The Abbey Newsletter

Volume 13, Number 5
Sep 1989

Paper Conservation Update: Bleaching and Fumigation

by Leslie M. Kruth
The author welcomes comments and discussion at: 145 Grove Drive, Portola Valley, CA 94028.

Revised excerpts from a talk given at the annual meeting of the American Institute for Conservation in 1988. Original title: "A Survey of Recent Scientific Research Which has Caused a Re-evaluation of Commonly Used Practices in Book and Paper Conservation." Reprinted with permission from the AIC Book and Paper Group Annual, vol. 7, 1988, p. 30-39.

When I was first approached to give this talk I saw it as a wonderful opportunity to speak with my colleagues--both scientists and practicing conservators--and review the literature on new developments and old developments I had forgotten. It soon became apparent that in a 20-minute presentation I could not hope to discuss all of the scientific research that has affected paper conservation practice. As a consequence I plan to give a very personal overview of historic developments and current research in four areas: bleaching, washing and deacidification, encapsulation, and fumigation. [Of these four, only bleaching and fumigation are covered in this excerpt for the Abbey Newsletter.]

However, before I talk about scientific research that has changed the way paper conservators do their work I would like to comment on research developments that are changing the way research is being conducted. Some of the most significant changes that have occurred in the scientific evaluation of the nature of paper involve the manner in which experimental data are analyzed and interpreted.

Paper conservators want to ask scientists about the effects of particular treatments on paper, and yet before we can get an answer to a practical treatment question, we find that the scientists are still trying to clarify how to evaluate the degradation of cellulose. For example, one of the standard industry tools for evaluating paper strength, and one that has been widely used by conservators and conservation scientists, is the fold endurance test. In simple terms, this test is administered by a machine which literally folds a piece of paper until it breaks. Antoinette Dwan discusses the limitations of this test at length in her paper in the Spring 1987 AIC Journal. While this test can be used as a quick indication of the state of degradation of a piece of paper, it is an imprecise measure and open to multiple interpretations. As a result it is not surprising that conservation scientists in recent years have moved away from fold endurance as the significant indicator of a paper's strength, and instead increasingly rely on a measure of the degree of polymerization or D.P.: a technique which looks at the length of the cellulose polymer chains. While this is not a new test, its application to paper conservation science for the analysis of degradation products is relatively new.

Another area that is being examined in detail is artificial aging conditions. The ideal artificial aging temperature for paper is an ongoing question--as it is for many materials. Different cellulose breakdown products are formed depending upon the temperature and humidity of the aging oven. Those products may not best represent the effects of natural aging. David Erhardt at the Conservation Analytical Lab of the Smithsonian reported his findings on the effects of artificial aging at the 1987 AIC Annual Meeting, and continues to research the problem. His research could change the future of paper testing and the meaning of previous research.

I raise these matters by way of an extended preamble because it is important that we all recognize at the outset that there are few truly settled issues in the areas I will be discussing. There are no easy answers. Paper conservators and scientists do not all agree on what techniques best achieve safe but effective treatments. Moreover, given the research now ongoing, what I have to say today may be moot by the time you read this article.


Bleaching is probably the most talked-about treatment in paper conservation. "To bleach, or not to bleach," that is the question. As Helen Burgess says: "It is difficult to conceive of a process which has more potential for the sheer destruction of artifacts."

Nevertheless, we do bleach! Over the last decade, chemical bleaches have been examined pretty closely and the general parameters of their safe use seen to be well defined. Bleaching must be done with the greatest respect for the artifact AND with the greatest attention to the method of application in order to make it as "safe" as possible.

The reactions which take place during bleaching are either chemical oxidation or reduction of the colored materials. It is difficult to confine this process solely to the stains you want to remove. Other components which make up the artifact, both the substrate and the medium, may be damaged by these same reactions. In addition to whitening, oxidizing agents cause changes that include a decrease in the degree of polymerization of the cellulose molecule, increase in carbonyl oxidation products, and the loss of a wide variety of physical strength properties. Nonetheless, oxidizing agents are often the only effective treatment against severe discoloration and have long been used in conservation treatments.

Some oxidizing bleaches which have fallen out of favor over the years are potassium permanganate and chloramine-T. The problem with chloramine-T is that it bonds very strongly to paper fiber and can be extremely difficult if not impossible to remove. There are better oxidizing bleaches available which do not have this problem. As for potassium permanganate, I do not know how widely it was used--I experimented with it once--it turned the paper sample bright purple-brown, then I added the acid which was the second step, and it became white again. It was 4th grade chemistry at its best. Chemically and esthetically it was terrifying to consider it as a conservation treatment.

Some of the oxidizing bleaches which are in use today include the following:

Recently, considerable attention has shifted to reducing bleaches. Helen Burgess has done research on reducing bleaches and has found sodium borohydride and tetramethyl and ethyl ammonium borohydride to be the safest for use in paper conservation. According to her research, there have been no scientific results which show negative chemical effects on cellulose as a result of bleaching with reducing agents. In fact, when particular reducing agents are used after oxidizing bleaches, as one would expect, they can actually reduce the state of oxidation of the paper, making it less vulnerable to future oxidation.

However, there are some very practical problems with the use of reducing bleaches in conservation. While they may be safe for the cellulosic polymers of paper, they can wreak havoc with the media and can pose serious problems to the physical structure of the sheet through the evolution of hydrogen gases. This can often be minimized by the use of the vacuum suction table and adjustment of the bleach mixture.

