Paper 33
When paper was first made in Medieval Europe in about 1200 AD, the best
source of easily separated cellulose fibres was rags made of cotton or linen.
Cotton occurs naturally as nearly pure cellulose in the fluffy seed heads of the
cotton plant and can be used for textile- or paper-making with little prepara-
tion. Linen fibres occur in the stems of the linen plant but the fibres have to
be freed from the stem matrix. Linen fibres are separated by a combination of
microbiological, chemical and mechanical methods by first softening the
stems of the plant by leaving them in water for several weeks (retting) and
then subjecting them to beating and further purification. Nowadays, little rag
paper is produced, but some is used by artists and calligraphers. Modern papers
are mostly made from wood cellulose fibres that are difficult to purify and
chemical processes have to be used which can cause significant damage to the
cellulose fibres/moleculeswhile effecting the separation.
Traditional Chinese and Japanese papers are made from cellulose fibres
derived from locally-occurring plants. The fibres are separated by processes
similar to those used on linen. Long fibres with relatively undamaged cellu-
lose produce strong papers with good permanence. The first papers ever made
are believed to have been made in China in 105 AD.
Paper is made from a suspension of fibres in water. The suspension is drained
through a fine mesh so the water drains away and the fibres are retained to
form a mat on the mesh. On drying, the fibres stick together and form a sheet
of paper. Many paper experts define paper as a material made by a process
that involves the separation of fibres into a suspension with subsequent for-
mation into a sheet.
It can be seen from the formula for cellulose that there are many hydroxyl
groups all along the chain. These –OH (hydroxyl) groups are attracted to one
another by the attraction of a partial negative charge on the oxygen atom for the
partial positive charge on the hydrogen atom (hydrogen bonding); this is the
principal mechanism explaining why paper fibres are held together in a sheet
of paper. Another mechanism is the tangling together of the long, thin fibres.
An important step in papermaking is the modification of the paper fibres
by “beating”. When undamaged, paper fibres are long and thin with relatively
little surface roughness. If damaged in the intended way, the ends and surfaces
of the fibres can be induced to fray into thinner subunits, a mental picture may
be of the way our hands have fingers at the ends. This process is called beating
because that is exactly how the process is traditionally done, by hitting the fibres
with big mallets. Beaten fibres have a much bigger area for the formation of
hydrogen bonds and generally form stronger sheets of paper. Industrially,
beating is performed in a machine in which the fibres pass between two moving
metal blades that subject the fibres to great stress and shear the fibres apart.
Handmade paper is formed by dipping a tray with a mesh base (the mould)
into a suspension of fibres in water, lifting the mould from the vat of fibre