Mechanisms of perception of laid lines in Japanese paper

Abstract

Laid lines are observed frequently in Japanese paper. For restoration of historic document paper, the laid line intervals should be consistent between the restoration paper and the original paper for harmonized appearance. Considering this prerequisite, the perception mechanisms of the laid lines are discussed. Laid lines can be easily observed against backlight. However, there are several cases where laid lines are not visible against backlight, but are clearly visible with diffuse reflected light. With paper sheets formed on screens with short bamboo splint pitches, laid lines were observed only with diffuse reflected light. Within these sheets, there was no fiber mass distribution or surface roughness that correlated with the periodicity of the laid lines. On the other hand, paper sheets produced using long splint pitches exhibited light transmission unevenness, fiber mass unevenness, and surface roughness. Microscopic observations using two-way low-angle illumination revealed the following mechanism. In the flow sheet-forming method, fibers are oriented strongly in the cross-splint direction in the first layer while some fibers rotate and become aligned along the interspaces between the splints in the second layer during dehydration. Bidirectionally oriented fibers perpendicular to each other result in a contrasting reflectivity perceived as laid lines.