Often called “rice paper” under the mistaken impression it was made of rice, pith paper comes from the pith of a plant, Tetrapanax papyriferus which is native to southern China and Taiwan. the pith is effectively the core of the plant stem which is surrounded by the outer harder ‘skin’.
Pith creates a thin, translucent paper and was made by pushing the white pith from the plant stem. After swelling and drying it was turned against a sharp thin blade to produce a continuous sheet. After cutting the sheets were pressed and cut to size.
Unlike paper made from a matted layer of crushed fibres, pith paper is composed of aligned plant cells and this leads to the requirement for special treatment in any conservation process.
Pith paper is extremely sensitive to moisture and expands and contracts rapidly in humid or dry conditions. This can lead to tears and mechanical damage if the sheet is restricted in movement in any way.
I’m sure there are perfect conditions of heat and humidity in which to keep pith paintings, but in the world of the average collector rather than a museum, there are certain practicalities to deal with. Certainly I’m not sure I would keep them in very hot and dry conditions like my lounge or potentially very moist conditions like my bathroom, but otherwise the key in a normal house is probably more about stable conditions than any particular humidity or temperature.
Pith paintings were traditionally mounted in albums by placing the painting inside a ‘pocket’ created by gluing ribbon (usually light blue) in a rectangle attached to a supporting paper sheet. This allowed the paper to move as it expanded and contracted without tearing. However, it appears the pith was often either accidently glued to the ribbon or firmly held by the ribbon to restrict free movement. The most common damage I have seen on pith paintings is either the corners being torn across from the ribbon being glued in each corner or if held firmly along one side that entire side can be torn from the rest of the picture. Also, as pages were turned in the album splits have occurred along the cell structure in the paper.
The bad news is that the opportunity to damage the paintings further continues once you have purchased the painting without careful handling, particularly for paintings still contained in albums or loose paintings that have not been mounted or framed. Even when framed it is obviously important that the movement of the pith painting has not been restricted as part of the mounting process and ideally acid free mounts should be used.