Expertise Photo: Photo Processes


EXPERTISE OF PHOTOGRAPHS DATING FROM 1839 TO 1939

Principal photographic processes

After the announcement in 1839 of the discovery by Jacques-Mandé Daguerre of the daguerreotype, a number of experiments, by artists and chemists around the globe, were undertaken to improve and refine the photographic process. Presented here are descriptions of the most successful photographic processes that were developed and used between 1839 and 1939; and thus produced the majority of vintage photographs found today.

Direct positive (single print)

These processes do not produce a negative that is used to create many copies of a print; the image is created directly on, or by using, the exposed plate (which is why these processes are called “single print” processes.)

Daguerreotype (1839 - 1850s): Was invented by Jacques Mandé Daguerre over the course of more than a decade and announced to the world at the French Académie des Sciences on January 7, 1839. The process consists of sensitizing a silver-plated copper sheet with iodine vapors. After exposure, the image is developed with heated mercury vapors and then fixed with sodium sulfate.
* An image of this type can be recognized because of its mirrored surface. If you hold the image at a certain angle, it should reflect your face.

Ambrotype (1850-1890s): Invented by Fredric Scott Archer in 1850, in this process, one side of a glass plate is covered with a thin layer of collodion. After exposure, the plate is developed and fixed, with the result being a negative. This negative could be used to produce Salt or Albumen prints (see next section for more details), but Archer discovered that the negative, if placed on a black background, could become the photo itself. The black background of an Ambrotype is often achieved by coating one side of the glass with a black varnish.
*The image can be mistaken for a daguerreotype, but it is lighter and the surface of the glass is not shiny. It is not uncommon for an Ambrotype to be hand-colored (as is the example presented here.)

Autochrome (1907 – 1930s): Process commercialized by the Lumière brothers in which green, red and blue granules of potato starch are mixed and then applied to a glass plate to form a filter. The plate is then coated with an emulsion sensitive to all colors. The non-coated side of the glass plate is exposed to the object being photographed and produces an image on the emulsion.
* When the developed autochrome is held up to the light, the colored granules are viewed in combination with the black-and-white image behind to create a color photograph.

Photographic prints (positives) obtained by using a negative

These processes are print processes; they discuss the types of photographic paper used to make prints from a negative.

Salt prints (1834 – 1860s; 1890s) : Invented in 1834 by William Henry Fox Talbot, this process allowed the creation of the first positive photographic prints. It consists of soaking a sheet of ordinary paper in a salt solution and then coating it with silver nitrate. The combination of these two elements produces a light-sensitive silver chloride. After exposure, the image is fixed by bathing it in a solution of ordinary table salt. The process was improved by Gustave Le Gray; he created numerous techniques to tone the images before fixing them. Salt prints were quite successful between 1840 and 1860 and were also used by photographers of the Pictorialist movement during the 1890s.
* The images have a matte surface and nuanced tints.

Albumen prints (1847 – 1890s) : Discovered in 1847 by Blanquart-Evrard and Nièpce de Saint-Victor, albumen prints replaced salt prints from 1855 on, and remained the most popular printing process until the 1890s. Paper covered with pure albumen and sodium chloride is bathed in silver nitrate to sensitize it. Once dry, the paper is placed under the negative and is exposed to light. The image that appeares during the exposure is tinted with gold chloride and then fixed with sodium thiosulfate.
* The albumen surface is slightly glossy and the whites in the image usually appear light yellow.

Gelatin-silver prints (1870 – today): Appearing in 1870, these papers are coated with gelatin containing light-sensitive silver salts. Industrially fabricated and ready for use, this process gradually replaced all other black-and-white photographic processes and is still in use today.
* The images are black-and-white, but, from time-to-time, have subtle coloration, such as a slight violet tint. They can be printed on paper ranging from matte to extremely shiny.

Cyanotype (1842-1880): This printing process, economical and easy to use, was invented by the English astronomer John Herschel in 1842. A sheet of ordinary paper is bathed in an iron salt solution and then dried. To obtain the image, the paper, with the negative on top, is placed in direct sunlight. After about 15 minutes, the iron salt solution exposed to the light begins to oxidize which causes it to turn blue. Rinsing under running water hastens the oxidation process and also washes away the non-exposed salts.
* The image is always various shades of cyan (a greenish blue), which is how the process got its name.

Negatives

These processes create a negative that can be used to create prints by utilizing one of the printing processes discussed above.


		Calotypes (1840 - 1850s): William Henry Fox Talbot discovered this process and used it to create the first negative in 1835. Over the following years, he improved the process and patented it in 1841. A sheet of paper is sensitized with silver nitrate and potassium idodide. After exposure, the negative is developed in silver nitrate and gallic acid, then fixed in sodium thiosulfate. The sheet of paper is often coated in wax, making the negative even more transparent. * Resembling a sheet of carbon paper, the image appears when held up to light. The fine layer of wax can be easily broken when folded.

		Collodion wet plates (1847-1870s): The fact that collodion could be used to produce photographic negatives was discovered in 1847. Collodion is a syrupy gel that is spread over a glass plate immediately before exposing the plate. Note: the negative produced here could be used to create an Ambrotype if desired. * The plates, stored in grooved wooden boxes, are often of differing thicknesses. When held to the light, the image appears dark brown.

		Gelatin dry plates (1880s-1920s): Industrially manufactured starting in 1871 but not seeing widespread use until the 1880s, these glass plates were treated with light-sensitive silver salts in gelatin; once the gelatin dried and hardened, the plates were ready for use and could be stored for months before exposure. They replaced collodion plates after 1880 * These glass plates, of equal thickness, are conserved in cardboard boxes bearing the name of the manufacturer. When held to the light, the image appears black and white.