These are the photographic processes used in the All Our Yesterdays selection of photographs. The first 100 years of chemical photography (1839-1939) boast a wide range of processes, of a startling morphological diversity. This was a period of continuous technical revolutions and new achievements, in which the counterpart to the technology – physical-chemical principles – nevertheless remained essential.
Of the hundreds of positive photographic processes available in this era, only few may be regarded as common practice. In this exhibition, the most often used among those are well represented:
- Albumen print (1850-1900) The predominant printing process of the 19th century. Through the photographic process of albumen print, a positive image is printed on paper. The emulsion holding the silver salts is the albumen, obtained from beaten egg whites applied on paper. Albumen prints were always adhered to a secondary support, usually cardboard.
- Collodion POP (1880-1920) Collodion photographic paper, also known as aristotype collodion, is coated with a baryta layer, to which a collodion emulsion is adhered. Collodion-based photographic papers belong to the so-called ‘printing out papers’ (POP): images are obtained by exposure to light, without requiring liquid solutions for development.
- Silver gelatin POP (1882-1930) Silver gelatin photographic paper, or aristotype gelatin, is coated with a baryta layer and paper. On this paper, a gelatin emulsion is adhered. Gelatin-based photographic papers range among the POP’s as well.
- Silver gelatin DOP (1885- 2000) The most important paper printing photographic process of the 20th century. The image is obtained by exposing a photographic paper for a short while under a negative in a copy frame or using an enlarger. Opposite to the POP gelatin papers, DOP silver gelatin requires chemical development of the latent image to obtain a photograph.
Some of the prints in this exhibition originate from a digitized negative. Negatives are manifold in our collections, especially those produced industrially and using – from 1880 onwards – dry plates. The two main processes in this period were wet collodion and silver gelatin.
- Wet collodion process (1851-1880) An artisanal process used until 1880, consisting of the application of a solution of cellulose nitrate in alcohol and ether to glass plates, in order to let them hold the silver salts. This procedure allowed for a significant reduction of exposure time and was widely used for 30 years for obtaining negatives. The main drawback was, that plates had to be prepared just before taking a photograph, and were to be developed immediately afterwards, when still wet. Therefore, outside photography required packing up a tent and all the necessary products to mix the emulsion and develop the image on location.
- Silver gelatin (1871-today) It is the negative – or gelatin dry plate – that stimulated instantaneous photography and mastered the market during the 20th century. In its early days, it consisted of a cadmium bromide solution, water and gelatine sensitized with silver nitrate and stretched over a glass plate. Later was discovered that heating the emulsion increased sensitivity and reduced the exposure time, even allowing for snapshots and images of moving subjects.
It is an early photographic process based on the additive colour system. Patented by the Lumière brothers in 1904, it essentially involved covering a glass plate with a coloured layer with equal parts of starch grains dyed red-orange, blue-violet and green. This process was in use until 1930, with a plastic base substituting the earlier glass carrier in the final stage.
The inclusion of photographic images on slides in the final third of the 19th century, gave a new impulse to projections with the magic lantern – the most popular, widespread, long-lasting, creative and versatile instrument for visual communication before cinema. The projection of photographs, in spite of being static, considerably increased the realism of the imagery. In some cases the images were coloured by hand, highlighting even stronger the close tie between picture and reality.