Kozo

Kozo (paper mulberry) is the most important fiber source for Japanese papermaking and, by extension, for Japanese woodblock printing. The plant, which grows rapidly as a cultivated crop across Japan, yields long, strong bast fibers from its inner bark that produce paper of remarkable durability and flexibility.

The fiber preparation process is labor-intensive: branches are steamed, the bark stripped, the outer bark removed, and the remaining white inner bark (shiroko) cooked in an alkaline solution to separate the fibers. These are then washed, any remaining impurities picked out by hand, and the cleaned fibers beaten into pulp. The long fiber length (up to 10mm, compared to 1-3mm for wood pulp) gives kozo paper its characteristic strength.

For woodblock printing, kozo paper's strength is essential — the paper must withstand repeated dampening and pressing against blocks without tearing or stretching. Its absorbent surface accepts water-based pigments beautifully, and the interlocked fiber structure allows the paper to mold precisely into the carved details of a block under baren pressure. The best-quality hosho paper used for fine woodblock printing is made from 100% kozo fiber. Lesser grades may blend kozo with wood pulp, resulting in less durable but more affordable paper.