A brief history of paper
Our word paper has its origin in the name of the papyrus plant that was pulped and pounded by the Egyptians to produce flat sheets which could be dried and decorated. The earliest paper has been dated to 2400 BCE but ink has been found that is 700 years older, suggesting that something existed upon which you could write.
The Chinese were making paper some centuries before the common era and in 105 CE Ts'ai Lun, a Chinese court official, links his name to the invention of paper by recording the instructions for making this material. Ts'ai mixed mulberry bark, hemp and rags with water, mashed it into pulp, pressed out the liquid, and hung the thin mat to dry in the sun. An early use was imperial toilet paper.
The technology of paper-making was slowly diffused westwards. The Arabs were making writing paper by the 8th century and Spain, France & Italy record papermaking mills in the 12th century. But it was not until 14th century that papermaking locations were recorded in England.
From the mid 14th century paper was made in Europe by pulping linen and canvas rags derived from flax and hemp plant fibre. This mix continued until the 18th century, when cotton rags were added to the mix. Rag pulp became an increasingly expensive source of fibre as the demand for paper increased to keep pace with the development of print technology, so around 1850 techniques were perfected for making pulp from trees.
Further processing is required to produce pulp that is white and smooth enough for printing, using wood as the source material. This process requires boiling the material in sulphuric acid, and results in a weaker, whiter pulp. ‘Sulphite pulp’ accounts for less than 10% of total pulp processing (1998) although processing technology has moved on dramatically in the last decade to mitigate the pollution generated by paper mills.
The first machine for grinding wood to pulp was patented in 1844. An elaborate mechanical and chemical process breaks wood into usable pulp. Thermo-Mechanical Pulping (TMP) softens the wood chips with steam before they are ground to leave long fibres intact. Chemo-Thermo-Mechanical Pulping (CTMP) is used to reduce hardwood to pulp using sulphuric acid before steaming.
Making paper from wood rather than rags has some drawbacks. The cellulose content of rags is almost 100% compared with wood which is about 50% cellulose by weight. As much as 30% of the balance in wood pulp can be lignin. Lignin causes wood-based papers to yellow and deteriorate when exposed to light. In 1883, a German, Carl Dahl, discovered that adding sodium sulphate to the soda used to process the wood produced a stronger pulp. This ‘kraft’ pulp is still used for cardboard and newsprint. Kraft simply means strength in German. The kraft process helped dissolve the troublesome lignin and better still, the chemicals and much of the energy were recovered for re-use.
During the industrial revolution paper was mass-produced by casting the pulp onto a moving belt of felt or a fine wire mesh. This technology gave the paper a ‘grain’. This arises from fibre orientation and dried-in strain. The strain is produced as the paper is quickly dried, trimmed and wound onto rolls while in tension.
Handmade papers have the fibres laid at random and allowed to dry flat so do not normally have a grain. To determine the direction of grain, cut a small square of paper then dampen one side with water. The grain runs along the ‘valley’ formed when the paper curls. Most of the expansion takes place across the grain as bonding is weaker. The likely cause is the relaxing of the strains trapped during the drying process.
Paper pulp manufacture uses some powerful chemicals. Paper makers attempt to
remove these to leave the paper with a pH of between 6 and 7, which is
considered acid-free. On the pH scale, which measures the degree of
acidity-alkalinity, 7 is neutral and anything lower is acidic. Acid-free paper
is vital for books and paper products which are intended to last. Paper that has
not had its acidity neutralised, such as newsprint, yellows and deteriorates.
Wood pulp papers are inferior in almost every way to rag-based papers and to vellum, which is derived from animal skin. The laws enacted by the British House of Commons are still recorded and stored on vellum. The law-givers of ancient Rome and the early German emperors took a similar view about the ephemeral nature of paper and demanded that a more durable medium, such as stone, was used.
Paper has become a hi-tech product. But printers need to understand what is an organic material to be able to turn it into books.
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