Humans often put on paper what is important to them—but paper is ephemeral. This makes it worth looking into the chemistry of paper disintegration and the restoration of old paper.

In this part, we will look at how deacidification can save old papers.

 

4 Deacidification Process

From a chemical point of view, rescuing acidic paper appears simple at first glance: The pH value must immediately be elevated to 7–9.

However, experience has shown that a lasting deacidification process must do more:

• Elevation of the pH value cannot be short-term; it must be permanent.
• An alkaline reserve must be applied to the paper to counteract future acid influence.
• Paper that is already damaged must be mechanically strengthened.

In the following, we will not be considering valuable original manuscripts, musical scores, or letters from famous personalities. Instead, we will be addressing the kilometer-high mountains of books and records dating from the years 1850–1970. For this reason, we will add another requirement for the mass deacidification process:

• It must be easy to carry out and inexpensive.

We may marvel at some examples of the incredible chemical fantasies archivists, conservators, and chemists have indulged in as they searched for suitable processes.

Many of the methods described below were developed for the conservation of books. Unlike books, official documents are always unique, meaning there is only one copy in the world.

 

4.1 Deacidification with Gases

The big advantage of gas fumigation is obvious: The paper does not come into contact with any solvent and cannot swell. Also, the ink used in writing, stamping, or typing, cannot run off [6]. Beginning as early as the 1950s, paper was treated with ammonia and later with other volatile amines (e.g., diethylamine, butylamine, cyclohexylamine, morpholine, and piperidine).

In all these cases, the pH value immediately rose. Unfortunately, any celebration was short-lived because the paper became acidic again within a few weeks. This mystery was not hard to solve. The ammonium ions formed during the neutralization process are in equilibrium with gaseous ammonia. This gaseous ammonia is constantly being released into the surrounding air until the paper returns to an acidic state (see Infobox 6).

 

Infobox 6. Reacidification after treatment with ammonia
(click here to expand)

 

When acidic paper is treated with gaseous ammonia in a reaction chamber, some of the ammonia dissolves in the paper’s moisture and immediately neutralizes any acid present. The pH value of the paper rises. However, the ammonium ions formed in this process remain in equilibrium with the gaseous ammonia. Outside of the reaction chamber – on the bookshelf or in the document drawer – the paper constantly releases gaseous ammonia until it becomes just as acidic as it was before fumigation.

 

Fumigation with ammonia:	NH3(gaseous) ⇌ NH3(dissolved)
Neutralization of sulfuric acid:	2 NH3 + H2SO4 ⇌ (NH4)2SO4
Dissociation of the salt:	(NH4)2SO4 ⇌ 2 NH4+ + SO42-
Acid-base equilibrium:	NH4+ ⇌ NH3(dissolved) + H+
Emission of ammonia:	NH3(dissolved) ⇌ NH3(gaseous) ↑
Figure B9. Chemical reactions involved in fumigation with ammonia.

 

 

A particularly original method of mass deacidification from a chemical point of view was an organometallics-based process developed by George Kelly and John Williams at the Library of Congress in 1977. Under reduced pressure, books were fumigated with gaseous diethylzinc (DEZ, boiling point 118 °C at atmospheric pressure). As diethylzinc diffuses into the paper, it reacts with any moisture, aluminum sulfate, sulfuric acid, and any other acids present. This forms insoluble zinc hydroxide, which is deposited in the paper, forming a good alkaline reserve [10] (see Infobox 7).

 

Infobox 7. Deacidification with Diethylzinc
(click here to expand)

 

When acidic paper is treated with gaseous ammonia in a reaction chamber, some of the ammonia dissolves in the paper’s moisture and immediately neutralizes any acid present. The pH value of the paper rises. However, the ammonium ions formed in this process remain in equilibrium with the gaseous ammonia. Outside of the reaction chamber – on the bookshelf or in the document drawer – the paper constantly releases gaseous ammonia until it becomes just as acidic as it was before fumigation.

 

2 Al(HSO4)3 + 3 Zn(C2H5)2 → Al2(SO4)3 + 3 ZnSO4 + 6 C2H6 ↑ H2SO4 + Zn(C2H5)2 → ZnSO4 + 2 C2H6 ↑ H2O + Zn(C2H5)2 → ZnO + 2 C2H6

Figure B10. Chemical reactions in the deacidification with diethylzinc.

 

 

Unfortunately, this process has a very big disadvantage: Diethylzinc is pyrophoric, meaning that it is spontaneously combustible in air. Despite stringent safety precautions, an incident occurred in a pilot plant at the NASA Goddard Space Flight Center during an operation carried out by Library of Congress staff on December 5, 1983.

