Kozo Mulberry Washi Paper Stretching & Rice Paste Sizing in Chochin Construction

The luminous skin of an authentic Japanese chochin owes its remarkable properties to kozo (Broussonetia papyrifera, or paper mulberry). Unlike wood-pulp papers derived from tree trunks through harsh chemical delignification, washi is harvested from the renewable inner bark (bast) of cultivated mulberry shrubs. These bast fibers are extraordinarily long—averaging 12 to 20 millimeters in length—with high tensile strength and natural helical flexibility.

During traditional winter papermaking (kan-suki), artisans boil the bark in wood-ash lye, rinse it in freezing river water to bleach it naturally (kawa-sarashi), and beat the fibers by hand with wooden mallets on granite anvils. This gentle mechanical treatment preserves the structural integrity of each cellulose molecule. Suspended in clean mountain water with natural plant mucilage (neri derived from the tororo-aoi plant), the intertwined fibers form an intricate web that can withstand thousands of folding cycles without weakening.

The adhesive that bonds delicate washi to the slippery outer skin of the bamboo ribs is shofu-nori—a refined, precipitated wheat starch paste. Unlike commercial flour pastes that contain gluten, proteins, and lipids that yellow with age and attract insects, shofu is manufactured by repeatedly washing wheat flour until all proteinaceous gluten is removed, leaving only pure, ultra-fine starch granules.

The artisan cooks this starch in pure water over a slow charcoal or gas flame, whisking continuously until the mixture undergoes phase transition into a translucent, silky gel. Once cooled, the paste is pressed through a fine horsehair sieve (koshi-ki) to eliminate any micro-lumps. Shofu-nori has a neutral pH, remains permanently flexible after drying, and possesses the crucial property of total reversibility: if a lantern paper tears years later, moisture applied to the seams instantly reactivates the paste, allowing the old skin to be peeled away cleanly without harming the underlying bamboo frame.

Applying flat sheets of paper onto a compound-curved, three-dimensional bamboo skeleton requires sophisticated geometric tailoring. The artisan cuts the washi into narrow, elongated vertical strips called gores (koma), whose convex curved edges correspond precisely to the radial curvature of the wooden mold.

The craftsman applies a thin, uniform line of shofu-nori along the bamboo higo ribs using a stiff bristle brush. The washi gore is aligned carefully from the top collar down to the base and pressed into contact. Using a flat, soft badger-hair smoothing brush (nade-bake), the craftsman works from the center of the strip outward, smoothing out trapped air bubbles and ensuring that the wet paper molds itself around the cylindrical contours of the bamboo ribs. Adjacent strips overlap by only 2 to 3 millimeters; master artisans often tear the paper edges with wet water-lines (mizukiri) rather than cutting them with shears, creating feathered fiber fringes that interlace invisibly when pasted.

The defining moment in chochin papering occurs during the drying phase. When first applied, the wetted washi is soft and slightly slack. As the moisture in the wheat paste evaporates into the surrounding air over a 24-hour drying period, the cellulose fibers undergo natural capillary contraction. The paper shrinks slightly in all directions, pulling itself drum-tight across the underlying bamboo rib skeleton.

This tight tensioning transforms the paper into an acoustic and optical diaphragm. Light striking the inner face of the paper encounters thousands of multi-layered, semi-translucent kozo fibers running in random orientations. Instead of transmitting light directly like clear glass, the fibers induce multi-directional Rayleigh and Mie scattering, softening harsh candle or bulb glare into a warm, omni-directional glow that bathes the surrounding architectural space in gentle, soothing amber light.