This talk was given by Céline Allain of the Bibliothèque nationale de France (BnF), after the lead author, Lucille Dessennes, also of the BnF, was unable to attend the conference.
In 2014, a pipe burst in the BnF, causing damage to 12,000 books, mostly from the 18th and 19th centuries. 360 of these 12,000 items contained coated papers, and when the disaster salvage/freeze drying contractors arrived on the scene, they would not accept books with coated pages for treatment.
The emergency team at the BnF instead had to use 6 freezers at the BnF to freeze-dry the 360 books with coated pages, although 51 of the 360 were too dry to be freeze-dried. Allain spoke to how difficult it was for the emergency team to accurately identify which books had coated pages—whether because the feel and look of the papers can vary or there might only be a few coated papers in a volume— and encouraged the audience to train emergency response teams to recognize coated papers beforehand. The difference is rarely as easy to identify as in the graphic below:
A common theme through the talk was the importance of keeping coated papers wet until they can be frozen. Even in the 2-3 hours it took the emergency team to arrive and place books in freezers, a number of books with coated papers had to be frozen “half-dried,” which limited the recovery outcome for these books. Had the books been kept wet and then frozen wet, they would have fared better.
Allain addressed the makeup of coated papers in order to explain why the pages should be kept wet: the coating (a mixture of pigments, binders, and other elements to improve opacity or water resistance) swells in the presence of water, readily attaches to the wet coatings of facing pages, and congeals into a “block” of stuck pages upon drying that cannot be separated without delamination of the paper surface. When the coated papers are still in a wet state, however, the pages can still be separated without loss of content.
The standard treatment for drying coated papers is freeze-drying (see below Further information), as long as there it is not a vacuum-thermal drying procedure. This allows the frozen water to sublimate.
For the 51 books that had been frozen half-dry, however, there were some that had blocked pages that needed to be un-blocked. The authors adapted a number of treatments to the books, including using a Teflon spatula to separate pages while still frozen.
The authors knew from previous research into the paper industry that the main binding agent in the papers was styrene-butadien latex (LSB in French, SB latex in English), which is soluble in tetrahydrofuran. Because the tetrahydrofuran’s toxicity made it too dangerous for use, Allain and Dessennes consulted the solubility triangle to arrive at a less toxic solvent. Using a mixture of toluene and ethanol (50/50 vol/vol), the conservators were able to attain equivalent solubility parameters and un-stick blocked pages of the affected books. The conservators brushed on the mixture, softening the SB latex, and then used a stiff spatula to separate the pages. The work is done in a fume hood. The authors noted that a large drawback is that the solvent can only be applied to specific areas of blockage and cannot be used on a large area or an entire book because the inks are frequently soluble in the solvent mixture.
Dessennes experimented with using the solvent in a solvent chamber, but speculates that because of the thickness of the block, that the vapors could not penetrate the interstices of the paper. Because of the limitations of the solvent applied as a liquid and in vapor form, Allain and Dessennes have plans to experiment with the solvent used in a low pressure environment.
“Effet de la lyophilisation sur le comportement mecanique et chimique du papier, du cuir et du parchemin” Flieder, Francoise; Leclerc, Françoise; Chahine, Claire
Carlsen, Soren. “Effects of Freeze drying on Paper,” IADA Preprints, 1999, p. 115-120.
David Tremain on Emergency Drying of Coated Papers http://cool.conservation-us.org/byauth/tremain/coated.html
NEDCC leaflet on Freeze-Drying: https://www.nedcc.org/free-resources/preservation-leaflets/3.-emergency-management/3.12-freezing-and-drying-wet-books-and-records
CCAHA on freeze-drying techniques: http://www.ccaha.org/uploads/media_items/ccaha-freezing-drying-techniques.original.pdf
NARA on efficacy drying techniques: http://www.archives.gov/preservation/conservation/drying-methods-02.html
LOC on drying techniques, what to do if collections get wet: http://www.loc.gov/preservation/emergprep/dry.html