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3. Cellulose supports

3.1 Paper: definition and composition
3.2. Other cellulose supports
3.3. Deterioration factors and preventive measures
3.4. Restoration techniques: materials and procedures

 

3.1 Paper: definition and composition

Paper is the main support for writing today; from the physical point of view it is simply a laminar conglomerate obtained by pulping fibres, normally vegetable fibres.

Its origins are very remote and, although the composition and manufacturing process have varied over the years, the essence has remained the same.

Paper as such appeared very probably in China in the second century A.D. Originally it was obtained from remnants of silk, cloth and plants, such as paper mulberry and ramie which were pounded, mixed with water and filtered through a kind of bamboo strainer which retained the pulp. This pulp, once dried, glued (generally using adhesives obtained from roots and algae) and polished was transformed into a surface suitable for receiving ink.

This Chinese system, jealously guarded, hardly spread beyond the area of its origins. It passed first to Korea and later to Japan (seventh century) where the methods of obtaining a paper of higher quality were perfected.

After the conquest of Samarkand in the eighth century the Arabs wrested the secret from Chinese prisoners who were paper-makers and spread the use of this support throughout their territories. It reached the West through Spain. In Europe paper is documented from the tenth century in Cordoba and Seville; from that time onwards it spread slowly across the rest of the continent and reached Russia in the eighteenth century, later than America where it was introduced by the Spaniards in the sixteenth century.

With the spread of the use of paper the raw materials employed in its manufacture changed. The Arabs used cotton fibres but in Europe the most usual ingredients were cotton or, more especially, hemp or linen rags.

In the manufacturing process the pieces of rag were rotted in lime to make extraction of the fibres easier. The fibres were then pounded with water-driven beaters, or, from the seventeenth century, with "hollanders", (a roll fitted with bars revolving in a cast-iron tub), these systems producing a more refined product.

The pulp obtained was extracted from the "hollander" using metal moulds which left their imprint on the paper which, once dry, was glued. At first, vegetable adhesives were used; later animal glues, and finally alum were introduced to harden the pulp.

Paper made in this way is of very high quality because of the fact that it is somewhat alkaline and consequently is protected against acidity, and on account of additives which are essentially harmless. Only the presence of alum usually presents conservation problems. The main characteristics of this paper are the impressions made by the moulds and the fact that it dilates evenly in all directions due to the regular distribution of the fibres.

The manual procedures gradually gave way to more mechanized methods which produced a paper with different characteristics known as endless paper. The first machine for this type of paper appeared at the end of the eighteenth century and is the basis of all modern machinery used in the paper industry.

Machines for producing paper in continuous rolls consist of an endless belt on which the pulp is deposited so that long strips of paper are obtained instead of sheets. The principal characteristic of continuous paper is that the fibres, due to the movement of the machine, adopt a mainly longitudinal direction which means that the paper dilates in a largely transversal sense. Endless paper is also characterized by having no water-mark, while hand-made paper produced with woven cloth moulds may not have a mark (vellum).

Coinciding with the appearance of endless paper some components began to change. For instance, the shortage of white rags led to the use of coloured rags which, as from the eighteenth century, could be bleached using chlorine substances. Pastes and animal glues were replaced by an alum-based preparation which was more advantageous because it could be mixed with the paper pulp and eliminate the glueing process.

The use of chlorine meant the deterioration of the components of paper as it favoured oxidation. Alum (a sulfuric acid salt) was also harmful since, when dissolved in water, it produces a strong acid reaction which destroys the alkaline reserve, harming the cellulose fibres.

But the problem of the quality of paper was aggravated when rags became scarce and a substitute had to be found for the principal raw material. Wood was used for the first time in the nineteenth century; the disadvantage of this new product was that it had a lower cellulose content and a higher lignin content. Lignin, which is present to a considerable degree in wood, is an element which contributes to the acidification and oxidation of paper.

The methods of obtaining wood pulp also affect the quality of the paper as, with so-called mechanical pulp, obtained by separating the fibres from the bark by abrasive methods, the paper obtained is of poorer quality with short uneven fibres.

