Current Research in Paper Conservation

CURRENT RESEARCH IN PAPER CONSERVATION

Francoise Flieder

Paper and its deterioration

The Centre de Recherches sur la Conservation des Documents Graphiques (CRCDG) founded in 1963 is a joint unit of the Centre National de la Recherche Scientifique/ Ministere de I'Education Nationale et de la Culture/Museum National d'Histoire Naturelle. Its function is to conduct research to ensure the preservation of the cultural heritage which consists of books, archives, drawings and photographs. Recently its field of investigation has been extended to archaeological leathers, in this laboratory more than twenty researchers, engineers and technicians endeavour to develop processes enabling documents to be preserved in their original state for as long as possible. They also accomplish the important task of identification so as to inform conservators of the exact nature of the materials to be restored and provide historians with information on the animal origin of leathers unearthed from excavation sites.

The research laboratory is divided into several sections, three o! which specialise in the conservation of the following materials: paper, leather, parchment,and photographic documents. A microbiology section has the objective of combatting destructive insects and microorganisms. In !he event of contamination, samples are taken, and after species identification, strict instructions are given to curators regarding disinfection and prevention. In addition, research has been carried out on the efficacy ol new molecules whose bactericidal and fungicidal powers make them suitable for use in the field of conservation. The section's activities involve not only all graphic documents but objects conserved in museums.

The CRCDG is also a consultant to those responsible for collections, especially in emergencies such as flooding; in addition, it has also a teaching function both at the higher education level and in continuing training for curators and conservators. Lastly, it is in contact with many of its foreign colleagues through various international bodies, in particular the Conservation Committee of the international Council of Museum. The willingness to cooperate is visible in the part which the centre plays within the European Economic Community, in the STEP programme (Science and Technology for Environmental Protection), one aspect of which is devoted to safeguarding trie European cultural heritage.

As paper is the essential constituent o! graphic documents, we have chosen to present some recent research carried out on the analysis, preservation and restoration of this material. To stop the damage caused by acidification over time and to develop protective techniques, the decomposition of different qualities of paper exposed to pollution was studied.

Aiming at long-term conservation, we have taken an interested in the stability of the optical brighteners added to paper during manufacturing to increase its whiteness and in the choice of a printing ink compatible with archival paper. Finally to improve some restoration techniques it seemed necessary to find a way of fixing unstable inks o! contemporary manuscripts before treating them.

Prior to 1650, paper was produced from white linen or Hemp rags washed in 'hard water1 containing alkaline carbonates (magnesium and calcium), bleached with wood ash and left to dry in the air. Most of these papers have stood the test of time and are in an excellent state of preservation today. Unfortunately, this manufacturing process changed. Around 1650, alum (potassium and aluminium sulphate) was added to pulp to harden the animal glue size but had the adverse effect of increasing acid content in particular through the formation of sulphuric acid.

Since the end of the eighteenth century, the ever increasing demand for paper required the use not only of white linen or hemp rags, but also coloured rags from

(a) H3CO Mg O COOCH3 + 2H2O ~> Mg(OH)2 + 2CH2OH

(b) H2O + CO2-->H2C03

(c) Mg(OH)2 + H2CO3-->MgCO3 + 2H2O

(d) MgCO3 ->MgO + CO2

(e) Mg(OH)2+ H2SO4 -> MgSO4 + 2H2O
(f) MgCO3 + H2SO4 --> MgSO4 + H2CO3
(g) MgO + H2SO4 -•> MgSO4 + H2O

which the colour had been removed to some extent. Many products were tried as bleaching agents until the discovery of chlorine in 1774 revolutionised paper bleaching techniques. Unfortunately, chlorine, in contact with moisture, produces strong acids which accelerate the destruction of paper. By the middle of the nineteenth century, great progress was made in paper manufacturing.Wood pulp gradually replaced rags. When insufficiently treated, wood pulp contains non-cellulose products such as lignin which, when in contact with light, heat and moisture, also produce acids.

