In some cases, such materials offer additional long-term protection against acids. Prior to about 1850, papers were produced from linen and cotton rags. Such papers are relatively stable and, when properly stored and handled, will last literally for centuries. After 1850, as the demand for papers and utilization of them increased, alternatives to linen and cotton rags began to be used in paper manufacture. From about 1860 on, chemical wood pulp became more and more used until today the vast majority of modern papers are made from chemically treated wood pulp.
Chemical treatment reduces the lignin content, and synthetic sizing agents are then added.
It is the decomposition of residual lignin in paper which, in the presence of heat and light, presents the major problem to individuals wishing to preserve the papers concerned. Decomposition of lignin produces a strong organic acid and also promotes the absorption of atmospheric acid, although some wood pulp paper is manufactured with the lignin removed resulting in the paper having only a slightly acidic pH and in the paper being acceptable to many conservation purposes. The resulting data are displayed in Table 1. The data in Table 1 are now old, and the study needs to be repeated. The conclusion from the study was that while a pH of 7 is ideal, any papers with a pH of 6, or 7, or 8 would be satisfactory for use with our stamps.