The paper industry has remedied the lignin problem, eliminating it by a chemical fibre-removal process. Foremost among these systems are the sulphate solution procedures in the production of so-called kraft paper which, due to its physical resistance, is of high quality.

A combination of these mechanical and chemical disintegration systems results in a semi-chemical pulp which is better than mechanical pulp and more economic than chemical pulp although of poorer quality.

Chemical pulp papers have not provided a solution to the problem of paper conservation for, although the harmful effects of lignin are removed, the effects of the chlorine elements, colophony and alum, persist.

The alternative is permanent-durable paper, made of good quality wood pulp, which has an alkaline reserve and is glued with stable resins.

The future of paper manufacture may be directed towards obtaining synthetic fibres which would produce a highly resistant material; an example of this might be the polyester used today as a substitute for sulphurized, vegetable paper.

Nowadays the composition of paper is highly complex as a multiplicity of additives are used which modify its characteristics, producing very different kinds of paper such as art paper and vegetable paper. These require widely differing treatment from the conservation point of view.

3.2. Other cellulose supports

Although paper is currently the writing surface par excellence, materials specific to particular periods and cultures which were also used as writing surfaces should not be overlooked.

Leaving aside certain materials which belong to the domain of archaeological conservation (stone, clay, metals, bone, ivory...) and writing supports of animal origin (parchment and tanned skins) which will be dealt with in other chapters, we shall concentrate on those materials of organic-vegetable origin (cellulose) which, in many cases, constitute a true forerunner of paper.

The first cellulose writing surface was probably tree bark as it was easily obtainable; among these supports we find palm leaves, typical of Hindu culture until the fifteenth century, bamboo canes in Chinese culture (from 500 B.C. to the fifteenth century) and wooden tablets which, waxed or stuccoed, were used in the Roman world and in Egypt from the fifth century B.C. until the Christian era.

Among the supports which already involve a certain manufacturing process are, firstly, felts which are the direct forerunner of paper. Apart from being characteristic of Chinese culture, they were also typical of other peoples who obtained a support for their writings from the pounding of rags or simply from tangled fibres.

Cloth has also been used as a writing surface. Examples of this are the silks used in the East and American aloe cloths (maguey and agave) used in Pre-Columbian cultures.

But the two cellulose supports of most importance because they were used habitually among peoples of a high cultural level and were disseminated to neighbouring cultures are papyrus and amate which will now be discussed in greater detail.

3.2.1 Papyrus

Papyrus - Cyperus papyrus - is a plant belonging to the cyperaceae family which grew naturally and in abundance along the banks of water courses in certain areas (extinct today). There are three varieties: C. nyloticus which grew on the banks of the Nile and in the Nile delta, C. syriaca, a species specific to Syria, and C. siciliaca, a species confined to Sicilyo. The "mates" which grow in Central America may be considered a variant.

The papyrus plant grows to a height of 4 metres and the stem is usually about 10 centimetres thick. It is smooth in appearance with a plume of elongated leaves and bears small greenish flowers. The stem section is cylindrical-triangular with concentric layers.

The Egyptians were the first to use papyrus as a writing surface. From the third millenium it appears in hieroglyphics, one example dating from the year 2,200 B.C.

From Egypt papyrus reached the Graeco-Roman world and from there it spread throughout Europe where it was used mainly until the eleventh century although its use continued within the Roman Church until the fifteenth century.

The process of manufacturing papyrus scrolls is known from the writings of classical authors such as Pliny. The usual procedure was probably something like this:

1. The stem of the plant was cut transversally into 30-50 cm. pieces and the cortex stripped.
2. Fine longitudinal strips were cut.
3. These were laid side by side, overlapping to form a flat surface.
4. One or more layers were laid transversally upon this first layer and always at right angles to it.
5. The surface was pounded with a wooden mallet and pressed under a heavy slab.
6. After being dried in the sun it was polished with a stone or bone polisher.

The joining of the strips to form a compact surface was aided by the sap given off by the plant when it was pounded. The sap also acted as size which rendered the sheet semi-apt for use as a writing surface. The polishing and, in some cases, liming process gave the surface the necessary characteristics to make it completely suitable to receive the ink.

