Why do Kozo papers have excellent longevity?

Every year, millions of books around the world slowly turned yellow and brittle. These damaged books were printed using machine-made paper and were made from wood pulp which became popular in the mid-19th century. Meanwhile, the oldest kozo paper (washi) made in Japan for census registration dates back to 702AD. To this day, the paper is still preserved in the Shoso-In Imperial Treasure Storehouse in Nara. There are also examples of Washi documents that are over 1,000 years old and are in great condition, showcasing its excellent longevity.

Fig. Jōyuishikiron (Kasuga-ban), Imperial Household Agency

The main reason for the longevity of kozo paper is that it contains little lignin, a chemical component that can cause discoloration and reduce the strength of the paper overtime. By nature, the long and slender kozo fiber is also stronger and more durable than wood fiber. Western-style paper made from wood pulp contains much higher amounts of lignin. In recent decades, chemical methods were used to remove lignin, but this can also damage the fibers.

Permanence and durability of a paper are also directly related to the processing and treatment the fibers receive during the pulping process. In the making of kozo paper, the fibers are cooked using one of at least four alkalis, which helps, to a degree, to get rid of the acids produced by the remaining lignin.

Researchers studied how the structure of kozo paper changes as it deteriorates. They found that the decrease in the degree of polymerization (DP) of cellulose in the kozo samples was related to the acidity of the paper. The higher DP of cellulose in kozo fibers compared to wood fibers makes the paper more permanent. As the paper ages, it becomes less flexible because some of the amorphous regions in the fibers recrystallize. However, this recrystallization happens less in kozo paper, which helps the paper maintain its strength and flexibility over time.