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Photographic Materials Conservation Catalog
Photographic Processes
Date: Initiated March 2012
Contributor: Saori Lewis


Historical Facts
Important Development Dates:
Historic Practitioners:
Widely used in commercial publication from around 1900 to the 1960s until offset lithography became more common.

Identification Characteristics
Image layer: Letterpress ink on paper
Color: Commonly black, but may be any color
Support: Paper
Analysis: Relatively large dots are visible via visual examination under moderate magnification (~15x) or often by naked eye. Due to the nature of relief printing, slight raise of ink may be present along the edges of the dots.

Basic Process Overview
A copper plate negative is prepared by coating with fish or animal glue mixed with light-sensitive bichromate. A photographic image is photographed through a glass screen with fine grid patterns on the copper plate negative. When the negative is developed to high-contrast, the smooth gradation of the original photograph is broken up into various sizes of dots. Since the coating on the negative hardens relative to the amount of exposure to light, shadows in the original image are converted into large dots and highlights become small or no dots. After un-exposed coating is washed away, the plate is etched, resulting in a relief block for printing. The plate is inked and pressed against paper, resulting in photographic image that is realized by varying sizes of pure-black dots.

Housing and Storage Considerations

  • Letterpress/half-tone prints should be stored in good quality paper-board folders, to protect from light, dust, and handling. Materials passing the Photographic Activity Test (P.A.T.) are suitable for use.
  • An ideal temperature and humidity are 68 degrees F (20 degrees C) +/- 2 degree drift and 50% +/-5% variation over 24 hours.

Emergency Recovery Tips

  • Handling wet paper can cause damage such as nicks, abrasions, and tearing.
  • Mold can develop on or within the papers.
  • Find step-by-step salvage technique for moldy paper in Preservation Leaflet 3.8: Emergency Salvage of Moldy Books and Paper[1]by Northeast Document Conservation Center (http://www.nedcc.org)

Electronic Media Resources: WEBLINKS, AUDIO, DIGITAL

Printed Resources: BOOKS, ARTICLES

  • Baldwin, Gordon. 1991. Looking at photographs: a guide to technical terms. Malibu, Calif: J. Paul Getty Museum in association with British Museum Press.
  • Benson, Richard. 2008. The printed picture. New York: Museum of Modern Art.
  • Gascoigne, Bamber. 2004. How to identify prints: a complete guide to manual and mechanical processes from woodcut to inkjet. New York, N.Y.: Thames & Hudson.
  • Reilly, James M. 1986. Care and identification of 19th-century photographic prints. Rochester, NY: Eastman Kodak Co.

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