However, the main cause of acidity in the paper from this period is due to the presence o! rosin which was used as a sizing agent. When combined with alum it forms an aluminium resinate which precipitates on the fibres and gives the paper good water resistance and imparts good writing quality. It has been proved, however, that this sizing process releases a substantial amount of sulphuric acid.

From this brief historical outline we can see why the papers of the eighteenth, nineteenth, and the beginning of this century have become brittle and often very coloured.

Effect of pollution on paper

Although the products used in the manufacture of paper are the source of its acidity, the damaging effects of external factors such as atmospheric pollution should not be neglected. Corrosive gases such as SO2 , and NO2 are absorbed by paper in quantities corresponding to its

porosity and on exposure to atmospheric humidity are converted into sulphuric and nitric acids. It is not only vital to neutralise She acidity of the celluiosic materials but also to prevent further acid deterioration by giving an alkaline reserve.

The awareness of this phenomenoa by chemists is not new, since it is found in the writings of Faraday, Calvert and Letbely as early as the 1850's. They noticed the adverse effects of acidity on books and attributed the atmospheric pollution to the combustion gases of lamps. Nevertheless, it was not until the early part of this century that neutralising methods were first developed.

Various compounds have been used for this purpose such as sails and oxides o! alkaline earth (Ba, Ca, Mg), and alkaline metals (Na, K, Li), borates, phosphates, aromatic and aliphatic amines, and alkaline salts of fatty acids. They can be used in aqueous or alcohol solutions and also in gaseous form.

Many so called 'deacidification' methods have been developed throughout the wood; soaking in alcoholic or aqueous solutions is limited to the treatment of small numbers of loose sheets or unbound documents. Considering the large quantities of books and periodicals requiring deacidification, it was necessary to develop mass treatment methods to meet some requirements; the first one being a good penetration of the products to the core of the documents piled up in the treatment autoclave. For this , gases or pressurised solutions were chosen. The compatibility of alkaline salls with the other constituent of the books (binding, glue, inks) has to be taken into consideration. Lastly, to save time and money, preference was given to treatments requiring a minimum of preliminaries. Very tittle research has been done on the phenomenon of the reacidification of treated paper over time and the mechanisms involved.

Thus we decided to include these two problems in a STEP research programme. This one was begun in July 1991 and will last three years. The goal is to determine the best storage conditions and the choice o! deacidification treatment. The sludy covers a large number of papers of different qualities, including most of those preserved in archives and libraries. Modern pure rag, 100% bleached chemical pulp and mechanical pulp papers have been selected as well as old papers of varying age and origins. All these materials were exposed to pollutant gases for two weeks (13 ppm SO2 and 25 ppm NO2 at 23°C and 50% RH). They were artificially aged for twelve days in an oven at 90°C and 50% RH.

A large number of chemical, spectrornetric, mechanical and microscopic tests were carried out on the untreated, polluted, deacidified and, deacidified then polluted, papers.

As regards the deacidification techniques, for

preference we chose mass methods, which are the only ones permitting the rapid treatment of very large collections presently in peril. Among the ten or so existing techniques, we decided to study the four which seemed !o us to be the most efficient.

Deacidification with magnesium methoxtde under pressure (technique developed by Weit o)

In 1977, Richard Smith designed a prototype in which he used magnesium methoxide under pressure. Two years later, Weit'o installed a pilot station in the Public Archives of Ottawa which has been operational since 1981. In this method of deacidification, methyl magnesium carbonate deposited on the fibres reacts with moisture in the atmosphere to form a mixture of salts (hydroxides (a), carbonate (b) and oxides (c)) which neutralise the acids present in the paper and give it an alkaline reserve (e) (l)

The documents are first dehydrated to ensure good penetration of the compound which otherwise would precipitate and leave on the surface an unsightly deposit of MgO. This process seemed very promising because it is easy to use and efficient. We improved it by replacing the rnethanol with ethanol whose compatibility with the writing was better and we calculated the concentration of active products and the exposure times. The results of this study were used in France by the National Library as a prototype at its restoration centre at Sable sur Sarthe in 1984. This station in service since 1987, treats about 100 books a day with good results.