The fact that several qualities of papyrus have been mentioned, depending upon whether it was made from the inner or outer parts of the stem, would seem to indicate possible variations in the manufacturing process. It is possible that with a longitudinal cut from the heart of the stem the internal layers would be separated out and whiter, softer papyrus could be obtained whereas the outer layers, which were darker in colour, were used for rougher supports.

The papyrus scroll as such could reach lengths of up to 30 metres when "pages" of 15-17 centimetres were joined together. These long strips were rolled onto a stick, which the Romans called the "umbilicum", so as to make handling easier and prevent tearing.

The ink most commonly used on this surface was lampblack bound with gum arabic although papyri exist with coloured illustrations.

3.2.2 Amate

Amate was the main writing surface used by the foremost Pre-Columbian American cultures, the Mayan and the Aztec. It may have originated earlier among some cultures which developed in the Gulf of Mexico. The oldest dates for amate come from the Teotihuacan culture in the first centuries of the Christian era and its use survived until the sixteenth century when it was replaced by paper.

Amate is a kind of felt (tangled fibres) obtained from a wild fig-tree of the same name which grows extensively in Central America, especially in Yucatan.

Botanically it belongs to the genus ficus and among the most valued varieties are f. cotinifolia, f. petiolaris and f. lancifolia. These trees reached gigantic proportions and from the trunk hung aerial roots which helped support the enormous structure; these roots were the material used to make amate.

Different chroniclers have furnished superficial descriptions of the processes which the Mayan, Aztec and other minor Meso-American tribal cultures used to transform the vegetable material obtained from these trees into "paper".

Perhaps the most complete description is that given by the naturalist Francisco Hernández in 1570. According to this author, the natives cut the thick branches and laid them in streams, weighted down by stones. They were soaked for several days to soften them. The outer bark was removed, and the wood was beaten on a flat surface with a grooved stone attached to a handle until it split open. The fibre was then separated from the wood and converted into a conglomerate which was cut into small pieces. It was then beaten with a flat stone until a fibre conglomerate was obtained; this was smoothed out to form sheets of varying size. When dry it assumed the characteristics of a thick, compact paper, although rougher than the paper made at the same time in Europe.

Other chroniclers differ slightly in the details of the manufacturing process and tell of boiled fibres to which a sticky substance was added as size, and a kind of "linewashing" process to close the pores of the amate and made it more suitable for use as a writing surface.

Research carried out on ancient amate codices has identified the fibres of several species of ficus although some also include maguey fibres. Ficus was probably preferred but, depending on the area, local plants such as maguey and presumably palm would be used.

Concerning the size, researchers are inclined to believe that it was obtained from the gluten of an orchid plant; it was used both to size the paper and to bind the colours.

With regard to the "linewashing" process applied to some codices, analyses indicate that it is a calcium carbonate of vegetable origin which was probably obtained from a bush found in the Yucatan. The white ashes obtained by combusion of this plant were perhaps used to cover some of the codices in order to achieve a smoother surface.

What seems to be unquestionable is that this type of "paper", before it was employed as a writing surface, was used to prepare offerings, adornments and vestments of a sacred nature and, by the less fortunate classes, to make clothing and blankets.

On the form of amate books, only accordion and folding documents are conserved; these are written on both sides and are protected by wooden covers so that their external appearance is quite similar to that of European books. Although sizes vary, they contained about 50 pages measuring 25 cm by 18 cm; laid out they could attain lengths of 10 to 15 metres.

The manufacturing method in modern times has changed slightly; the fibres, having been washed in running water, are boiled in a pot with wood ash and quicklime, washed again and finally beaten to achieve even maceration. The product is then smoothed out and polished.

Some peoples in East Asia and Oceania used supports similar to amate such as bark-cloth (tape) obtained from a mulberry tree in Hawaii, Tahiti and among other Pacific peoples.

3.3. Deterioration factors and preventive measures

It must be borne in mind that any substance, especially of an organic nature, is potentially perishable.

Preventive measures (preservation) are aimed at halting, as far as possible, the deterioration process which can only be tackled by, first, analysis of the causes of alteration and, subsequently, application of a method capable of eradicating these causes.

