Aspects of Deacidification Technology - a Source Bibliography

ASPECTS OF DEACIDIFICATION TECHNOLOGY - A SOURCE BIBLIOGRAPHY

R.C. Morrison

Introduction

Beginning with studies performed in the 1930s at the US Bureau of Standards and the US National Archives and Records Service (79); and culminating in the 1950s and 1960s with the in-depth studies of Barrow (26-28, 82, 83), Wilson (78, 79) and several scientists in the Soviet Union (29-33, 44, 45, 62, 64), it has been concluded that acid in paper - either inherent or due to environmental factors - is the foremost cause for the deterioration of library and archival materials.

During the last fifteen years scientists and conservators have been conducting studies to determine the best methods of deacidifying paper and depositing an alkaline buffer within the fibre network. Unfortunately, Australian conservators have had limited access to the results of these studies because the reports have been published in journals not readily available in this country.

A source bibliography compiled from the publications of the last sixty years would provide conservators with the location of these reports, and would allow them to assemble copies so that they may establish a working file on deacidification technology.

In searching the literature one discovers that a useful bibliography must include three specific topics if the entire scope and application of these deacidification methods are to be understood. The bibliography must contain sources of pH analytical techniques and methods of accelerated ageing as well as the deacidification techniques themselves.

Determination of pH

Any chemical which can produce a proton (H+) is an acid - acids donate protons. (In water, each proton is associated with a water molecule forming the hydroniura ion, H30+). Any chemical which can react with protons can neutralise acids - they can accept protons. The number (concentrations) of protons in an aqueous solution can be determined using a numerical scale (0-14) called the pH (4, 5, 8, 11, 17). A low value for the pH indicates a high concentration of hydronium ions and therefore, a high concentration of acid.

Neutralisation is the reaction between a proton donor and a proton acceptor. Neutralisation produces a pH=7, the raid-point on the numerical scale of pH. To prevent degradation of paper, the paper conservator must neutralise the acids present in the paper before they can react with the cellulose fibres. In addition, an excess amount of this acid neutraliser must be added to prevent future paper degradation, if and when more acids are produced. This excess acid neutraliser is called an alkaline reserve (or buffer), and gives the paper a pH greater than 7.

By definition then, deacidification of paper is a process of neutralising the acids present and depositing an alkaline buffer reserve. To determine whether a paper is acidic and requiring deacidification; or to determine whether the deacidification process has deposited a suitable alkaline reserve, the paper conservator must undertake pH analysis (9, 10, 12).

There are many methods of determining this pH, either directly on the document which is a limited possibility) or as a reasonable approximation using surrogate materials or methods. One method is to use dyestuffs whose colour will change according to the pH (2). These dyestuffs are used either in solution (2, 6, 11, 13, 11) or in paper of plastic strips (2). Invariably these dyestuffs can permanently stain paper and should not be applied directly to a document. They are suitable, however, as a rapid means of analysing paper to be used in a restoration process.

A slightly more accurate means of paper pH determination is the use of the flat-surface electrode (2, 3, 7, 10, 16). Because water is involved in this technique there is a possibility that a valuable paper will be stained during the analysis. This staining, however, is only a concern if the document cannot be subsequently immersed in water.

Unfortunately, the use of dyestuff pH indicators or the flat-surface electrode will only determine the pH of the paper at the spot tested, and does not determine the pH of the entire piece of paper. This drawback is overcome by extracting the acid from a sample of the paper and measuring the pH of the extraction solution using a glass electrode and pH meter (1,2, 12, 15). The method, however, lnvolves the destruction of the paper sample and should not be applied in the determination of the pH of a book page or document.

It should be obvious that the direct determination of the pH of books and documents is rarely possible. The paper conservator must rely on supposition (knowledge of the source and date of manufacture of the paper) or on experience (familiarity with the brittleness and colour of acidic paper). But, all of the above pH methods (and especially the extraction method) can be performed on materials used in conservation to determine their suitability.

