Egg Tempera and Bone Glue Binders on Chalk Whitewash in Petrykivka Wall Murals

The radiant, luminous visual brilliance of Petrykivka murals is inseparable from the unique chemical substrate upon which they are painted: traditional chalk whitewash. The walls of the Ukrainian khata were constructed from natural earth materials—either sun-dried clay-and-straw bricks (saman) or wattle-and-daub woven onto heavy timber posts. To protect these earthen structures from atmospheric moisture and insect infestation, village homemakers applied seasonal coats of mineral whitewash.

The whitewash was compounded from two primary calcium compounds: slaked lime (calcium hydroxide, Ca(OH)2) and finely powdered kreida (natural chalk or calcium carbonate, CaCO3). When brushed onto the wall, the slaked lime absorbs carbon dioxide (CO2) from the surrounding air in a natural chemical process known as carbonation, converting into insoluble calcium carbonate crystals. The addition of chalk creates an intensely luminous, velvety-matte white surface with millions of microscopic capillary pores, providing the ideal mechanical tooth to anchor water-based paint binders.

To paint upon this absorbent mineral ground, Petrykivka artists formulated egg tempera. While European Renaissance masters used egg tempera on wooden gesso panels, Ukrainian village women adapted the medium to the demanding environment of the heated clay cottage. The craftsman strictly separates the egg yolk from the albumen (egg white), rolling the yolk across paper towels to remove all traces of white before puncturing the vitelline membrane.

Egg yolk is nature's most sophisticated pre-formed oil-in-water emulsion. It contains approximately 50% water, 30% lipids (including neutral fats and cholesterol), and 20% proteins (primarily vitellin and livetin), naturally emulsified by lecithin—a powerful phospholipid surfactant. When mulled with dry earth pigments, lecithin suspends the dense mineral particles in liquid suspension. When applied to the lime plaster, the water evaporates first, allowing the lipids and proteins to crosslink through oxidative polymerization over several weeks, transforming into an insoluble, water-resistant, and flexible paint film that will never re-dissolve during subsequent seasonal cleaning.

When painting the masonry oven (pech)—which endures intense thermal cycling during winter baking and heating—standard egg tempera or animal hide glues are subjected to extreme stress. Animal collagen glues tend to dry out, embrittle, and scorch at elevated temperatures, leading to flaking paint. To solve this engineering challenge, Petrykivka artisans introduced a remarkable natural botanical binder: vyshnevyy kley (sour cherry-tree gum).

Harvested during the hot midsummer months from natural wounds in the bark of native sour cherry trees (Prunus cerasus), this gum weeps in golden, translucent tears. Chemically composed of complex branched arabinogalactan polysaccharides, cherry gum is completely water-soluble yet dries to an exceptionally tough, elastic, and glossy film. When blended into egg tempera, cherry gum acts as a natural heat-stabilizer, allowing the paint film to expand and contract with the thermal movements of the clay-and-brick oven up to 120°C without cracking, peeling, or discoloring.

In secondary architectural zones—such as the exposed squared pine ceiling joists (sosraby) and high-wear entrance doorways—artisans fortified their paints with stolyarnyy kley (animal bone and hide glue) or milk casein.

Bone glue, manufactured by simmering cattle bones to extract pure collagen peptides, was applied as a warm, dilute pre-sizing wash over chalky whitewash surfaces. This wash deeply penetrates loose chalk particles, binding them into a coherent, rock-hard substrate that prevents powdering. For exterior wall murals exposed to driving rain, homemakers blended fresh skim milk curd (cottage cheese) with slaked lime putty to produce calcium caseinate. This ancient water-insoluble binder crosslinks into a weatherproof, stone-like matrix that enabled Petrykivka decorative friezes to survive harsh steppe winters on the exterior facades of the cottage.