Preservation is a priority in the process of conservation and even when restoration is carried out it is necessary to insist on the need for preventive treatment to avoid any future damage.

Examining the origin of the causes, we may identify external causes (present in the environment surrounding the document) and internal causes (deriving from the materials of which the documents are made).

External causes are extraneous to the work itself and are mostly foreseeable as they are an integral part of the microclimate surrounding the document. Preventive measures are mostly directed at these causes, which are termed natural causes, because alterations of an internal nature are not unfrequently encouraged by environmental aggression.

Among these natural causes the alterations caused by the binomial temperature-humidity are of great importance. Water is an essential element for the good conservation of cellulose materials since the fibres are bound together by means of semi-chemical bonds in which water helps to form the hydrogen bridges which hold the cellulose molecules together.

Lack of humidity will lead to the partial breakdown of these interfibre bonds thus making the document fragile. Dryness, furthermore, also makes the adhesives crack.

Excess humidity causes decomposition by hydrolysis as well as provoking acid formation, weakening the size and softening the adhesives.

Abrupt changes in temperature and humidity produce dilation, exfoliation and cracking in archive materials and micro-organisms proliferate when temperature and humidity levels are very high.

The microclimate which is suitable for cellulose materials lies between 50-60% RH and 16-21°C.

Prevention of these factors should be taken into account from the moment of siting and construction of the building which is to house the graphic documentation. There should be no dampness in the subsoil; foundations, outside walls, roofs, interior walls and ceilings should be waterproofed and flooring should preferably be terrazzo or a waterproof material to prevent hygroscopicity. Cavity walls are recommended as well as the use of insulating materials. Moreover, it is advisable to house the documents above the ground level of the building.

Temperature and humidity should be controlled by air-conditioning provided that it is kept working constantly. The best system, however, is natural ventilation with ascending air entering at the lower end of the building and emerging at the opposite upper end.

In order to avoid condensation shelving should not be placed against walls, and should be raised at least six centimetres above floor level.

To control humidity in show-cases, dampness-control devices may be used or hygroscopic products such as silica gel in the proportion of 1-3 Kg. per cubic metre. This is the idea for show-cases or small areas.

Light is another important factor in deterioration processes. It may give rise to chemical changes (especially ultraviolet radiation) and physical changes (especially infrared rays). Physical changes which occur as a result of heating caused by lighting are yellowing, increased chemical reactions, and internal vibrations which in turn produce molecular movement and, finally, disintegration.

Chemical changes are due to photolysis which breaks down the molecular chains, producing fragility and disintegration of the document, and photo-oxidation which appears once oxygen has been released during the disintegration process. This oxygen can act on its own in discolouring processes (yellowing the paper and discolouring the ink) or it can form new harmful molecules such as oxides, acids, hydrogen peroxide etc. which are, in turn, reinforced by excess heat and humidity.

The less light there is in the building the better the documents are preserved; an intensity of 50 lux seems to be appropriate for it does not harm the cellulose material and allows the works on the shelves to be identified and handled. Windows in the building should be few, so as to reduce sunlight, and ultraviolet filters (varnishes, plastic sheets...) should be fitted on the windows. Fluorescent lighting is the least harmful provided that filters are installed; show-cases should always have indirect lighting.

Another cause of physical-environmental change are vibrations which may lead to the disintegration of the objects. Preventive measures involve siting the building on firm, non-seismic ground far from railways, airports, etc. where there is acoustic peace. Should this be impossible, these problems should be solved in each particular case by the methods used in the building construction and shelving and show-cases should be provided with shock-absorbing supports (rubber).

Another group of natural causes are those of a physicalmechanical nature, brought about by handling, poor installations, etc. which give rise to tearing and staining, etc. Prevention is ensured by adequate storage and the use of reprographic techniques · (microfilm, microfiche...) to reduce frequency of use and, consequently, deterioration of the originals.

Chemical-environmental causes are due to atmospheric pollution which carries particles detrimental to the documents, such as sulphur dioxide which, combined with dampness and catalyzed by metal particles, can form sulphuric acid. The elements carried by dust, smoke and fumes (spores, metals, salts, gases...) may act as abrasives, catalyzers, agents of biological pollution, and so on.