Deacidification with zincdiethyl (technique developed by Akzo)2

It also reacts with the water contained in paper giving off ethane antizincoxide (c) which is immedately deposited on the fibres giving the paper an alkaline reserve of up to 1.5% ZnO.

In 1970, J Williams and G Kelly presented a deacidification method using the gas zinc diethyl (ZDE) it reacts with the acids in the paper (sulphuric and acetic acids) and converts thern into neutral non-volatile sulphates (a) and acetates (b) and releasing ethane, an inflammable, asphyxiating gas.

Nevertheless, in its liquid form it is pyrophoric. As a result, the treatment must be done at a low pressure and in a nitrogen flow. The operation is carried out in a large capacity autoclave in a vacuum without preselection of the documents but dehydrating them before treatment. 5000 books can be treated in a single operation.

Deacidification with magnesium butoxytriglycolate (MG3) (technique developed by Lithco-FMC)3

This method is similar to the one used in France. The active product is an aikoxylated compound, solubilised in chloroftuorocarbon. which has the dual effect of neutralising and reinforcing the paper. Prior dehydration of the documents is necessary; 150 volumes are treated by a cycle of four to six hours.

deacidification with amine ethylenes (technique .developed by Book Preservation Association BPA}4

This is a gaseous process presently in the experimental stage, it is realised in an autoclave in a vacuum by combining the action of gaseous ammonia (NH3 ) and ethylene oxide (CH2 -0-CH2). The products formed are a mixture of mono-, di- and tri-ethanolamine (NH2CH2CH2OH; NH(CH2CH2OH(CH2CH2OH)2 which are basic amines capable of neutralising the acids contained in the papers and constituting an alkaline reserve. No preselection or pretreatment of the documents has to be done.This method is rapid (twenty-four hours), inexpensive and easy. The products deposited on the fibres are non toxic. However, there remains the problem of ethylene oxide neutralisation upon removal from the autoclave. Data on the efficacy of the technique is unavailable.

The first part of the STEP programme therefore consisted in defining the experimental conditions as wet! as the characteristics of all the papers used in the experiments. We now begin the study of the mechanisms involved during the degradation of the different papers exposed to pollution. The last part of this work will concentrate on a behavioural comparison between the papers deacidified by the four methods described above.

Influence of optical brighteners on paper permanence

As mentioned earlier, some papers sized with rosin in an acid environment have greatly deteriorated. Research has been carried out for many years to determine the best characteristics for a permanent paper in order to guarantee the life expectancy of valuable documents.

An ISO5 standard is being developed to qualify conditions of permanence, it has been found that this kind of paper should be 100% bleached chemical wood pulp free of lignin, sized in a neutral medium and filled with an equivalent of 2% calcium carbonate. Its pH should be around 8. The possibility of optical brighteners being present is tolerated.

Optical brighteners are substances that neutralise the slight yellow colouration of paper thus making it whiter. While in the past, blue natural pigments were used, nowadays fluorescent substances derived from stilbene are employed. They absorb ultraviolet radiation re-emitting visible blue light, so that paper has a whiter appearance. As these substances are sensitive to heat and light they were studied for their long-term effect on cellulose. The optical, mechanical and chemical properties of different papers with brighteners were analysed and the results compared to those obtained from identical papers without brighteners. The study was carried out on several qualities of paper supplied by different manufacturers. All were made from bleached chemical pulps, sized in a neutral medium and filled with calcium carbonate except for 100% cotton linters paper (with no size or filler) which was included to study the behaviour of pure cellulose.

Measurements of the optical, mechanical and chemical parameters were carried out before and after two types of artificial ageing: wet heat (80°C. 65% RH for twenty

four days) and light (150 hours on each side of the paper under a xenon lamp at 30°C, 50% RH). There is no doubt that optical brighteners improve paper brightness. During ageing (especially with light) brightened papers lose more whiteness than non-brightened ones but are still whiter.

