sea/son ... the SEASON

,,, thousands of projects in many contexts,

In these Easter cleaning between books magazines and papers photographs would

deepen a technique photography much used but little known ...

" Silver gelatin DOP "

Silver gelatin DOP is based on the light sensitivity of silver halides, which are suspended in a gelatin binder on a baryta paper support. DOPs made their first appearance in the mid 1880s and became the dominate printing process of the twentieth century. Due to the complexity of the process, silver gelatin papers have always been a manufactured product. As chemists’ understanding of silver halide chemistry increased over time, papers could be manufactured with a variety of characteristics, including varying light sensitivities, speeds, and tonal ranges. Papers were also produced with a variety of surface sheens, textures, paper thickness and paper tints. Optical brightening agents were introduced to papers beginning in the 1960s, which serve to make the highlights brighter providing papers with more contrast.

"Silver gelatin DOPs have a paper support. Most prints made before 1970 have fiber-based paper supports of varying paper weights. The standard weights are described by manufacturers in different ways including: heavy, medium, and light weight; single-weight, double weight, lightweight, extra thin. Water-resistant papers coated with cellulose acetate were made before the late 1960s. These papers were used in photo booths, as ID pictures, and for printing map reproductions, among other things that required fast processing. In the late 1960s papers coated with polyethylene, called resin-coated (RC) papers, were introduced. RC papers are coated on the back with clear polyethylene and on the front, directly below the emulsion, with pigmented polyethylene and are easily identified by their smooth plastic back. Specialty metallic papers were marketed until the 1970s. These have a special surface that give the print a metallic appearance". 

Significant advancements were made throughout the twentieth century to silver gelatin emulsion making for both negative and print materials. During manufacturing of the emulsion, silver nitrate is combined with a halide (usually a combination of bromide and chloride, though silver iodide papers were also made) in the presence of gelatin with an excess of halide present. The gelatin slows the formation of the crystals allowing for smaller, more uniform crystals to form and react with impurities (sulfur) in the gelatin, all of which make the silver halide crystals more light sensitive. The emulsion is then heated in a process called Ostwald Ripening which increases silver halide sensitivity and creates more uniform silver halide crystals. The next step is to remove impurities from the gelatin. Initially emulsions were shredded into noodles, washed to remove impurities, and then heated and re-melted. Later emulsion making required a complicated procedure of flocculation which required altering the pH of the emulsion causing the silver halides to precipitate out. The silver halides were washed and dispersed back into the emulsion. Another way of extracting impurities is through reverse osmosis through a thin membrane. Finally additional sensitizing chemicals and other additives are added. Gelatin is the perfect binder for silver halide crystals; it has the ability to swell allowing the penetration of processing solutions, but is tough and resistant to abrasions when dry. The gelatin emulsion was then machine coated onto a baryta paper support.

 A final thin layer of hardened gelatin was applied to act as a protective layer called an overcoat, also called a supercoat or topcoat.

DOPs can be contact printed or printed by enlargement through a negative. During exposure, a latent image is formed where light strikes the paper. Development reduces the silver ions in the latent image to visible silver particles in an oxidation-reduction reaction. Development is followed by a stop bath, which halts development and keeps the following fixing bath from being contaminated with developing solution. Next, unexposed silver halides are removed in a fixing solution, usually sodium thiosulfate, which dissolves silver halide crystals into a water soluble compound. Finally the print is washed thoroughly to remove residual processing chemicals and by products produced during fixing.

Photographers may choose to tone prints to alter the image color and/or to increase the stability of the print. Popular toners include gold, polysulfides, and selenium or a combination of sulfide and selenium.  Gold toning replaces part of the silver image with a more noble metal (gold). Gold toning usually produces a cooler neutral image tone of blue-black. Selenium and sulfide toners create a compound with silver that is more stable than silver alone. Image tones generally range from sepia, brown, purple, and purple-brown. This can be done by indirect toning in which after fixing the silver image is bleached and then immersed in the toning solution. The sulfide solution reacts with the silver halides to form silver sulfide. Direct toning does not require bleaching. Dye toning converts the silver image to a dye mordant that attracts dye from a dye solution. Finally, metal ferricyanide toning converts the silver image into silver ferricyanide complex which is then converted to a ferricyanide salt of a different metal (iron, copper, uranium). Dye toning and metal ferricyanide toning can result in a diverse rainbow of image colors.