This talk was given by Sarah Bertalan, as the culmination of a career observing foxing and reverse foxing during her conservation practice. It is such an interesting topic that there was too much information to squeeze into 30 minutes, and Bertalan left us hanging without a conclusion but I will provide some links to articles that she mentioned at the end of the talk summary, as well as to the conservation wiki on foxing. I hope that she will publish in the BPG Annual Postprints, as I’m sure the majority of the attendees of her talk would agree! It is a topic that I find so interesting, and I even compiled my own literature review of foxing while in graduate school!
Bertalan’s observation of 19th and 20th century papers has led her to propose that foxing, the “reddish, brownish, roundish stains that occur in a random pattern,” is not caused by mold or by metal inclusions, but rather by inorganic additives that were added to the paper. It is widely reported that treating foxing stains can frustratingly lead to their reappearance within a relatively short amount of time. Bertalan also considers the capacity for 19th and 20th century papers to discolor over their entire surface, not just in in the staining we associate with foxing. In such cases, the stains may be extensive but superficial, and the condition would be due to contact with catalyzing, acidic materials, not migration of degradation products, as is seen for matburn. Reverse foxing is a term that remains undefined, and the cause is unknown. White spots, as if negative images of the dendrite-like reddish brown foxing spots, will appear and be visible in normal light, often after a paper has been treated. Reverse foxing has been identified frequently on Van Gelder Zonen papers.
Inorganic additives were added to papers in the 19th and 20th centuries to achieve specific results. Additives such as minerals and metal oxides were added to modify the surface and texture, to act as fillers and opacifiers, brighteners and to aid in ink retention. The additives are extremely reactive and act as salts, catalyzing acid-base reactions. While foxing was not visible immediately after the paper was made, elevated humidity, changing pH and daylight would provide the environment to form foxing. Contemporaries were aware of the effect of humidity on papers, notably the appearance of foxing stains.
Bertalan observes that the sensitivity of papers coated or immersed in metal salts to light is well documented, as the earliest photographs were made with paper coated in metal salts. The supporting evidence Bertalan presents for inorganic particles causing foxing is the presence of opaque zones that correspond to foxing stains when paper is seen in transmitted light. Furthermore, when foxing is not visible in normal light, opaque dendrite-like inclusions in the paper can be seen in UV light as well as transmitted light. Even when the reddish-brown stains have been washed out of the paper, the opaque regions still remain when viewed in UV light.
Resources:
http://www.conservation-wiki.com/wiki/Foxing_(PCC)
Soyeon Choi Literature Review on Foxing (you must use your AIC sign-in to access the article) http://www.maneyonline.com/doi/pdfplus/10.1179/019713607806112378
Browning, B.L. Analysis of Paper 1977.
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AIC Blog Archives: Conservators Converse
Former Blog of the American Institute for Conservation