Optical brighteners do not significantly modify the mechanical and chemical behaviour during ageing of the treated papers (the pH of brightened papers is always slightly higher than that of non-brightened ones). Conversely, papers made from rag pulp deteriorated "much more when containing optical brighteners.

Printing ink stability

The compatibility of modern printing inks with permanent paper, and the stability of documents printed with these materials was studied with the intention of preserving documents of artistic or historic value.

Basic black printing ink, used in offset printing which is the most widely used process in France, was chosen for the study. Samples were selected from among the principal ink manufacturers on the French market(Basf, Brancher, Grant, Lorileux, Sicpa, Sun Chemical).

For each sample, several preliminary tests were performed to characterise the inks and to determine their compatibility with permanent paper. Measurements of viscosity, rigidity, tack6, and printability make it possible to anticipate any risks of picking (if tack is too high), inkfly7 (if viscosity is too low), and blotting8 (ii drying is too long).

All these preliminary tests showed that the fifteen
selected inks were compatible with permanent paper.
Next, real size machine prints were made with each ink
on permanent paper at l'Ecole de Papeterie de Grenoble,
All existing types of offset printing were represented:
flatness, half-tones, text and control bars for each sample.
One hundred A4 format copies were printed and their
stability was examined before and alter two types of
artificial ageing: wet heat (twenty-one days in an oven at
90°C, 60% RH) and light (300 hours under a xenon lamp
at 30°C, 50 % RH)......

The following tests were used: on a black disc, recording of trichromatic values; on half tone plates, colour differences, measurement of black optical density, reflectance and contrast; on the flatness, measurement of pH levels of cold aqueous extract.

At this point in the research we can conclude that after wet heat or light ageing neither the colour nor the density of the fifteen inks showed significant modification. Furthermore, the pH levels of printed papers remained unchanged (close to 9), which indicates a large alkaline reserve. Thus the black ink offset prints on permanent paper retain their visual and chemical qualities before and after artificial ageing. The bonding capa-:ny seems satisfactory.

Simultaneously, in order so explore the full range of oifsel Sour-colour printing (cyan, magenta, yellow and black) we are studying the behaviour of coloured ink samples using the same tests. In fact, although there are some very fast blue inks, the pigments used for yellows and magentas are less resistant to light.

ink fixing of contemporary manuscripts

Most of the numerous methods of paper restoration in use throughout the world for the past several decades, require in most cases immersion of the document ir aqueous or organic solutions. For example, in the leal casting technique, paper pulp suspended in water and deposited in the gaps by means of rapid suction; the document must remain immersed in the water for several minutes.

in addition, the deacidification methods used most often in the workshops for the treatment of rare or targe documents or even small quantities of sheets, require the use of aqueous or alcohol solutions. Other restoration processes, which we cannot describe here (sizing, bleaching, etc) also call for the use of different solvents. Inks used in ancient manuscripts (carbon or ferrogallic inks) are generally insoluble, but the same does not apply to inks used in some contemporary manuscripts (fountain pen or felt-tip pen inks, etc). Therefore, the problem of fixing them before treatment had to be resolved.

For this, substances had to be selected which were suitable yet considered harmless with regard to the paper. Few articles treat this particular problem. Recently, K Bredereck stressed the importance of the ionic nature of the colourants found in inks. The conclusions of this study show that different chemicals (condensates of dicyandiamide and formaldehyde; polyethylene imine and di- or polyhalides; salts of stearyl dimethylbenzyl ammonium) give good results in fixing.

We employed Sandoz products (Dermafix S, Indosol E50, Tergolan GS, Sandofix WE), generally used for fixing dyes. A preliminary study was carried out on contemporary archive documents comprising different types of ink soluble in water or organic solvents. The concentrations, immersion times and drying methods giving the best fixing were determined for each product. We also examined the effect of deacidification, using magnesium bicarbonate, on the inks thus treated, as some of these fixing agents are acidic.