DIFFERENCES IN IMAGE TONALITY PRODUCED BY DIFFERENT TONING PROTOCOLS FOR MATTE COLLODION PHOTOGRAPHS
2 BRIEF HISTORY OF PRINTING PROCESSES
The first photographic printing paper, known as salted paper, was introduced in 1839 by William Henry Fox Talbot. It consisted of a sheet of paper soaked in a dilute solution of table salt and sensitized with a strong silver nitrate solution. The sensitized paper darkened rapidly when exposed to daylight, and the image appeared spontaneously without requiring chemical development. When exposure was judged sufficient, unexposed light-sensitive silver chlorides were eliminated in a fixing bath of sodium thiosulfate, or “hypo,” followed by a final rinse in water. Papers of this type are known as printing out papers (POPs) as opposed to developing-out papers (DOPs), which require chemical reduction of the silver salts to silver metal for the image to appear. The image of POPs is composed of photolytic silver, which literally means “separated by light.” Silver images occur in three basic structural forms: photolytic silver, physically developed silver, and filamentary silver. “Photolytic silver has the smallest particle size and produces red or brown image color. Size is directly proportional to the amount of light received during exposure” (Reilly 1986, 15). If not well processed or if exposed to an unfavorable environment, salted papers would fade and lose highlight detail over time.
In 1850, the French photographer Louis Désiré Blanquart-Evrard introduced a new printing material, the albumen paper that had a slightly glossy surface and provided images with much greater density range and contrast than the salted paper. Albumen paper was prepared by floating a sheet of paper on the surface of a beaten egg white solution containing ammonium chloride before sensitizing with silver nitrate. Later, the desire for more detail and greater sheen brought about double-coated paper with a heavy gloss. By the turn of the century, public taste had swung to the other extreme, and “matte” or even “rough” albumen papers were offered (Photo-Miniature 1907, 265). Albumen prints were processed in the same way as salted papers. The heretofore reddish brown hue of the image, which was considered inartistic and displeasing to the eye, could be altered to a more agreeable brown or dark purple by the use of a gold toner. Gold toning, or gilding, was introduced to photography in 1841 by Hippolyte Fizeau to improve the image contrast and stability of daguerreotypes (Fizeau 1841). The technique was adopted for paper prints in 1847 (Reilly 1986, 5). During the process of toning silver prints, some of the silver atoms are replaced with atoms of the toning metal that become distributed within the crystalline structure of the silver. The change in image color is a function of the alteration of particles' shape, size, and composition (Reilly 1986, 23). Toning also improves the image stability because silver alloys are much more resistant to oxidative-reductive deterioration than silver alone. This deterioration occurs when substances that oxidize the image silver (such as hydrogen peroxide, thiourea, and others) convert the metallic silver atoms to silver ions. The ions migrate away from the original site of the silver grains and are eventually reduced back to metallic, elemental silver. Albumen paper rapidly became the standard printing support used by photographers, but it was soon noticed that albumen prints could be unstable, showing a tendency to fade and discolor within a few years. The search for a new medium that would have the same desirable properties of albumen, without its instability, became the aim of the efforts of many photographers.
Carbon printing, introduced by Alphonse Poitevin in 1855, seemed to possess all the stability requirements. The light-sensitive media consist of a layer of dichromated gelatin containing a pigment. The dichromated colloid, selectively hardened by light through the negative, is then washed in warm water, where the unhardened areas dissolve away, leaving a positive image of pigmented gelatin. Carbon prints are very stable, and any color of pigment can be used. Although the process was improved successively by A. Fargier in 1860 and Joseph Swan in 1864, it was still delicate and costly, with many manipulations, and never became very popular for commercial portraiture (Nadeau 1982, 6).
The platinum print, or platinotype, came into use around 1880. The process was patented and introduced to the market by the Englishman Williams Willis in 1873 (Willis 1873). It consists of coating a sized sheet of paper with a light-sensitive solution of iron and platinum salts. After exposure to light, the paper is developed in a bath of potassium oxalate and then immersed in an acid bath that eliminates residual light-sensitive iron compounds from the paper. The final image is composed of metallic platinum. Platinum prints have a matte surface and a soft gray-black color, and are praised for their wide tonal range. However, inflation in the price of the metal imported from Russia starting in the 1880s drove the price so high that platinum paper cost about three times more than other papers (Aristo Eagle 1906). Platinotypes became too costly for general commercial use.
The pre-eminence of albumen paper ultimately concluded with the arrival of emulsion papers, that is, papers precoated with a sensitized layer of collodion or gelatin binder containing suspensions of light-sensitive silver salts. These papers were industrially produced, which improved their quality and uniformity (fig. 1). They were also more sensitive than albumen paper and provided finer detail. Two types of light-sensitive salts were used in the emulsions: silver chloride and/or silver bromide. Collodion and gelatin chloride POPs became the dominant material from the late 1880s to the late 1910s. They were offered in a wide range of surfaces, image colors, and contrasts and did not share albumen's tendency to yellow (Reilly 1986, 10). Another type of silver chloride paper, known as “gaslight” or development paper, was introduced to the market by the Nepera Chemical Co. in 1893, under the name of Velox. The paper, coated with a gelatin silver chloride emulsion, was sensitive enough to be printed under artificial light (gaslight). The image was developed chemically (Nadeau 1989, 114).
The development of continuous-roll coating machinery for photographic papers lowered the cost of the newer papers and improved their quality and uniformity. Consistency in the results was one of the marketing argument of the industry. Advertising published in Aristo Eagle, 1908.
Gelatin silver bromide paper was introduced at the same time as bromide dry-plate negative. More sensitive than chloride papers, it could be printed with a very short exposure to daylight or with any artificial light. The image was brought out through development. Bromide paper was essentially used for enlargements. Although it was commercially available in the mid-1880s, it did not become popular until much later. Commercial photographers were used to controlling the exposure time visually, as the image was appearing to the printing frame, and to working the paper throughout in daylight. They found the trial-and-error method for bromide DOPs cost them time and money; the paper was more expensive and did not produce the wide range of tones that were possible with chloride POPs. However, bromide paper eventually became the dominant printing material and is still in use today.