PITH PAPER
A partial reprint from “Conservation of Pith Paper” by Penny Jenkins, Paper Conservation News, March 1995
Introduction
Conservators are often asked to deal with pictures on pith paper. I have had regular inquiries about handling pith paper and the following is a brief practical overview.
Pith Paper
Pith paper or rice paper, a misnomer as it is sometimes known, is made from the inner pith of the plant Tetrapanex papyrifera, which is a member of the Araliaceae family. The plant is a shrub native to Southern China and Taiwan. As well as a painting material, pith paper was used to make artificial flowers and surgical dressings.
Morphology of Pith Paper
Unlike paper made from a matted layer of macerated fibers, pith paper is composed of parenchymytous plant cells and needs special considerations in conservation.
Pith is the name given to the tissue located in the central region of the plant stem and is made up of large food storing and conducting cells known as parenchyma and collenchyma tissue. Their function is to support the plant and conduct water-soluble nutrients both longitudinally and laterally. This ability is determined by complex cell arrangements that are very responsive to moisture. Collenchyma has long overlapping cells to give strength and plasticity so the cells can extend and adjust to the growth of the plant. The behavior of damp pith paper reflects these properties.
Preparation of Pith Paper
The white pith is pushed out from the plant stem and then placed inside hollow sections of bamboo where it swells and moulds itself as it dries into a cylindrical baton. Later the baton is removed and sheets of pith paper are made by hand turning the long side of the pith cylinder against a sharp thin knife to produce a continuous sheet. One can often see vertical lines that correspond to turns of the blade or reel of pith. After cutting, the sheets are pressed flat and trimmed to size.
Problems of Conservation of Pith Paper
Because of its structure, pith paper has an alarming sensitivity to moisture and its tendency to expand and contract can lead to serious mechanical damage if the sheet is restricted in movement in any way. Once torn or degraded pith paper can be very brittle and delicate. Traditional methods of mounting such as by tabs of glue at corners or setting into albums can make them particularly vulnerable if they should be exposed to changes in humidity. (The more humid Orient may well by more conducive to pith display methods). Corners may become detached and vertical or horizontal tears depending on the grain direction may appear every time the page is flexed. Pith paper does become weaker and less flexible with age.
Handling Pith Paper
The light spongy paper can easily be damaged by the lightest pressure from a finger or sharp object, to leave a permanent indentation. Great care must be taken not to crush the sheet and pictures should be rested on an additional support to avoid excess handling.
Removal from Traditional Supports
Chinese export pith paintings are traditionally mounted into albums with decorative silk bindings. The painted pith sheets are generally tipped onto an album page with a starch paste or light animal glue and each painting is bordered with a narrow (1 cm) paper-backed silk ribbon around the four edges to give further protection. The glues can usually be softened with a light application of water and the painting gently eased away with a thin bone folder.
Surface Cleaning
Pith paper cannot be surfaced cleaned with any abrasive methods. The dry surface is far too delicate to withstand erasers.
Mounting and Display
If possible the pith sheets should be individually mounted to protect their surfaces. One method of supporting the pith sheets without restricting their movement, is an adaptation of the old Far Eastern method. The painting is positioned on an acid free sheet without any adhesive. The four edges are overlapped and held down with border ribbons or strips of coloured Japanese paper that are pasted down beyond the painting itself. This in effect cradles the picture and simulates the traditional presentation style. This package can then be placed in an acid-free mount and stored in a controlled environment.
