Paper and Books
Paper is one of the most commonly used materials, with much of the history of mankind recorded on this medium. While the introduction of computers heralded promises of a ‘paperless society’ this has not happened. Hard copy back-ups are still used and stored in archives, printing, copying and scanning of documents has continued and paper waste has continued to increase.
The word paper is derived from papyrus, a long-stemmed plant Cyperus papyrus. The method that used this plant as a writing support was developed by the Egyptians about 4000 years ago. Although paper, as it is known today, was developed by the Chinese around 2000 years ago, it was not introduced into Europe until the 12th century. The Chinese used the mortar and pestle method of paper making, where cloth or bark was rinsed with water and then reduced to a pulp by grinding with the mortar and pestle. In Europe, hammers driven by water power were used to produce pulp from cotton or linen and water by beating these materials in stone or metal-lined wooden troughs.
Beating is a very important part of the paper making process as it increases the area for inter-fibre bonding, the strength and the flexibility of the paper. Paper made without mechanical treatment is usually weak. Occasionally straw from wheat and rice plants was added to cotton and linen but this produced an inferior, weaker paper. Very high quality paper is produced from cotton or linen and because of a shortage of the raw materials this type of paper was highly valued.
It was not until the 1850s that paper was produced from ground wood pulp. Wood pulp consists of both fibrous and non-fibrous components. This wood pulp paper, which usually consisted of 80 % ground wood and 20 % cellulose, was both cheap and of low quality.
The low quality and low durability of wood pulp paper are due to the rapid breakdown of the non-fibrous components when exposed to air and light. Newspaper, for example, which is made from low grade wood pulp, yellows very rapidly when exposed to light. Prolonged exposure leads to increased yellowing and brittleness.
Techniques developed in the early 20th century to separate the fibrous and the non-fibrous components of wood pulp led to the production of higher quality paper.
Usually paper is made up of the following combination of materials:
- raw plant matter;
- sizing agents;
- fillers; and
- brighteners and colouring agents.
The basic raw materials of paper are fibres derived from wood and other plants. Good quality paper contains a high proportion of cellulose molecules with a correspondingly low lignin content. Cellulose is a polymeric substance made up of linked glucose units. The arrangement of these units and of the resultant polymeric chains determine the overall quality and mechanical properties of a paper sheet. In good quality rag paper about 1000 glucose units are linked.
High cellulose, low lignin papers are usually prepared from cotton rag or linters, whereas paper with a higher lignin content is derived mostly from wood pulp. Depending on the required use for the paper, wood pulp may be chemically treated to produce papers of varying qualities.
Paper is sized so that it repels liquids such as inks, making it suitable for writing and printing. A higher degree of size is used for the purpose of writing, to prevent ink from feathering with a lower degree used for printing so that ink can be absorbed as quickly as possible.
Traditionally sizing was carried out as a separate step. By the second half of the 19th century however, it became possible to add the sizing agents directly to the pulp (internal sizing). Combinations of alum and rosin were originally used in this process, but as these sizing agents elevated acid levels in the paper, the paper deteriorated more quickly. Synthetic cellulose components are now used as sizing agents to overcome the acidity problems, allowing pH neutral and alkaline papers to be produced.
Clay, titanium oxide and calcium carbonate are among the most commonly used fillers. These materials, added during stock preparation, serve to produce different finishes including calendared paper.
Although indigo was used originally as a colouring agent it has been replaced by synthetic fluorescent dyes and optical brighteners.
Types of Paper
Paper types are many and varied. Paper quality depends on the raw materials and chemicals used in processing and the actual production practices used. Archival quality papers, made from rag material and which use an alkaline processing system, last longer than those made from the low quality wood pulp used for newspapers. As described earlier, the latter discolour and degrade rapidly.
A list of paper and board types, which describes their components and chemical characteristics, is supplied in Appendix 6.