Exploring

Inherent Vice: Structures

Photographic Prints: Black and White

For a discussion of photographic negatives, see Session 3: Deterioration of Film-Based and Electronic Media.

Among the materials collected and organized by cultural institutions, photographs are the most treasured and heavily used. Extensive technical and aesthetic changes have affected photography since 1839, when Louis Daguerre developed a practical method for creating permanent images.

Each photographic process has unique characteristics that affect its deterioration, storage, and handling. In order to care properly for photographic collections, you must understand how photographs are made and learn how to identify their various types. You must also become familiar with problems related to their physical and chemical composition.

The Basic Photographic Process

Photographs are composite objects, consisting of a base (also called a support), a binder, and an image-forming substance. To produce a photographic image, light-sensitive materials such as silver salts are applied to a support made of paper, cloth, plastic, or metal, and exposed to light. This forms either a direct visible image (known as printing out) or a latent image, which can be developed using a chemical developer (known as developing out). The image is "fixed" in order to remove excess light-sensitive material and stop the darkening process. It is then washed to remove residual fixer.

Image-Forming Materials

As photographic techniques developed, a number of different light-sensitive materials were used to form the image. The most important was metallic silver, which was used in most 19th century photographic prints (including salted paper prints and albumen prints) and is still used in modern black and white prints. Differences in appearance in the different types of prints are due to differences in the physical form of the silver particles.

Other types of image-forming materials used during the 19th century included platinum (platinotypes, which are silver gray in color and quite stable), iron salts (cyanotypes, which have a distinctive blue color and are very subject to fading due to light exposure and contact with alkaline materials), pigments (carbon prints, woodburytypes, and gum bichromate prints, all of which used inorganic pigments and have excellent image stability), and dyes (used for hand coloring and tinting of binder and support layers and very unstable when exposed to light).

Silver images exhibit several characteristic types of deterioration:

Sulfiding—This is the formation of silver sulfide when silver reacts with sources of sulphur. This can result from the presence of sulphur in the atmosphere, but it is most often due to inadequate washing that leaves behind residual processing chemicals (fixers) in the print. The result is yellowing, staining, and fading of the image, usually beginning with the highlights.

Redox—The visual results of redox reactions vary depending on the type of image, but generally they include loss of highlight detail, fading, and change of image color to reddish, yellow, or yellow-brown. Silver mirroring (formation of a bluish metallic sheen on the surface of the image) is also a result of oxidation-reduction, and is particularly characteristic of silver images suspended in a gelatin binder.

Binders

Various materials have been used as binders (in which the image-forming material is suspended) for photographic prints. Albumen (egg white) tends toward chemical reactions that cause it to yellow over time, which is exacerbated by the presence of residual fixative and mounting on poor-quality board. Albumen also becomes brittle and subject to cracking. Collodion (cellulose nitrate) is not flexible and becomes brittle, leading to cracks in the emulsion, but otherwise is fairly chemically stable and does not yellow. Gum arabic was used as a binder for gum bichromate prints, which are stable but rare because they were difficult to produce. Gelatin, the most common binder material, is made from animal proteins. It is fairly stable chemically but is quite sensitive to moisture in the air.

Supports

Nineteenth century prints generally had a primary support of high quality stable paper, although the secondary support was often poor quality (usually board was used for very thin prints, such as albumen prints). In the later 19th century, the primary paper support was often coated with barium sulfate in gelatin (or in collodion, for collodion printing-out paper); this was known as the baryta layer. This layer was very stable, protected the paper from light damage, and provided some protection against damaging substances from poor-quality secondary supports.

Resin-coated (actually polyethylene-coated) paper was introduced as a support for photographic prints during World War II and became commercially available during the 1970s. This type of paper shortened photographic processing time because it required less washing and it was sturdy, allowing processing to be completely mechanized. Most modern color prints are printed on resin-coated paper, which consists of a paper base between two layers of polyethylene, with titanium dioxide added to the emulsion-side layer.

Many early prints on resin-coated paper that were framed and exhibited developed cracking of the emulsion layer over time. Some black and white resin-coated prints on exhibit also developed redox, yellowing, and silver mirroring. These problems were due to the titanium dioxide in the emulsion (which facilitated the formation of an oxidizing agent on exposure to light), and they occurred most commonly in framed photographs where the oxidizing agent could not dissipate. Stabilizers and antioxidants were added to later resin-coated papers during manufacturing to combat this type of deterioration, but this does not completely eliminate the risk of damage.

Daguerreotypes, Ambrotypes, and Tintypes

Strictly speaking, these photographic images are not paper-based, but they are positive images and are frequently found within collections of photographic prints. Daguerreotypes, positive images on thin copper plate with a silver coating, were popular between 1840 and 1860. They are usually found in small hinged cases and are quite fragile and subject to tarnishing due to exposure to pollutants. Ambrotypes (popular in the late 1850s) and tintypes (produced throughout the late 19th century) are both silver images in a collodion binder, but ambrotypes have a glass support sometimes coated with paint or lacquer, while tintypes have a thin lacquered iron support. Ambrotypes sometimes suffer chips in their original coating material. Tintypes are easily bent, sometimes causing the emulsion to crack.

James M. Reilly's Care and Identification of 19th Century Photographic Prints, (see Additional Resources) provides more information on why prints deteriorate and how to identify photographic processes.