pre-heat the soaked casein.
- While the soaked casein is heating, dissolve 1 oz. (32 g) borax in 9 fl. oz. (250 ml) hot water*.
- Stir the hot borax solution into the soaked casein when it reaches 120 °F (48 °C).
- Continue stirring the casein solution while heating it to 180 °F (82 °C).
- Cook at that temperature for at least 15 minutes with continuous stirring.
- Allow the mixture to stand to cool to room temperature and store it in a refrigerator.
- Add 6 cups (1.4 liters) of water for use.
How to Use
Use only on rigid, porous surfaces. Casein is brittle, so flexible supports, such as stretched canvas, should not be used. Prime the surface with diluted casein binder. Next, mix chalk and titanium dioxide or zinc oxide white pigment with the casein binder to create casein gesso. Apply gesso as usual and sand thoroughly dry coats (approximately one day) as needed.
Preparing Casein Colors
Mix pigments with distilled water into a paste. Add an equal amount of casein binder to the paste. Thin with water to the desired consistency. Test your paint by applying it to a piece of cardboard. If some pigment rubs off after an hour, you need more binder. If it cracks, reduce the binder content by adding more pigment and water.
Storing Casein Binder
Store the prepared binder in the refrigerator and warm it to room temperature before use. Keeps for 2–3 weeks.
* Use distilled water to prepare the binder to reduce microbial contamination and prevent mold.