好文一篇:Historic Photographic Processes

 

Historic Photographic Processes

The Albumen Print

Invented in 1850 by Louis Désiré Blanquart-Evrard, the albumen print became the dominant photographic printing process for nearly fifty years. Most of the albumen prints in the A. D. White Architectural Photographs Collection were produced between 1865 and 1895.

Technical definition:
First, a thin piece of paper is coated with an emulsion containing both egg white (albumen) and salt (usually sodium chloride). A subsequent immersion in a bath of silver nitrate renders the paper light-sensitive. The paper is next dried in the dark, then placed in a frame under a glass negative (most often, it was a glass negative with a collodion emulsion) and exposed in direct sunlight until the image achieves the proper level of darkness (from a few minutes to an hour, depending on light conditions). Albumen prints are thus
contact printed, or placed in direct contact with the negative. Since the image emerges as a direct result of exposure to light and without the aid of a developing solution, the albumen print is a Printed-Out (rather than Developed-Out) photograph. A bath of sodium thiosulfate then fixes the print’s exposure and prevents further darkening. Finally, gold toning improves the photograph’s tone and helps protect it from fading.

General Attributes
During the first stage of preparation, the viscous albumen coating fills in the pores of the paper and produces an even, slightly glossy surface. Because the albumen covers the paper fibers so smoothly, the process is particularly well suited to capturing fine detail. On very close examination, however, the surface may be covered with tiny fissures, as the albumen layer sometimes cracks as it dries. Although albumen prints are highly prone to fading, the general tone is yellowish, with cream-colored highlights and deep chocolate brown shadows. They can range from reddish-brown to purplish-blue.

Dates
1855-1920 (although most albumen prints in the A. D. White Architectural Photographs Collection were produced between 1865 and 1895)

Primary Characteristics of Albumen Prints

&S226; Silver print (photo-sensitive element is silver)
&S226; 2-layer structure (paper support & albumen layer)
&S226; Made primarily from glass plate negatives with collodion emulsion
&S226; Usually mounted on paper or board to prevent curling
&S226; Red-brown or purple image tone
&S226; cracking and yellowing of binder
&S226; Surface gloss
&S226; Cannot produce true black and white tones

The Gelatin Silver Print

In 1873, Peter Mawdsley invented the first photographic paper with a gelatin emulsion, and commercially-produced gelatin silver printing papers were available by 1885. Gelatin, an animal protein, is used as an emulsion, to bind light sensitive silver salts (usually silver bromides or silver chlorides) to a paper or other support. Unlike the albumen print, which is a printing-out process, the gelatin print is a developing-out process. After a brief exposure to a negative (under an enlarger), the print is immersed in chemicals to allow the image to develop, or emerge fully. Typically, the photographic materials in a gelatin silver print are extremely sensitive to light. Gelatin silver prints replaced albumen prints as the most popular photographic process by 1895 because they were much more stable, did not have a tendency to yellow, and were far easier to produce.

Dates
1885 to present (although most gelatin silver prints in the A. D. White Collection were produced between 1900 and 1930)

Primary Characteristics of Gelatin Silver Prints

&S226; Silver print (photo-sensitive element is silver)
&S226; 3-layer structure (support, gelatin binder and baryta layer)
&S226; Glossy, matte, or textured surface
&S226;
Silver mirroring common in dark areas and edges
&S226; May be toned
&S226; Fiber-based paper support or Plastic, resin-coated support
&S226; Paper fibers invisible
&S226; Can attain true black and white tones

The Cyanotype

Sir John Herschel invented the cyanotype process in 1842, rather early in the history of photography. While both albumen and silver gelatin prints rely on the light sensitivity of silver, cyanotypes are produced by light sensitive iron salts.

Technical definition:
A piece of paper is first sensitized with a solution of ferric ammonium citrate (an iron salt) and potassium ferricyanide (a crystalline iron salt) and dried. The prepared paper is then
contact printed, or placed in direct contact with the negative, and exposed to sunlight until an image begins to appear on the paper (usually about fifteen minutes). As contact prints, they are always the same size as their negatives. In the final step, the print is washed in water to oxidize the iron salts and draw out the cyanotype’s brilliant blue color.

General Attributes
The cyanotype is named for its rich blue-green hue, cyan. Cyanotype prints have no emulsion; the light-sensitive iron salts have been infused into the paper fibers, unlike either albumen or gelatin silver prints. Cyanotypes were far simpler and les expensive to produce, which made them a favorite method for turn-of-the-century amateurs who wanted to make proofs of their negatives. The architectural blueprint is a variation of this photographic process.

Dates
1842 to present (although most cyanotypes in the A. D. White Architectural Photographs Collection were produced between 1880 and 1900)

Primary Characteristics of Cyanotypes

&S226; Non-silver print (photo-sensitive element is iron)
&S226; Blue color
&S226; One-layer structure
&S226; Paper fibers visible
&S226; Matte surface

发表评论

电子邮件地址不会被公开。 必填项已用*标注