black-and-white photographic processes

These can be divided into those that produce camera originals, including negatives, and those that produce copies. The overwhelming dominance of silver halide in both areas can lead to unwarranted neglect of alternative processes—though the familiar negative-positive process, established in its essentials by Henry Talbot in the early 1830s, has more twists and turns than many realize.

By far the most common black-and-white process involves the formation, through the influence of light, of a latent image in a silver halide (chloride, bromide, or iodide) ‘emulsion’. To be accurate, the ‘emulsion’ is a suspension of silver halide in gelatin, though the term ‘emulsion’ is too well entrenched to be overthrown. Instead of gelatin, collodion can be used, and indeed was used in the past; but the disadvantage is that the material must be exposed while still wet, hence the term ‘wet-plate’ photography. It is also worth knowing that in photographic papers, though far less in films, natural gelatin (derived from bones and hides) is supplemented by synthetic latexes: supplemented rather than supplanted, because organic constituents of gelatin (especially sulphur compounds) contribute greatly to the sensitivity of the ‘emulsion’.

The latent image is then developed by a reducing agent that reduces silver halide to metallic silver in direct proportion to the intensity of the latent image. The very earliest processes obtained an image without development via printing out, where the influence of the light alone produced the image: a technique still used with ‘printing-out paper’ (POP). The exposure required to give a ‘printed-out’ image is however several orders of magnitude greater than that required to give a latent image for development: up to about a million times greater. Unconverted silver halide is then dissolved out of the emulsion (fixing). A final wash removes anything in the emulsion that poses a threat to the stability of the image: by-products of processing, and the fixer itself. The result is the familiar negative, in which tones are reversed: the brightest parts of the original scene are the darkest parts of the negative.

A variant on the negative process is chromogenic development, so called because it works in a similar way to colour films. Dye precursors are incorporated in the emulsion, and are converted during processing to dyes in direct proportion to the intensity of the halide image. The halide image can then be bleached away, leaving an extremely fine-grained dye image which will not, however, be as sharp as a halide image.

In practice, a ‘negative’ can be made to appear ‘positive’ by at least two simple tricks. One is to make a conventional negative, but lack it with black varnish or even black paper: this is the ‘ambrotype’ or ‘melainotype’ devised by Frederick Scott Archer and Peter W. Fry in 1851. The other is to coat the sensitive medium on to a shiny black surface: traditionally, japanned sheet iron (hence ‘ferrotype’) or ‘tinplate’ (hence ‘tintype’), a process invented by A. A. Martin in 1852. Both ambrotypes and tintypes were used for rapid, low-cost portraits, affordable by all but the poorest. Often poorly fixed, many are badly faded today, some even to invisibility.

The daguerreotype (1835-7) is another direct-positive silver halide process. Here, the halide is formed directly on a thin sheet of polished silver, normally plated or affixed to a copper support. The ‘developer’ is mercury vapour, and the image is seen against the mirror-like background of the plate. The disadvantages of the daguerreotype are that it is slow and expensive; that each image is unique (and laterally reversed); that the mercury vapour used for processing is toxic; and that the image is very fragile indeed, and can be destroyed with a fingertip. This explains its rarity and appeal to collectors.

Other direct-positive processes are mostly clever adaptations of the halide negative-positive process. For direct reversal materials (normally black-and-white slides) the negative is formed in the usual way, then bleached out. This leaves unexposed silver halide in inverse proportion to the original negative. This is then re-exposed to light and developed (or it can be chemically fogged) to create a positive image. Traditional peel-apart Polaroid materials actually form a negative, which is then transferred by chemical diffusion to the positive, after which the negative is discarded or (in some films) recovered: ‘integral’ materials form transient images en route to the final negative, but again work essentially by dye diffusion.

Armed with a camera original negative, there are numerous ways to make copies. Overwhelmingly the most popular is a simple repetition of the silver halide process, so that the negative is again reversed and becomes a positive: this is the principle of what used to be called developing-out papers, though similar emulsions can be coated on glass to give positive lantern slides. Silver halide printing-out papers also have a small but loyal following: the extremely low sensitivity is far less of a disadvantage at the printing stage than at the camera stage. Most silver halide papers use a gelatin ‘emulsion’ that is similar in many ways to a film ‘emulsion’, though typically a couple of orders of magnitude slower (some are slower still), but other supports have been used: salted-paper printing papers are sensitized by forming silver halide on the surface of, and in the fibres of, good-quality writing paper, while albumen prints used egg white. This held more silver and therefore gave a more vigorous and contrasty image. Albumen papers were devised by L. D. Blanquart-Évrard in 1850; in 1851 he started developing prints, instead of printing them out.

The paper ‘emulsion’ is normally coated on top of a stratum of whitener to increase the brightness of the highlights, and therefore the overall brightness range of the print. In traditional fibre-based papers the normal brightener layer is baryta (barium sulphate) but with resin-coated papers titanium oxide is used.

Both alternative and early silver-based processes like daguerrotypy enjoyed a considerable revival in the later 20th century.

— Roger W. Hicks

Bibliography

• Crawford, W., The Keepers of Light: A History and Working Guide to Early Photographic Processes (1979).
• Hicks, R., and Schultz, F., The Black and White Handbook: The Ultimate Guide to Monochrome Techniques (1997).
• Jacobson, R. E. (ed.), The Manual of Photography (9th edn. 2000)