Biodegradable paint is hardly new, but it is enjoying a resurgence within the green-living community, because it is safe to use, easy on the environment and does not contain toxic, volatile, organic compounds -- the noxious vapors that can cause illness and environmental damage. Two common methods exist for making biodegradable paint: sidewalk paint and milk paint. Sidewalk paint, which traditionally was more commonly used outdoors than indoors, is not only a staple of playgrounds but is increasingly a favorite of crafters. Milk paint has been around since ancient Egypt and found increased use during the Renaissance, Colonial-era America and the Civil War era, up to the present day. The milk protein binds to pigment just as polymers or oils do to the pigments in commercial paints. Milk paint is valued for its saturated color and a translucent finish.
Measure the 1/4 cup of cornstarch and pour it into a small plastic bowl, gradually stirring in the 1/4 cup of cold water. Stir until the consistency is smooth and paste-like.
Add the six to eight drops of your choice of food coloring into the mixture. Stir until the color is even.
Check the consistency of your mixture. If the paint feels too dry, gradually add a little more water, stirring constantly. If the paint is too thin, add a few teaspoons of cornstarch, stirring constantly. Repeat this process for each paint color you wish to make.
Squeeze a lemon into a large bowl and add 1 quart of skim milk, stirring constantly. Leave the mixture overnight at room temperature. This should induce the mixture to curdle, which is exactly what you want.
Pour the mixture through a cheesecloth-lined sieve to separate the curds from the whey. Add 4 tablespoons of dry color pigment to the curd, stirring gently until the solution is well mixed and the paint is well disbursed. Wear a mask for this step. You can use acrylic paint from art stores instead of powdered pigment, if you wish.
Add the pigment one drop at a time, stirring constantly. When the pigment is of the hue you desire, apply the paint immediately, as milk paint spoils quickly and should be applied within a few hours after mixing. Once the paint is dry, the sour odor of the milk paint will disappear.