The traditional binder in tempera paint is egg yolk. The yolk acts as an emulsion that binds pigment particles together and adheres them to a painting surface. But “tempera” has come to describe a broader family of paints, and modern commercial versions often use plant-based binders like gum arabic instead of egg. The specific binder determines how the paint handles, how it dries, and how long it lasts.
Why Egg Yolk Works as a Binder
Egg yolk is a natural oil-in-water emulsion, meaning it contains tiny droplets of fat suspended in a watery liquid. That dual nature is exactly what makes it useful as a paint binder. The water allows the paint to be thinned and applied smoothly, while the fats and proteins lock pigment in place as the paint dries.
The yolk’s fat content is roughly 65% triglycerides (the same type of fat found in cooking oils), about 30% phospholipids (which act as natural emulsifiers, keeping the oil and water mixed), and 4 to 5% cholesterol. The protein component includes dense clusters called granules, tiny spheres between 0.3 and 2 micrometers across, made primarily of high-density lipoproteins. Together, these fats and proteins form a film around each pigment particle that hardens into a durable, slightly lustrous surface.
How Egg Tempera Dries and Hardens
Egg tempera dries in two distinct phases. The first is fast: water evaporates from the paint within minutes, leaving a dry-to-the-touch surface. This is why egg tempera is known for its quick working time and why artists typically paint in thin, layered strokes rather than thick applications.
The second phase is slower and chemical. Over days and weeks, the unsaturated fats in the yolk undergo oxidative cross-linking, a process similar to how linseed oil hardens in oil painting. The fat molecules react with oxygen in the air and bond together into a rigid polymer network. This aging process is what gives centuries-old egg tempera paintings their remarkable durability. Panel paintings from the 1300s and 1400s survive in excellent condition largely because of how thoroughly the egg binder polymerized over time.
How Traditional Egg Tempera Is Mixed
Making egg tempera from scratch is straightforward but requires fresh preparation. The basic method involves separating the yolk from the white, puncturing the yolk sac, and draining the pure yolk into a container. The membrane itself is discarded since it can leave lumps in the paint.
The yolk is then mixed with roughly equal parts cold water. Dry pigment is ground separately with a small amount of water to form a smooth paste, and then the yolk mixture is stirred into the pigment paste in approximately equal proportions. The goal is a paint that dries with a soft luster. If a test stroke looks chalky, more egg mixture is needed. If it looks too shiny, more pigment paste balances it out. A few drops of vinegar or clove oil can be added to slow spoilage, extending the mixture’s usable life to about a week. Without a preservative, egg tempera needs to be mixed fresh for each painting session.
Casein: The Milk-Based Alternative
Casein tempera uses proteins extracted from milk instead of egg. Casein proteins, primarily four types found in cow’s milk, form durable, elastic, and strongly adherent films when they dry. The resulting paint is more water-resistant than pure egg tempera once fully cured, and it dries to a matte, velvety finish rather than the slight sheen of egg.
Casein binder is made by curdling milk (usually with an acid like vinegar), isolating the protein solids, and then dissolving them in an alkaline solution such as ammonia or borax dissolved in water. The paint handles differently from egg tempera: it’s slightly thicker, more opaque, and can be applied in heavier strokes. It was commonly used for decorative painting, murals, and illustration work throughout the 19th and early 20th centuries.
Tempera Grassa: Adding Oil to the Egg
During the second half of the 1400s, many painters began experimenting with adding drying oils to their egg binder, creating what’s called tempera grassa, or “fat tempera.” This hybrid aimed to combine the fast-drying precision of egg tempera with the smoother blending and richer color depth of oil paint.
Most tempera grassa recipes combine one part egg yolk with one part of an oily medium, typically a mix of drying oils like linseed, walnut, or poppy oil and natural resins like dammar or mastic varnish. A typical recipe might call for two parts whole egg, four parts distilled water, one part stand oil, and one part dammar varnish. The resins adjust the paint’s brilliance, hardness, and viscosity. Tempera grassa played an important transitional role in European painting as artists moved from pure egg tempera toward the oil painting techniques that dominated from the 1500s onward.
What’s in Modern Commercial Tempera
The bottles of tempera paint sold in art supply and school supply stores bear little resemblance to traditional egg tempera. Modern store-bought tempera typically contains no egg at all. The most common binder in commercial tempera, especially the solid cake form popular in classrooms, is gum arabic. This is a natural gum harvested from acacia trees, the same binder used in watercolor paints.
Gum arabic dissolves in water, is nontoxic, and has an extremely long shelf life, which makes it practical for school settings where allergens like egg and milk are a concern. Some liquid tempera formulations use other water-soluble binders like starch, dextrin (a carbohydrate derived from starch), or cellulose-based thickeners. These commercial paints are designed to be washable, inexpensive, and safe for children. They dry quickly to a matte, opaque finish but lack the permanence and film strength of egg or casein tempera. For this reason, professional artists who want the qualities of true tempera still mix their own from egg yolk and dry pigment.
How the Binder Affects the Finished Painting
The choice of binder determines almost everything about how tempera paint behaves. Egg yolk produces a semi-translucent film with a soft sheen that deepens in richness as the paint ages and polymerizes. It rewards thin, precise layering and can achieve luminous effects when built up over a bright white ground. The trade-off is a short working time and the need for fresh preparation.
Casein dries harder and more matte, with greater water resistance once cured. It handles heavier application better than egg but can become brittle on flexible surfaces. Tempera grassa offers more blending time and a richer surface quality, bridging the gap between tempera and oil. Commercial gum arabic tempera is the easiest to use and store, but it remains water-soluble even after drying and lacks long-term durability, making it suitable for student work and temporary projects rather than fine art intended to last.