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4. Parchment

4.1 Definition and composition
4.2 Deterioration factors and preservation
4.3 Restoration techniques: materials and procedures

 

4.1 Definition and composition

Parchment is a semi-tanned skin used as a writing surface in Asia Minor as far back as 200 B.C.

According to Pliny, this support was invented in the city of Pergamum to take the place of papyrus, the importation of which was forbidden due to the rivalry between the famous libraries at Alexandria and Pergamum.

Whatever its origins, the fact is that parchment, due to the ease with which the raw material could be obtained and its clear advantages as a writing surface, gradually replaced papyrus. Its use as a writing material only declined with the general use of paper in Europe (about the fourteenth century, depending on the area).

Very little is known about how parchment was prepared in ancient times but the procedure cannot have been so very different from that used by monks in the Middle Ages.

Lamb, goat or calf skin was used, being soaked in lime water for three days to make removal of fat, flesh and hair easier. Then the hair was scraped off completely using a sharp instrument.

Finally, the skin was dried on stretchers. Once dry (and in some cases slightly dampened), the process was completed by sandpapering both surfaces to ensure even thickness and to smooth both the right side and the reverse side of the skin which was now true parchment.

When this type of support was destined for use only as a writing surface, it was whitened using gesso dust which eliminated grease. When it was to be used for illumination, the parchment was covered with a talc preparation in order to provide an opaque background against which the colours would stand out more clearly.

The outcome of these processes was a writing support which was less flexible than papyrus but which was pleasing to the eye and agreeable to the touch; both sides could be used to write on and mistakes could be eliminated simply by scraping them off.

These properties, together with the resistance and strength of parchment, had a decisive influence on the format of books and scrolls for the innovation of the square book had begun as far back as the first century A.D.

Physiologically, parchment is the inner layer of skin (dermis) and each side has quite different characteristics; the outer layer, the skin or hyaline side, is more compact and darker in hue, yellowish in colour and grainy to the touch. This was the side of the parchment which was preferred for writing. The inner layer, whiter and more fleshy, is the side which was in contact with the fattier parts of the live animal.

The finest parchment used more especially for exquisite books of small size is that made from the skin of newly born or unborn animals (vitela uterine). In this case the skin is so fine and transparent that the hyaline layer can scarcely be distinguished from the fleshier side which is little developed.

Of all the elements which make up the living skin, the only ones which are still present in a parchment are the insoluble proteins and water. The proteins are grouped together forming fibres of considerable physical consistency and excellent hydration capacity; unlike paper, the fibres are not crossed or tangled. The only element which binds them together is water which, as in paper, forms intermolecular bonds which join the fibres chemically mainly by means of hydrogen bridges. This as we shall see later means that the role of water and, more specifically, humidity is of prime importance in the conservation of parchments.

4.2 Deterioration factors and preservation

In general terms, it may be said that parchment is affected by the same factors that damage cellulose materials; but as far as the internal aspect is concerned, since it is a protein-based material, it is less prone to the risk of deterioration and is more resistant to natural ageing.

Abrupt changes in humidity and temperature which may distort the surface and degrade its appearance are the worst enemies of parchment; as it is a semi-tanned skin, the greatest problem is its physical-chemical instability due to its sensitivity and requirements in regard to the binomial temperature-humidity.

Parchment is characterized by its high degree of hygroscopicity for, as was explained in the case of certain cellulose materials, its fibres are held together by the action of water molecules. These molecules are formed by oxygen and hydrogen atoms in adjacent fibres and thanks to these chemical bonds the protein fibres are held together cohesively so long as the hygrometric balance is not disrupted.

The phenomenon of chemical adhesion is the same as for cellulose fibres with the particular feature that in parchment the fibres are not tangled and there is no adhesive (such as are used to size paper) to aid mechanical adhesion.

Skin has the property of being flexible due to the natural disposition of the protein fibres but, when the hygrometric balance is lost due to lack of humidity, they become rigid; dessication leads to isolation of the protein filaments (linking by means of hydrogen bonds is lost) and this separation reduces flexibility, promoting cracking, exfoliation and even disintegration of the support.

In the case of complete saturation with water the excessive number of water molecules transform the fibres into aggravated gelatines; decomposition of the parchment will be further aggravated by the effect of hydrolysis.

As far as preservation techniques are concerned, those mentioned for cellulose materials are effective; hydroscopic stabilization of the materials using polyethylene glycol may be added to them. Furthermore, in order better to preserve the parchment from the effects of temperature and humidity when subject to very aggressive microclimates, the surface may be protected using microcrystalline waxes.

In any event, the ideal microclimate is achieved by maintaining the relative humidity between 60 and 50% and the temperature between 18 and 22C. Parchment, although sensitive to hydrothermic changes, adapts quite well to its surroundings but suffers serious damage when the temperature and relative humidity exceed 40C and 70% respectively.

Alteration due to chemical causes is less frequent than with paper. The problem of acidity is practically non-existent because in pseudo-tanning alkaline substances (lime) are used. However, parchments which are acidy due to a poor manufacturing process, the action of micro-organisms or atmospheric pollution do occur.

To prevent acidification due to amospheric agents, apart from an air-filtering system, use may be made of a solution based on potassium lactate applied directly to the parchment as a preventive measure (described later in the section dealing with the treatment of tanned hides).

Alkalinity, more common in parchment than in paper, leads to yellowing but this effect, although generally due to excess lime, may also be due to the action of bacteria, the existence of fatty acids resulting from a defective tanning process which fails to eliminate fats thoroughly or to pollution, especially if dust carries iron particles which are transformed into a coloured hydroxide.

With regard to alterations due to biological causes, the alkaline nature of the material inhibits the action of many micro-organisms, but if the environment favours their proliferation, the parchment may be attacked both by these agents and by insects.

An effect which is more damaging to parchments than to paper is surface dirt. It is more difficult to eliminate because it penetrates more deeply, partly due to the formation of fatty acids and also because the structure of parchment is conducive to the incrustration of dirt between the pores of the fleshy layer and in the grain of the hyaline layer.


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