Curling is a planar distortion in a paper support in response to changes in relative humidity (Reilly 1986). Factors such as fiber strength, internal bonding, and fiber orientation may affect curling.
Warping, cockling, buckling
Synonyms in English
Reilly (1986)discusses curling in relation to photographs and any work on machine-made paper. The process of paper manufacture creates a predominant fiber alignment, called the machine direction or grain direction. This provides a directional orientation to the physical properties of the paper. The direction perpendicular to it is the cross direction. Due to this predetermination, the fibers will expand much more in one direction than the other when exposed to moisture. Fluctuations in relative humidity will accelerate curling tendencies.
Some types of photographs, such as Albumen prints, are especially susceptible to curling. The binder and the paper support are connected yet respond to environmental factors differently, expanding and contracting at different degrees and rates. When the binder expands and contracts at a different rate than the paper support, compressive and tensile strengths are created which can damage the photograph (Reilly 1986). Curling, especially tight curling, may cause a series of parallel cracks because the binder is not flexible enough to withstand the stresses.
Types of Curl
as described by Brenda Bernier (2004):
- T curl: Across-grain or transverse curl.
- L curl: direction of curl is longitudinal or parallel to grain.
- D curl: diagonal curl or two opposing corners.
- C curl: cupping or all four corners.
Bernier, Brenda. 2004. “Issues in Humidification and Drying of Gelatin Silver Prints” Topics in Photographic Preservation, Vol.11. Compiled by Brenda Bernier. Washington: American Institute for Conservation, Photographic Materials Group. Pp. 6-16.
Kiely, Helen U. 1927. The effect of moisture on paper. Holyoke, MA: American Writing Paper Company.
Reilly, James M. 1986. Care and Identification of 19th-CenturyPhotographic Prints. Rochester, NY:Silver Pixel Press.
Van der Reyden, Dianne. 1992. Recent Scientific Research in Paper Conservation. Journal of the American Institute of Conservation 33(3):117-138. http://www.jstor.org/stable/3179619
Vitale, Timothy and Paul Messier. 1994. Physical and Mechanical Properties of Albumen Photographs. Journal of the American Institute of Conservation 33(3):279-299. http://www.jstor.org/stable/3179638