Discoloration and foxing (scattered dark spots) in paper are some of the most common reasons that clients bring paper artifacts to a conservator for restoration. These conditions are often cleaned with oxidative bleaches. While such treatments are generally quite effective, if iron is present in the paper oxidative bleaching can actually make the discoloration worse. Part 1 of this article describes the problems that iron can cause in paper; Part 2 will describe some solutions that are making their way into the paper conservators repertoire.
Iron is sometimes, but not always, present in paper. When iron is present in paper, it can sometimes, but not always, cause discoloration and foxing. Because iron is a catalyst, traditional oxidative bleaching methods used by conservators to reduce discoloration can, after an incubation period of a few weeks or months, actually accelerate discoloration, making a bad situation worse. Iron can exist in many forms. As anyone with a car, boat or carbon steel kitchen knife knows, metallic iron (Fe) is not particularly stable; it likes to oxidize into a broad array of black, white, uncoloured and multicoloured compounds, including rust.
Iron can exist in more than one oxidation state. The most common oxidation states are Fe(II) in which metallic iron has lost two electrons and Fe(III), where metallic iron has lost three electrons. Fe(III) is the least reactive form of iron but both Fe(III) and metallic iron can, under the right environmental circumstances, convert to the very reactive Fe(II) form.
Iron can be introduced into the paper intentionally, as in the case of inks and other media, or as unintended contaminants. Iron can be found in unpurified wood pulp, the water used to make the paper, low grade chalk fillers, rosin and papermakers alum. Bits of metallic iron that rub together and break off from paper-making machines also find their way into paper sheets.
Traditional methods of treating discoloured paper include oxidative bleaches that contain or generate peroxides (for example, hydrogen peroxide, sodium perborate and light). Peroxides react with and solublize the damaged and discoloured portions of the cellulose molecule, allowing them to be washed away. Although these oxidative bleaches can be very effective, Fe(II) can (but not always) catalyze the peroxides to trigger the dreaded Fenton Reaction. In a few weeks or months, the previously cleaned paper can become heavily spotted and discoloured. This situation is obviously something to be avoided, but its occurrence cannot always be predicted.
How, then, to avoid such a situation? Is it possible to determine if paper contains iron before embarking on oxidative bleaching treatments?
There are visual and chemical tests for detecting iron in paper before treatment but, unfortunately, these tests are not foolproof. Iron particles can be hidden deep within the paper sheet; oxides may not be concentrated enough to give positive reactions when tested with chemical reagents or they may be unevenly distributed in the paper sheet, leading to false negative test results.
What, then, is the best way to safely reduce discoloration in paper that may contain iron, detected or not? And once discoloration has been reduced, how can paper be protected from residual iron that may have been left behind? There are no simple answers to these questions, but my research over the past year has given me some leads which I will share in the next article.
Previously: Caring for Public Collections: A Condition Survey
Next issue: Iron in Paper: Problems and Current Solutions Part 2