Feb 19 2025
Washi, the traditional handmade paper of Japan, holds a revered place in both cultural heritage and artistic expression. Among its various types, kozo paper stands out for its exceptional strength, versatility, and enduring significance. The term washi refers to handmade paper specifically from Japan and encompasses various paper types and their manufacturing techniques. Kozo is one of the three primary fibers that are used in the production of Japanese paper along with mitsumata and gampi. Kozo paper, also referred to as mulberry paper, is derived from the species Broussonetia papyrifera.
Kozo paper’s durability, pH neutrality, and aesthetic qualities make it indispensable in fields like conservation and decorative arts. Out of the three primary fiber types, kozo has the longest fiber structure and creates the strongest paper. Broussonetia papyrifera is cultivated with relative ease and the plant can survive while regenerating its farmed branches for decades. The native range of this species is India to Korea and Indo-China.
To prepare kozo paper, branches of Broussonetia papyrifera are harvested and trimmed to uniform lengths before they are steamed for several hours. The steaming process allows the brown outer bark and two layers of the inner bark to be loosened and separated from the wood core. The outer layers are the raw material used in the preparation of the fibers while the inner core is reserved for firewood or used in other crafts. The brown outermost bark is meticulously shaved away from the lighter inner layer. The green middle layer, rokubu-heguri, can either be removed or remain if one wishes to make a toned paper. Following the removal of the brown bark, the fibers are washed and cooked in an alkaline solution (usually created by the addition of wood ash, soda ash, or lime) to further break down the fibers. The residue of the alkaline wash makes the finished paper mildly alkaline, serving to deter future acid deterioration that occurs in the aging of paper.
Once again, the fibers are rinsed and then set out to be bleached in the sun or by a chemical bleaching accelerant. The fibers are then pounded to separate the individual strands of fiber -- this process can be done mechanically or manually. The separated strands are added to a thickened slurry mixture, usually achieved by the addition of tororo-aoi (Hibiscus sp.) or noriutsugi (Hydrangea sp.). Finally, to be formed into a uniform sheet, a mesh screen is placed within a frame and dipped into the slurry vat. Using a repeated rocking motion side-to-side, the kozo fibers are distributed and evenly spread across the sheet. The sheets are piled upon each other and pressed to expel any extra water. And then, they are placed upon a flat surface to dry.
Kozo paper is an integral material in the field of conservation because of its slightly alkaline composition and structural qualities. The featured photos are from a booklet containing the various kozo paper samples that conservators utilize in the Preservation and Conservation Department at the LuEsther T. Mertz Library. The long fibers of Broussonetia papyrifera make it an ideal tool for conservators as it is applied as a thin layer while providing reinforcement to fragile or compromised material. Conservators select the paper type depending on the needs of the project. For instance, a conservator may select a transparent sample of kozo paper and apply it in thin layers to ensure that the surface of an object remains visible to viewers. Alternatively, they may choose a paper that has been dyed a specific color to conceal its appearance on an object.
