Four drying methods (air drying, vacuum freeze drying, vacuum packing, and drying in a Vacme press with Zorbix), and one sterilization technique (ethylene oxide) retained essentially all of the pre-treatment mechanical integrity of the book papers tested and were deemed non-damaging. By contrast, samples from the thermally dried and gamma-irradiated books lost 19% and 24%, respectively, of their mechanical integrity (Fig. 1), and were determined to be inappropriate for treating water-damaged books of permanent retention value.
Physical cockling in treated material was most successfully minimized when book paper was pressed as the moisture was removed. Vacuum packing and Vacme press drying best achieved this ideal but at different levels of fiscal investment. The initial cost of a vacuum packing
machine may prove prohibitive for many collecting institutions in addition to which numerous, disposable polyester vacuum pouches were required for each book dried.11 The Vacme press, conversely, is so inexpensive that even relatively poor libraries might consider acquiring one as a precautionary measure in the event a limited numbers of rare books ever require drying. Used in conjunction with Zorbix the Vacme press can significantly reduce drying time, but in trials conducted outside of this study the Vacme press also proved effective when readily available
newsprint (printed or unprinted) was used as interleaving.
Air drying, which remains the most commonly applied book drying technique due to its low setup cost, produced far better results in terms of physical flatness when semi-dried books were pressed overnight, followed by further air drying the next day. Mold did not form inside the
books given the limited duration of this damp pressing, but intermittent exposure to freely circulating air is requisite and, of course, this unrestrained period promotes further page cockling.12 A final long pressing as the book reaches its dry state helps reduce this recurring
Air drying, vacuum packing, and drying in a Vacme press with Zorbix are all labor intensive methods best monitored by trained technicians handling relatively small batches of books (e.g., <100 volumes at a time). Given sufficient freezer capacity to forestall microbial growth, these
three techniques can be applied to several hundred books by simply thawing manageable batches prior to treatment.
Vacuum freeze drying remains the most efficient method for drying large quantities of books (e.g., >500 volumes), especially for books printed on coated stock paper.13 Unrestrained vacuum freeze drying, however, produced the greatest amount of cockling in the book papers tested. While not tested as part of this study, examples of vacuum freeze dried books pressed during sublimation were examined at the British Library and were shown to produce excellent results. The British Library’s small vacuum freeze drying chamber has been modified with a mechanical jack inside the chamber that is used to manually squeeze books between thin steel plates. The chamber must be opened every two days and the jack’s pressure increased to compensate for decreasing book thickness caused by sublimating ice crystals. While approximately doubling the drying time and significantly increasing labors costs, this approach to vacuum freeze drying produces far better results than books are dried without constraint. Consequently, it is recommended that consumers insist commercial vendors flatten permanent retention collections during the sublimation process and that they be prepared to compensate them for the added time and effort involved.
During the past three decades sterilization of cultural property has largely been downplayed as a technical option. Instead, conservators have learned to rely on environmental stabilization to return mold to a dormant state before removing the desiccated spores with a small vacuum
aspirator or vacuum cleaner equipped with a HEPA filter.14 The literature does a poor job, however, of identifying appropriate technical options when sterilization must seriously be considered. As noted by Fausta Gallo in 1978, sterilization should be reserved for “cases in which arresting infection and infestations is an unarguable necessity.”15 This can occur, for example, when significant delays in a recovery cause wet collections to mold excessively, or when floodwaters contain contamination such as sewage or other biological hazards. Ignoring sterilization in such circumstances, even when dealing with irreplaceable collections, can pose potential health risks to future users and result in long-term liability issues for recovery specialists and collecting institutions.
Gamma irradiation continues to suggest a promising alternative to ethylene oxide sterilization, but as reported by Butterfield in 198716 and confirmed in the present study, this approach damages book paper to an unacceptable degree. The mechanism by which ionizing gamma
radiation kills microbes and renders spores nonviable simultaneously cleaves the cellulosic (and other polysaccharidic) chains from which paper derives its mechanical integrity. Lower levels of damage have been reported in experimental settings where paper was treated with low levels of
gamma radiation,17 but it remains to be demonstrated that significant cellulose degradation can be avoided at doses that yield effective sterilization, or that this approach is commercially viable. Consequently, the authors cannot recommend gamma irradiation over ethylene oxide for sterilizing books of enduring cultural significance under any circumstances we can envision.
The use of ethylene oxide remains controversial in the U.S. Its detractors concede, however, that loss of lignocellulosic mechanical integrity as a result of ethylene oxide fumigation is not the issue. Objections focus instead on potential latent effects to book components other than paper
such as plastics, adhesives, skin-derived materials, and media, as well as possible health risks associated with ethylene oxide off-gassing. The first concern has limited relevance in the context of a flooding incident whose scope and severity are so extreme that sterilization would need to be
considered. Bindings in the recovery phase of such an event are quite often discarded, and the authors are unaware of any well-grounded studies reporting replicable findings of significant ethylene oxide-induced alteration of media likely to be encountered in books that would be considered for mass sterilization.18 The present study corroborates the research of Flieder (1999)19 and Gallo that found that book paper thoroughly sterilized by a commercial dosage of ethylene oxide remains mechanically undamaged.
An influential study performed at the Library of Congress by Hengemihle, Weberg and Shahani (1995) determined that following sterilization, “off-gassing of ethylene oxide by library materials is a reality.”20 This finding is often misconstrued to suggest that ethylene oxide is inappropriate for use in conservation, but in fact, the study simply concludes that “fumigated materials should be added to the collections only after the ethylene oxide concentration is decisively under 1 ppm.”21 This conclusion mirrors U.S. workplace safety standards implemented nearly a decade before (in 1986) that mandate a 24-hour off-gassing period following sterilization to permit ethylene oxide concentrations to fall below 1 ppm.22 This legal standard is adhered to by licensed U.S. contractors and is readily achieved because EtO is a volatile and reactive gas that dissipates quickly, reacting with atmospheric water molecules to yield more benign species.
Both gamma irradiated and ethylene oxide sterilized books contained a residual and, by consensus, objectionable odor. It is speculated this smell related to decomposing mold spores within the books. Whether this could be mitigated by surface vacuuming, followed aeration or exposure to absorbent media (e.g., activated carbon, potassium permanganate, or baking soda) in a confined space is outside the scope of the present study. Clearly, neither method is ideal. Further research into alternative mass sterilization techniques is desperately needed, including but not limited to an investigation into plasma fumigation.23