A residue of liquid diethylzinc was left in the reaction chamber. As the chamber was opened, it ignited and triggered the sprinkler system. The water cut off the automatic controls for pumping out the supply lines. Because it was unclear if more liquid diethylzinc was present in these lines, a special unit from the army was called in to shoot at the lines from a distance. Some diethylzinc was indeed still present and immediately ignited. The fire was kept under control, but the facility was irreparably damaged.

After this incident and comprehensive investigations, the Library of Congress delegated further development to Akzo, the company that produced diethylzinc. In 1994, Akzo abandoned the project. Their official reason was that the capital costs were so high that facilities of this type could not operate profitably.

 

4.2 Liquid Phase Deacidification

Since gaseous deacidification agents did not meet expectations, alternative liquid-phase methods were tested. Initial deacidification experiments were based on aqueous solutions of alkaline earth carbonates [11]. Very small magnesium oxide particles were suspended in water, and the paper was soaked in this liquid (Bookkeeper process). In non-aqueous solvents, a wide variety of alkoxides were tested: magnesium methoxide Mg(OCH₃)₂ in methanol or a methanol/Freon mixture (Weit’o process), magnesium triethylene glycolate Mg[O(CH₂CH₂O)₃-(CH₂)₃CH₃]₂ in Freon (Lithco process), and mixed alkoxides of magnesium and titanium in hexamethyldisiloxane (Batelle process).

 

4.3 Stabilization of Damaged Paper

Deacidification and the introduction of an alkaline reserve protect paper against further degradation, but the paper remains brittle. For this reason, methods were developed to mechanically stabilize the paper in concert with deacidification. One original idea was to establish a polymer network within the cavities between the cellulose fibrils. To achieve this, the paper was soaked in monomeric methyl or ethyl methacrylate, which was subsequently polymerized with low dose γ radiation.

The addition of alkyl methacrylates with basic amino groups in their side chain provides simultaneous deacidification. This method even restores good material properties to very brittle paper (British Library Process) [12].

Other polymers can also strengthen the damaged cellulose network. According to a process developed by the Austrian National Library, good deacidification can be achieved with an aqueous solution of calcium hydroxide accompanied by the addition of methylcellulose.

Methylcellulose is a chemically modified version of cellulose in which some of the hydroxyl groups are converted to methoxy groups by a reaction with methyl chloride or other reagents. Because some of the hydroxyl groups are missing, polymeric methylcellulose is less polar, but can still form hydrogen bridges to the native cellulose chains, improving the mechanical stability of the paper.

 

4.4 Comparison of Deacidification Processes

Critical comparisons of the different mass deacidification processes have been undertaken by several institutions, showing that all the methods have strengths and weaknesses, depending on the type of material being treated and its condition. All the experts were in agreement that inaction is the worst solution: “In any way, it is better to deacidify than to do nothing at all.”

 

5 An Example: The Bückeburg Conservation Process

In the following section, we will more closely examine a deacidification process known as the Bückeburg process. The scientific basis of this process stems from work carried out in the State Archives of Lower Saxony in Bückeburg, Germany. Originally, the archival materials were passed through three different baths. The process has now been taken over and commercialized by the company Neschen AG, who refined it to a one-bath system. When running at full capacity, the newest large-scale plant, which was installed in the German Federal Archive in Berlin-Hoppegarten (former site of the Stasi, or Ministry for State Security, encryption division), can conserve over 3 million sheets of A4 paper in a year.

Once processed, the paper in the archival materials is guaranteed to have a pH value between 7 and 9. At the same time, the process produces an alkaline reserve that is equivalent to that in new alkaline paper (equivalent to 1–2% CaCO₃). Integrated re-sizing with methylcellulose mechanically strengthens the paper by about 30 %.

In the Bückeburg process, individual pages are dipped into a single bath for one minute. This bath serves several functions:

• Fixation of the printing and stamping inks:
  The water-soluble cationic, mostly blue, purple, and black inks adhere very well to paper due to interactions with the acidic, negatively charged groups. However, anionic red and green inks adhere poorly. By adding appropriately charged fixing agents, both cationic and anionic inks can be fixed for the deacidification bath.
• Neutralization of the acid and creation of an alkaline reserve:
  The acid in aged paper is neutralized with magnesium hydrogen carbonate Mg(HCO₃)₂. In addition to carbon dioxide, the neutralization reaction produces insoluble magnesium hydroxide and magnesium oxide, which creates an alkaline reserve in the paper.
• Mechanical strengthening:
  In the deacidification bath, the paper is simultaneously re-sized with methylcellulose. As it dries, the methylcellulose binds to the cellulose fibrils by means of hydrogen bridges, improving the mechanical stability of the paper by about 30 % (see Infobox 8).