The most effective preventive measure to stop particles and fumes from entering is a filter system (activated, dry or semi-dry carbon filters...).

One final cause of external and natural alteration, extending into catastrophic causes, is biological contamination. Alteration brought about by living things (micromammals, insects, fungi and bacteria) is regarded as a real scourge.

Micromammals and insects usually attack paper, causing holes and dirt. Alterations produced by micro-organisms generally give rise to stains of varied hue and produce a very characteristic softening of the cellulose support.

Control of insect pests can begin from the stage of manufacture of the components which make up the graphic document by including repellent substances, especially in the natural adhesives which are so "appetizing" to many insects.

But this defence right from the manufacturing stage lies outside the control of the conservation specialist who must search for other means of preventing damage.

The main defence against biological factors is control of the environment using systems which make life difficult or impossible for the organisms. The combination of high temperatures and dampness is to be avoided as this is an environment in which such organisms thrive. Dust and dirt are also to be avoided, as well as poor ventilation, lack of light, hidden nooks and crannies where insects and rodents can conceal themselves, and direct means of access to the outside through which they may enter.

Periodic checks are a must and new materials and objects should not be introduced unless it is certain that they are free from contamination. In any case, products for preventive treatment must be available. (See 3,4,5 and Table 1. Disinfection and elimination of insects) so as to inhibit the presence and activity of pests.

Apart from natural dangers, risks from catastrophes must also be foreseen, although they are to some degree unpredictable, such as floods, fires, vandalism...

Although these factors cannot be controlled totally, there are certain steps which can be taken to minimize their effects.

Fire precautions involve the use of non-inflammable fittings and fire-resistant construction materials. Wooden floors should be avoided and shelving should be metallic and treated with antioxidants. Wall, floor and ceiling materials should be resistant to fire for a period of at least two hours. Appropriate well-maintained electrical conduits should be the rule so as to remove, as far as possible, any risk of an outbreak of fire, and a lightning conductor should be installed. Fire walls and doors will inhibit the spread of flames.

Doors reinforced with double steel sheeting with an insulating material insert should be used and they should be loose enough in the frame to allow for dilation. Fire escapes or, better still, fire-chutes should be provided to make rapid evacuation of the documents possible.

Fire detectors, detectors of changes in ionization and fire extinguishing systems are necessary: where possible portable extinguishers should be of the polyvalent powder type, while fixed installations should be of the halogen kind.

Precautions in regard to flooding were commented upon in the section on prevention of dampness in the installations and in the structure of the building. In the event of a disaster it is recommended that damp books and documents should be lyophilized or, as a last resort, frozen until they can be restored, thus avoiding other alteration factors which might arise with the damp accumulated after a flood.

The internal causes which alter paper depend upon the components used during the manufacturing process (raw materials and additives). The risks are different according to the materials involved and depend directly on the degree of technical evolution, the greatest risk being to continuous paper as opposed to hand-made paper.

Early hand-made paper contains hardly any components liable to cause deterioration except in odd circumstances when metallic particles may have been deposited on the paper pulp; besides causing stains through oxidation, they can give rise to harmful chemical processes.

With regard to paper made in more modern times (from the eighteenth century onwards) the addition of chlorine substances, alum, colophony and in particular the use of wood pulp itself have contributed to the alteration of the support due to chemical causes (oxidation and acidity) with results such as a reduction in mechanical resistance and yellowing.

Acidity is the most harmful element for cellulose supports as its effects are not obvious until the harm has been done.

The best preventive measure for acidity is to control it through deacidification of those documents with a pH of less than 7 with substances which will give the document an alkaline reserve.

Finally, it must be remembered that often the format of the document or its specific features may be potential causes of alteration. When documents are very large or anomalous in size, creases and tears are frequent. In other cases, complements such as suspended seals also constitute risks. Poor binding, fastenings, etc. are also a frequent cause of tears and stains... The only preventive measures are proper installations suitable for the document and the substitution of the original by a copy for general use.


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