Lastly, pH analysis is useful in determining whether a deacidification process has been correctly performed. Kelly (145) has published a paper which considers this application and relevant data has been provided by Bristow (92).

Ageing Properties of Paper

The durability of paper is determined by its ability to withstand the physical wear and tear which accompanies its use. The permanence of paper is determined by its ability to retain its original chemical qualities during its storage. Although permanence may be thought to include retention of durability, many papers may be durable but not permanent, or permanent but not durable. Durability is predominantly a function of the manufacturing process and permanence is predominantly a function of its chemical composition and its storage environment.

Ideally, allpapers used in library, archive and artistic materials should be both ' permanent and durable. Realistically though, permanence of the paper used in these materials is the more important aspect. And because permanence is a time factor, the ageing characteristics of paper must be understood by the paper conservator.

The conservator should have knowledge of the methodsfor studying the natural ageing characteristics of paper, and how these characteristics can be rapidly approximated through the use of artificial, accelerated ageing techniques. In fact, accelerated ageing techniques are important for the determination of (a) the suitability of materials to be used in the restoration of paper, and (b) whether the proposed deacidification method is going to enhance or detract from the permanence of the paper. The latter is also important in understanding the claims researcher has made for a particular deacidification method.

Studies into the general permanence of paper covering the years 1885 to 1963 have been compiled by Byrne (38). Specific investigations into the natural ageing of paper have been reviewed by Alekseeva (18-20), Barrow (26), Kathpalia (51), Kleinert (52), Langwell (54, 55), Perl'shtein (62), Rasch (67) and Wilson (76, 79).

Although recent studies (24, 42, 48, 49, 57, 60, 68) have been devoted to the use of the Arrhenius equation and activation energy in determining paper ageing qualities, the dry heat ageing of paper at 100±2°C. for 72 hours (an equivalent of 25 years of natural ageing) is still the accepted method for studying this quality. This test was first proposed by Rasch (66, 67) in 1931 and has been studied extensively by Baer (23), Barrow (25-28, 39), Belen'kaya (3D, Flyate (45), Perl'shtein (63), Roberson (68), Skall (73) and others (45, 50, 53, 64, 65, 69). One aspect of this dry heat ageing method which has produced considerable criticism is the effect of humidity in the ageing process. Many researchers question whether desiccation of the paper during dry heat ageing contributes tothe degradation, or on the other hand, whether this ageing method is at all valid

because the natural ageing of paper occurs in an atmosphere which contains water vapour as a natural element. Although the recent papers by Wilson (76, 79) seem to support the dry heat ageing method, the work of others (29, 30, 36, 37, 13, 58-60, 70, 72, 75, 77, 80) lead to the conclusion that accelerated ageing methods still require further study.

Deacidification of Paper - General Considerations

As stated previously, paper deacidification is a two-fold process involving neutralisation of acids present in paper and a deposition of an alkaline buffer reserve to prevent acid hydrolysis in the future. The result should produce within the paper a minimum pH of 7.5 and a maximum pH of 9.5.

To neutralise all of the acids within a paper, one must treat the paper with chemicals which will react - with the acids and form inertsalts as the products. This acid-neutralising chemical, however, must itself be inert to any other constituent within the paper.

In order to prevent the formation of more acid, either from inherent or absorbed acid-producing chemicals, the acid neutraliser should be of a larger quantity than Is required for the initial neutralisation process. This excess amount of chemical must remain within the paper and be unreactive to anything but acid. It will be consumed only if sufficient acid can form to react with all of it.

The simple dusting or coating of papers with an alkaline chemical will not prolong their storage life because acids within the paper will continue to degrade the cellulose fibre network.

Aqueous Deacidification Methods

Because there are no suitable, effective and simple deacidification processes which can be performed without wetting paper, aqueous systems of deacidification are most commonly used in paper conservation. The reactions involve the use of inorganic chemicals to neutralise the acids and deposit an alkaline buffer. The buffer most commonly deposited is a carbonate salt.