 

Infobox 8. Steps of the Bückeburg Paper Deacidification Process
(click here to expand)

 

Written materials are very heterogeneous, and every page must be individually examined to determine if it can go through the conservation bath without damage. To prevent rust formation, metallic components such as paper clips and staples are removed. Materials that have been glued on, such as stamps, telegram strips, and paper clippings, are secured with a protective film. In a touchless process with an inkjet printer, every page is labeled with a file or inventory number and a page number.

Subsequently, photographs and documents with wax or seals, as well as very brittle and torn pages are set aside for hand conservation. Particularly important and frequently used written materials are preserved on microfilm before conservation.

The documents are individually laid on a sieve belt in four rows. Once the documents are in place on the first sieve belt, a second sieve belt covers them and holds them securely in place as they are processed through the machine.

Over about seven minutes, the documents secured within the double sieve belt are moved through the deacidification bath, which is cooled to 13 °C. The coarse-meshed transport sieves and additional nozzles ensure complete and intensive saturation.

In the bath, the ink is fixed to the paper, the paper is deacidified, an adequate alkaline reserve is established, and the paper is strengthened through sizing with methylcellulose—all in a single step! The documents are then passed over five drying cylinders at varying temperatures and air dried at temperatures that reach a maximum of 60 °C.

An air extraction and circulating system provides an optimal distribution of warm air and removes humid air. At the distribution station, the documents are removed by hand. The total time for processing one document is about 18 min.

After passing through the machine, the pages are somewhat swollen and slightly wrinkled. They can be flattened in heated presses. Overall, the volume of the paper increases by 3–5 % through this process. In the final step, the mechanically deacidified documents are reunited with the hand-preserved pages and checked for completeness, page by page.

 

After installation of this equipment in the German Federal Archive, the first documents treated—deacidified and strengthened—between the years 2001 and 2003 were the Ministerial files of the GDR (German Democratic Republic). This encompassed 550 continuous meters of documents consisting of 3.2 million pages, with a cost of a little over EUR 1,000 per meter of documents.

Deacidification of these documents was particularly urgent, because the paper used by the cabinet had a high wood content and was of poor quality. Currently awaiting deacidification in the Federal Archive are the collections of the “Politburo of the Socialist Unity Party of Germany”, the “Reich Chancellery”, and the “Federal Chancellery”. It is hoped that these tasks can be completed within the next ten to twelve years. At that point, however, only 1 % of the total collections of the Federal archives will have been secured for future generations.

 

6 Modern Paper

Years ago, libraries reached agreements with publishers to ensure that books would only be printed on paper that is resistant to aging and has an adequate alkaline reserve. When stored properly, this paper will most likely not need to be deacidified in future centuries. This makes deacidification a time-limited task.

Written records from the 1970s pose particular problems because the use of non-age-resistant recycled paper was heavily promoted and often mandated by German administrations. The industry standard for “Paper for Documents and Print Materials” (DIN EN ISO 9706) is strict, and recycled paper rarely meets it. This is no surprise, because recycled paper is a mysterious cocktail of acidic and wood-containing paper with unknown sizing, residual inks, etc. The preservation of valuable documents from this particular era will keep archivists busy over the next few decades [13].

 

7 Tackling the Problem

All official documents and books written or printed between 1850 and 1970 are threatened with disintegration due to acidic sizing and the use of paper containing lignin. Aging acidic paper does not immediately crumble into dust, as is described in some exaggerated publications, but the situation is certainly bad enough. Some of the paper is so brittle that normal handling by users of archives and libraries cannot be allowed. It is bitterly ironic that with today’s nearly unlimited information storage possibilities, we can only helplessly watch as time destroys our valuable cultural artifacts.

However, it is useless to complain. In the fourth century C.E., we faced a similar situation: All the classic literature of the Romans and Greeks was disintegrating on crumbling papyrus scrolls. In a tremendous effort, Constantine the Great and his son, Constantine II, had 100,000 works copied onto parchment [14].

Things don’t look as bad today because many works can definitely be preserved for future generations. Modern technology is of great use here, but we should not just start throwing away the originals after digitization to solve pressing space problems.

Originals are irreplaceable and must—if they seem valuable to us—be preserved. Many deacidification processes have been refined, some have proven themselves and guarantee the survival of documents for future centuries. When the time comes, others will decide their fate.

