In the digital age of the 21st century, our reliance on paper is rapidly declining. Smartphone or tablet screens and electronic paper displays like those of Amazon's Kindle are becoming the norm for everything from cinema tickets to best-selling books. But our history is written on paper and we face an ever-growing urgency to preserve paper-based artefacts before they are lost forever.

About one third of the paper items in large libraries are too brittle to handle, with another third in need of attention over the coming century. Chemistry is at the heart of paper conservation, but as a science, paper conservation is a relatively new field. As educators we must include paper chemistry and conservation in our curricula to encourage future generations of chemists to take up the challenge.

What is paper?

The principal component of paper is cellulose, which is effectively a polymer of β-D-glucose. Hydrogen bonding between cellulose chains sticks them together to form fibrils, which further associate to form fibres, the basis of the structure of paper (see image). Raw cellulose fibres are extracted from plant sources and suspended in baths of water. Pulling an appropriately sized mesh through the suspension forms a mat of interwoven cellulose fibres as the water drains away. The remaining water is removed through drying and pressing, which bonds the fibres together into a sheet.

Laboratory filter paper consists of almost pure cellulose, but anyone who has ever tried writing on it will know how poor it is for this purpose, and how weak it is when wet. Additives are therefore included to strengthen the interaction between the fibres. Traditional additives include gelatine and aluminium sulfate, which strengthen the paper and prevent ink from running. There are several sources of instructions for paper making as a classroom activity (see Further reading).

SEM image of paper fibres

Paper fibres

Source: © Susumu Nishinaga/Science Photo Library

The principal component of paper is cellulose. Hydrogen bonding between cellulose chains sticks them together to form fibrils, which futher associate to form paper fibres

Paper in Europe was originally made from cellulose sourced from linen and cotton rags. This made strong paper structures, owing to the long cellulose chains. The degree of polymerisation - a measure of the average number of glucose molecules in a polymer chain – is high for papers made from linen (3500) and cotton (1000–3000), and this means the chains are tightly bound into the fibrils and fibres by extensive hydrogen bonding.

However, following the invention of the printing press and the enormous surge in demand for paper in the nineteenth century, most paper in our hands today is made from cellulose extracted from wood pulp. Cotton and linen sourced cellulose is now usually reserved for special purposes such as banknotes and artists' materials. While wood is a much more readily available source, the resulting paper has shorter cellulose chains (with a degree of polymerisation around 600–1000) and a weaker structure.

Wood also contains a variety of other carbohydrates and lignin. Lignin is a three-dimensional polymeric material that gives woody plants their physical strength. However, it reduces the strength of paper by interfering with the way the cellulose fibres assemble. For low value paper items, such as newspapers, cheap books and ephemera (material produced for one-off use but now of historical importance), the wood pulps would only be minimally purified to remove lignin. This means they are often the most fragile and rapidly deteriorating materials.