Historically, carbonates have been the most prevalent buffering constituent in paper, but usually more by accident than by intent. Pulp beating and papermaking were carried out in carbonate-containing stone vessels. Water usually contains dissolved carbonates and large quantities of water have always been required for the production of paper. Rags were a common paper fibre source and were processed with limewater which reverts to calcium carbonate in air. And finally, limestone (calcium carbonate) has been used for centuries to whiten paper and act as a filler to make paper more opaque and heavier (paper has always been sold by weight). It is only within the last 50 years that carbonates have been investigated for the deliberate preservation of paper.

Research by Otto Schierholtz in the 1930s (105-107) produced patents for a process of deacidifying paper using bicarbonates prepared by bubbling carbon dioxide into aqueous mixtures of the carbonates and hydroxides of barium, strontium and calcium. During the next 30 years William Barrow developed a two-stage system of deacidification (82-89). The process involved the neutralisation of acid paper by immersion in a saturated calcium hydroxide solution followed by immersion in a calcium bicarbonate solution which produced an alkaline reserve of calcium carbonate after the paper had dried.

This process has been thoroughly investigated by many researchers (90, 91, 94, 97, 104, 142, 143, 147, 152, 155).

The US National Archives and Records Service developed a one-stage deacidification system in the 1950s- - the most recent discussion of this process has been reported by Wilson (109). As with the Barrow process, this method, using a magnesium bicarbonate solution, has been well investigated (81, 83, 85, 91, 91, 97, 100, 101, 104, 112, 113, 117, 118, 152).

Other aqueous deacidification systems (96, 97, 104, 108) and some methods of

preparing paper containing an alkaline buffer (93, 103) have been reported, but further investigation of these proposals is required.

Non-aqueous Deacidification Methods

Conservators have used aqueous deacidification methods because of the advantages of these processes. The waterwill wash out soluble acids, degradation by-products and products resultant from the neutralisation reaction. The wetting of the paper, and its subsequent drying under pressure, tends to increase the fibre-to-fibre bonding, thereby strengthening the paper overall. Lastly, the chemicals and equipment used in the aqueous processes are inexpensive and readily available.

There are certain limitations in the use of these processes, however. Whole books cannot be treated with aqueous solutions because the bindings will be physically disrupted due to the swelling of the paper and the dissolution of the adhesives. Equally important, documents, books and works of art on paper may contain water-soluble components which would prevent their deacidification with aqueous solutions.

Within the last 25 years, several non-aqueous deacidification processes have been developed based on the dissolution of a deacidifying agent in organic solvents. These systems can be used provided the chemicals and solvents are inert to all constituents of the paper other than the acids. These processes must neutralise the acids and deposit an inert residual buffer; the solvents must be inert, readily volatile and have a minimum health risk (150); and the solution should be usable as a spray, immersion, brushing or vapour process.

A.D. Baynes-Cope (113) has developed a non-aqueous-deacidification system based on barium hydroxide- dissolved in methanol. The process has been investigated by Agrawal (110), Flieder (143), Ruggles (148), Saclier (119) and Wachter (152). Barium compounds are toxic and methanol is toxic and flammable. Baynes-Cope has described the limitation of this process to single documents (113).

Hayworth (114) describes the non-aqueous deacidification of paper using a commercial product, 'Regnal 7'; but Baer (112) has reported that the product is not very effective.

Smith has developed a non-aqueous deacidifier based on the alkoxlde, magnesium methoxide (120-124). Alkoxides are produced by the reaction of metals with alcohols, are very alkaline, and will neutralise acids. They also react with water to form metal hydroxides and the parent alcohol. Smith's deacidifier is commercially available as 'Wei T'O I' consisting of magnesium methoxide in methanol further diluted with a chlorofluorohydrocarbon solvent. The latter solvent functions to inhibit the flamrnability of the methanol and to increase the volatility of the solvents after application of the solution to the. paper. As the treated paper dries, any excess magnesium methoxide reacts with moisture in the air and is converted to magnesium hydroxide. Over a period of 48 hours, the hydroxide further reacts with atmospheric carbon dioxide to form magnesium carbonate as a buffer-within the paper.

