Two major problems associated with old paper manufactured during mid nineteenth century onward are:
- Brittleness
- Yellowing or darkening
Breaking of paper due to brittleness (loss of flexibility to bend without breaking) generally occurs in very old paper which was manufactured from wood pulp technology.
Other feature found in such old paper getting brittle, is yellowing or browning.
Paper becomes acidic either by absorbing pollutants such as sulfur dioxide and nitrogen oxides, or because of manufacturing technique and components added during manufacturing. Traditionally, aluminium sulfate (or ‘papermaker’s alum’) was added to harden, or ‘size‘, the paper. While this gave initial strength, it was also a source of acidity. Most paper made around or after 1850 is acidic due to manufacturing additives.
The β-acetal oxygen bridge joining the glucose molecules of cellulose together is susceptible to acid hydrolysis. This breaks the chains and weakens the fibres. Paper that decomposes this way becomes hard and brittle, and disintegrates easily.
The yellowing that people typically associate with old books is unique to wood pulp paper, and it is a result of lignin, the polymer that is responsible for binding cellulose fibers together. Lignin is an integral part of wood itself and essential to the pulping and binding process of wood pulp paper, so it is an unavoidable ingredient. When lignin is exposed to the environment, it undergoes oxidation, causing it to destabilize and absorb more light, in effect turning it yellow. Although it is possible to preserve wood pulp paper by keeping it completely unexposed to sunlight, it takes only the briefest exposure to begin the oxidation process, which cannot be reversed.