 

7.1 Save It, Copy It, or Toss It?

Official records are not works of art, but a document with a signature, seal, or hand-written notes in the margins has the special allure of the original. Regardless of this charm, we cannot keep everything. If we view every record from the Chancellery of the Reich, the People’s Parliament, and the German Bundestag as a piece of our cultural heritage, must we go through great effort and cost to preserve the brittle originals, or are film or digital copies sufficient? Unfortunately, copying does not solve the conservation problem, it just shifts it from paper to a different storage medium (film, magnetic tape).

There is one non-physical storage method: memorization. François Truffaut’s film “Fahrenheit 451” (based on the novel of the same name by Ray Bradbury) is about an authoritarian society in which all books are burned by the firemen. A small group of “Book People” live hidden in the forest, attempting to preserve the content of books by memorizing them.

Sometimes we end up exchanging one problem for another—the silver gelatine film commonly used in the past ended up being less age-resistant than paper. Many people are banking on digital document storage, but despite impressive advantages, experience warns us to be cautious. Older magnetic tapes made of nitrocellulose or cellulose acetate became brittle through hydrolysis, and magnetic materials lose their magnetization over time. In addition, external fields (from motors, elevators, etc.) can also irretrievably erase data.

For security reasons, magnetic tapes are now re-copied every ten years, and only copies of the “original” magnetic backup copy are allowed to be used.

The main problem with digital information storage lies in the short lifespan of the hard- and software [16]. As an example, when the American National Archive wanted to archive data from the 1960 census in 1975, only two devices were left in the world that could read the original magnetic tape (!) [17], and one of these was already in the Smithsonian Museum. In fact, not all the data could be retrieved, unlike the census of 1860, whose data was kept on paper and can still be read today with no problem [18].

The software used for coding the information also has a limited lifespan. Popular software undergoes dozens of upgrades over a ten-year span, and it is often impossible for the latest version to read data from three versions prior without error. Consequently, archives must read their entire store of digital data with old hard- and software and store it again with the newest technology. Who pays for this this? Perhaps printing on high-quality paper would be more secure and economical in the long term?

 

Acknowledgments

I wish to thank several colleagues who helped me to delve into the complex background of this fascinating area of chemistry with patience and advice: Dr. H. Bansa, former editor of the journal Restaurator and former Director of the Institute of Conservation in Munich, Germany, Dr. A.-C. Brandt of the Bibliothèque Nationale in Paris, France, Dr. R Hofman at the German Federal Archives in Berlin and Koblenz, Germany, and Dr. H. Kleifeld and Dr. W. Markiewicz at Neschen AG in Bückeburg, Germany. I thank Neschen AG for the use of photographic materials.

 

References

[10] A. N. Maclnnes, A. R. Barren, Spectroscopic evaluation of the efficacy of two mass deacidification processes for paper, J. Mater. Chem. 1992, 2, 1049–1056. https://doi.org/10.1039/JM9920201049

[11] A. D. Baynes-Cope, The Non-aqueous Deacidification Documents, Restaurator 1969, 1, 2–9. https://doi.org/10.1515/rest.1970.1.1.2

[12] D. W. G. Clements, 1988 Paper Preservation Symposium, Capital Hilton, Washington, D.C., Oct. 19–21, Technical Association of the Pulp and Paper Industry (TAPPI) Proceedings, Tappi Press, Atlanta, USA, 1988.

[13] R. S. Kamzelak, Schrift- und Kulturgut in Gefahr: Chancen und Risiken der Massenentsäuerung, www.bibliophilie.de. (archived version accessed December 4, 2023)

[14] K. Kleve, Preserving The Intellectual Heritage–Preface, Council on Library and Information Resources, clir.org. (accessed December 4, 2023)

[15] N. Baker, Der Eckenknick, 2005, Rowohlt Verlag, Hamburg, Germany. ISBN: 9783498006266

[16] H. Bansa, The New Media: Means for Better Preservation or Special Preservation Problems?, Restaurator 1991, 12, 219–232. https://doi.org/10.1515/rest.1991.12.4.219

[17] F. H. Westheimer in Durability and Change (Eds. W.E. Krumbein et al.), Dahlem Workshop Report, Wiley, Chichester, UK, 1974.

[18] H. Cerutti, Von sterbenden Büchern und digitalen Verlockungen, unimagazin zürich 1995, 3.

---

The article has been published in German as:

• Papierkonservierung: Chemie kontra Papierzerfall,
  Klaus Roth,
  Chem. unserer Zeit 2006, 40, 54–62.
  https://doi.org/10.1002/ciuz.200600376

and was translated by Caroll Pohl-Ferry.

---

Chemistry Takes on Paper Conservation – Part 1

The beginnings of paper production—from rags to wood

Chemistry Takes on Paper Conservation – Part 2

The chemical causes of paper disintegration

---

See similar articles by Klaus Roth published on ChemistryViews.org