This system has received much discussion in the literature (111, 115-118, 143, 148). One reported problem with the solution is its extreme sensitivity to moisture which creates many problems in its storage and use. Spray bottles and other spray apparatus quickly become clogged, and bulk quantities of the solution decrease in their concentration of methoxide because of the formation of insoluble magnesium hydroxide.

Kelly (115, 116) has chemically modified magnesium methoxide to overcome this water sensitivity and to extend the storage-life of these solutions to at least six months. The process involves the bubbling of carbon dioxide through a solution of 8% magnesium methoxide in methanol to produce methyl magnesium carbonate. The precise structure of this carbonate has not been determined, but its properties as a deacidifier are similar to that of magnesium methoxide.

Methyl magnesium carbonate is soluble in many organic solvents, including chlorofluorohydrocarbons. It can be used as a spray or in an immersion bath. As the

treated paper dries, magnesium hydroxide forms and is converted to a reserve buffer of magnesium carbonate within 48 hours.

A conservator with suitable facilities can prepare magnesium methoxide, bubble C02 into the prepared solution, and produce the methyl magnesium carbonate. The deacidification solution is commercially available as 'Wei T'O II', also.

Vapour Phase Deacidification Methods

Nearly all deacidification systems presently in use by conservators involve the wetting and drying of paper. Although the drying of wet, degraded paper has some advantages, the drying may cause physical stresses which can permanently damage a fibre network already weakened by deterioration. Deacidification processes involving the reaction of gaseous acid neutralisers, which are readily converted into solid alkaline buffers, are most desirable. Such systems not only overcome the problems associated with the wetting and drying of paper, but also allow for the deacidification of large quantities of whole books. This would provide the means for the mass deacidification of entire collections housed in libraries and archives.

Many systems of this nature have been researched, but their developments have not been refined to the point where they can be readily used. Each proposed process has one or more disadvantages which severely limit their applicability. Some of these disadvantages include the use of toxic or hazardous chemicals, complex or highly sophisticated equipment, or have limited use. Other systems have not been thoroughly investigated to discover the problem areas, or they have been proven to be relatively ineffective.

Langwell (129-131) has developed a vapour phase deacidification process based on the sublimation of cyclohexylamine carbonate, but Dupuls (126) and Nielsen (133) have reported adverse results.

Knubben (127) has made claim to devising a process based on the use of sodium hydroxide vapours and ozone which not only deacidifies paper but sterilises, bleaches, strengthens, re-sizes and decreases the porosity of the paper. Unfortunately, only one independent report concerning this system could be found in the literature (Wachter 152).

Williams and Kelly (136, 137) describe a system using organo-metalllc compounds to deacidify and buffer books and paper, but the process involves the use of hazardous chemicals and sophisticated equipment. The system is still undergoing study.

Smith (134) has recently reported the use of magnesium methoxide in a liquified gas mass deacidification system for books and paper. Likewise, Kusterer (128) and Walker (135) report on the use of alkaline vapour deacidification processes.

Conclusion

A search of the literature has revealed no less than fifteen different methods of deacidifying paper. Nearly all of these processes have had one or more reported disadvantage; or in some cases, the systems have not been investigated sufficiently to determine their suitability. Therefore, the paper conservator must keep abreast of the literature in order to correctly decide on the most suitable deacidification processes. This means the conservator must go beyond the conservation literature and become familiar with sources of information contained within the chemical literature.

In addition, to more fully understand all aspects of deacidification technology, the conservator must continue to read reports concerned with the pH analysis and the natural or artificial ageing of paper.

Conservation

CanberraCollege of Advanced Education

BIBLIOGRAPHY

The following bibliography has been compiled from the chemical literature and from conservation publications covering the period January 1927 through May 1978. The reader is generally referred to the original publication which is readily available at most acdemic libraries, through a library-loan system, or by request from the publisher. Papers which have appeared in obscure publications, or in foreign publications which may be difficult to obtain, are accompanied with a reference to a more readily available abstract publication.