Needless to say, the easy answer to what bleach to use does not exist, and the limitations of any bleaching agent can be quite troubling. Despite the limitations of reducing bleaches, I have noticed that many conservators are using them more now than in the past, especially for spot bleaching.

I could not finish my discussion of bleaches without telling all of you who are not paper conservators why it is that paper conservators all carry dark glasses in their tool bags. It is because the bleach that has really captured the interest of paper conservators in the last few years is light bleaching. The process basically involves the bleaching of paper with light--either sunlight or artificial light--in the presence of alkalized water, either a bath or locally wet paper. Light bleaching is an oxidizing process. In the 18th century it was also the only bleach available. Now light bleaching is back, in spite of, or perhaps because of, the advances of chemistry.

Wholly apart from its scientifically provable merit, many conservators use light bleaching for very simple reasons: it is nonchemical, it is easy to control, and it is amazingly effective. Of course it is photo-energy, and while it may seem more wholesome to humans, we already know that photo-energy can be a source of degradation when paper is exposed to it in dry conditions.

Whether or not light bleaching is the safest of the oxidizing bleaches has not been finally determined. Several scientists have been researching the effects of light on paper from different angles.

Robert Feller has been trying to identify what the components are that are being bleached or darkened by light in mostly dry conditions. He has been looking at model compounds that are representative of the components which would be reactive to light and heat in a paper substrate. Subjecting known sugars and known lignin-like compounds to light, he has been able to determine the effects of wave length, temperature, and pH on brightening and on post-irradiation darkening. His testing in dry conditions, however, have not proven to be representative of light bleaching under wet conditions. Dr. Feller most recently reported on results from light bleaching paper samples immersed in water (Ottawa CCI Conference, Fall 1988). His experiments indicate that light bleaching of lignin-free papers in wet conditions does not cause damage to the cellulose structure. The light degradation which is seen on dry paper may in fact be significantly attributable to thermal causes.

Further testing is still necessary, but Dr. Feller's work confirms some earlier informal tests done by Keiko Keyes with Santucci that have not been published. Their results which were analyzed using D.P. tests on good quality, non-lignin-containing papers, exposed to the California sun, showed that there was no appreciable degradation of the fibers. Their test did not examine unpurified wood-pulp or severely degraded papers.

Dr. Feller's results also follow on the heels of Diane Van der Reyden's work at CAL. Risking oversimplification here, I request that the reader read her results as published elsewhere in the Book and Paper Group Annual, vol. 7. She examined the effects of light bleaching in water baths on paper of mixed fiber content. Tensile strength measurements were taken on test samples and no significant change in strength was found that could be attributed to light exposure. The tests did find a significant change in strength characteristics between samples which had been placed in water and those which had not been water treated. Whether this is due to the alkaline material present in the water or just the effects of washing is not clear and is reminiscent of the results of Tang and Jones on wash water quality.

Paper conservators can not get information fast enough about the effects of bleaching. At times we feel that we have so few tools in our arsenal to improve the state of a badly damaged work of art. More relevant scientific data would certainly make us feel more secure about our treat-merits.


Ideas about fumigation to kill or inhibit mold growth on paper have changed a great deal in recent years. It used to be that every lab had a thymol chamber where screen racks sat over a low wattage light bulb which heated a dish of thymol. The accepted treatment was to put the mold damaged article in the chamber for a few weeks and then, when you removed it, to brush off the dead mold. Presto, your artifact was protected--or at least the mold was dead. Well, apparently not.

Research has found that in addition to being hazardous to your health, thymol is not effective in killing the broad spectrum of mold that we thought it would kill. Mary Wood Lee gave an inkling of this when she mentioned at the Baltimore AIC meeting that placing mold infested articles into a thymol chamber just gave the mold more time to grow and spread!

In recent years various researchers and conservators have shared with us their favorite fumigants. Some like thymol. Some favor orthophenylphenol. Ethylene oxide will sterilize anything (including the staff). Biblio-cryobionomics has been suggested, as have vacuum treatments, radiation, and carbon dioxide. I am sure I have missed some.

While it has always been known that the way to prevent mold growth is to control the environment, the current thinking is that controlling the environment is also the most effective way to stop mold. According to Tom Parker s (of Pest Control Services, Inc.) research, what stimulates a spore to germinate is not the moisture content of the substrate, but the relative humidity in the immediately surrounding area. If the RH is less than 65%, a spore cannot germinate. That means that if an object with am active mold growth is subjected to a RH below 65% it will be killed. The spores may not be killed, but the feeling among many researchers is that we live in a spore-filled world anyway, and killing a few will not eliminate the problem--regulating the environment will.

Certainly if the intrepid conservator is still intent on killing spores, there seem to be much less toxic agents available than thymol and ethylene oxide. For example, Richard Smith has suggested the use of the vacuum chamber which he believes will cause mold spores to burst.

It may seem strange to think that a non-treatment may be the best treatment for a problem like mold, but research into fumigation has increasingly rendered the fumigation chamber a thing of the past.

I would like to thank all of the individuals listed in this article for doing the research that they do in conservation--it is much needed. In addition, I would like to thank those who so generously gave of their time to help me clarify the scientific data and the issues which are being debated in our specialty. Helen Burgess, Diane Van der Reyden, Robert Feller, Tom Parker, Keiko Keyes, Tim Vitale, Chandru Shahani, Susan Lee-Bechtold, Tom Albro and the paper conservators at LC were of invaluable assistance to me.

In conclusion I would like to urge all of my fellow paper conservators to take a conservation scientist to lunch--or vice versa.

 [Contents]  [Search]  [Abbey]

[Search all CoOL documents]