Glue-Tempera and Animal-Hide Gelatin Priming on Korean Pine

In the material science of traditional Asian tempera painting, the master adhesive medium is agyo (아교, 阿膠)—animal-hide gelatin glue. For over two millennia, Korean artisans extracted this proteinaceous binder from the connective tissues, cartilage, and skins of cattle (Ugyo) or, in coastal regions, from the swim bladders of fish (Eogyo). At the molecular level, agyo is composed of collagen—a structural protein characterized by a tight triple-helix polymer chain rich in glycine, proline, and hydroxyproline amino acids.

When animal hides are simmered in pure water at carefully controlled temperatures between 50°C and 60°C, the intermolecular cross-links of the collagen dissolve, releasing the triple-helix chains into solution as gelatin. The thermal discipline of this extraction is absolute: if the temperature rises above 70°C, or if the glue is allowed to boil, the delicate peptide bonds break down irreversibly through thermal hydrolysis. Boiled agyo loses its long-chain elasticity, resulting in a brittle, weak glue with negligible tensile strength. Properly extracted agyo, upon cooling, solidifies into a firm, translucent, amber-toned jelly that can be dried into hard slabs and stored indefinitely without degradation.

The timber used in grand Korean architectural monuments is predominantly Korean red pine (Pinus densiflora). While renowned for its monumental structural girth and natural curved beauty, red pine presents severe challenges to the painter. Pine is an anisotropic conifer with high resin content, porous cellular tracheids, and dense resinous knots (songjin) that continue to weep volatile terpenes for years after felling.

Before painting, the master Dancheongjang executes rigorous surface preparation. The timber faces are scraped with curved hand scrapers to remove mill glaze and loose surface fibers, opening the cellular pores. Next comes knot sealing: every resinous knot and pitch vein is carefully torched with hot irons to crystallize volatile turpentine, followed by two generous coats of natural dewaxed blonde shellac (or aged, highly concentrated agyo sizing). Once sealed, the entire timber surface is saturated with a warm, dilute priming wash (pojeong) containing approximately 5% agyo dissolved in water. The warm liquid is drawn deep into the cellular lumen of the wood via capillary action, stabilizing the soft springwood tracheids and equalizing surface suction across the entire beam.

In contemporary commercial paint, synthetic acrylic or vinyl resins encapsulate pigment particles in a continuous plastic film. In traditional Danchong, the paint is a pure glue-tempera. The dry, hand-ground mineral pigment is placed in a shallow ceramic dish (jongji), and a measured volume of warm, 50°C agyo solution is added. The artisan mulls the paste thoroughly using clean fingers or a glass muller, ensuring that every individual microscopic mineral crystal is coated in a paper-thin sheath of warm protein solution.

The ratio of agyo to pigment is a subtle art learned through years of sensory apprenticeship. Different mineral pigments possess vastly different specific gravities and oil/glue absorption capacities. Dense, heavy mineral ores—such as cinnabar (HgS) and red lead (Pb₃O₄)—require a higher concentration of glue (typically a 10% to 12% solution) to prevent the heavy particles from settling out or chalking when dry. Lighter earth ochres and organic soots require a leaner glue solution (around 6% to 8%); excessive glue in an ochre paint creates high surface tension that will cause the paint layer to curl, crack, and pull away from the timber as it dries. When compounded with mathematical perfection, the tempera glides effortlessly from the horsehair brush, leaving an opaque, velvet-matte mineral field.

The ultimate technical brilliance of agyo glue-tempera lies in its reversible, micro-porous physical nature. Unlike impermeable modern paints that suffocate the wood, dry agyo tempera forms a breathable, open-network matrix. As the water evaporates, the collagen proteins form a flexible three-dimensional web that binds the mineral crystals to one another and anchors them into the cellular tracheids of the pine, leaving microscopic air channels between the pigment grains.

This micro-porosity allows the underlying timber to breathe. When summer monsoon humidity surges, moisture vapor in the air can enter and exit the timber through the paint film without building up hydrostatic vapor pressure. During sub-zero winter freezes, the flexible collagen protein network expands and contracts harmoniously with the dimensional movement of the pine, preventing the cracking and peeling that inevitably destroys rigid synthetic coatings. Furthermore, should restoration be required centuries later, the paint film remains completely reversible: it can be consolidated with fresh warm agyo or gently dissolved with warm water compresses without damaging the precious underlying historical timber, ensuring the survival of royal craftsmanship across generations.