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Photogravure Conservation
Chemical

FOR A WORK OF ART OR CULTURAL ARTIFACT, virtually any chemical interaction with the environment is a potential form of deterioration. For photogravures, the most susceptible component is the paper base. The chemical degradation of paper is most often catalyzed by long-term exposure to elevated relative humidity and light. Moisture from the air interacts with cellulose to break chemical bonds along the polymer chain and to produce acidic deterioration products that then serve to catalyze additional deterioration. With time, these changes are usually manifested in the gradual overall darkening of the paper base and increasing brittleness. Exposure to light will have a similar effect caused by the photo oxidation of cellulose and the accumulation of chemical deterioration products. Temperature is a significant variable governing the rate of any chemical reaction, including those catalyzed by moisture and light. Over the years, museums and other cultural institutions have developed guidelines to take these factors into account. A common recommendation for museum storage of works on paper is a range of 30% to 50% relative humidity (RH) and temperature not to exceed 68°F (20°C). As damage caused by light is mainly due to duration of exposure and intensity, most institutions will limit light levels to 5-9 foot-candles (54 -97 lux) and rotate exhibitions of works on paper on and off display after several months.

Housing materials, such as mats, interleaving papers, frame backings and boxes can greatly assist or undermine the preservation of photogravures. Paper-based enclosures should be lignin-free (lignin is an acidic resin found in wood) and composed of cotton fiber ("rag") or highly purified wood pulp. The addition of an alkaline buffer to paper enclosures is recommended to help counteract acid-based deterioration. The importance of good quality interleaving paper for intaglio prints in storage is essential as the linseed oil ink medium used for these prints can be fairly acidic and cause localized staining among stacked prints. Plastic sleeves, used in addition to or as an alternative to paper enclosures, should be composed of chemically inert materials such as polyester, polypropylene or polyethylene. Plastic sleeves of unknown composition should be discarded. Whenever possible, handling should be limited to the print enclosure as finger oils can cause staining over time and embed surface dirt. If prints do require direct handling, the use of white cotton gloves is recommended. On display, frames should incorporate a glazing material that filters for ultraviolet. These higher energy wavelengths (relative to visible light) catalyze proportionally greater degrees of deterioration. Good quality housing materials, including frames, museum cases and other storage boxes also can significantly reduce the impact of fluctuations in relatively humidity. This attribute can be a significant benefit to the private collector that is unequipped to maintain a constant level of moderate relative humidity.


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Special thanks to expert conservator Paul Messier for contributing his expertise to this project.