Origins of digital effects

Digital editing offers the ability to add various digital processing effects to images, but we are often asked about the origins of some of these digital effects and how they stem from chemical reactions that occurred in analogue darkrooms.

There are many different methods, chemicals and dilutions that can be used for toning prints, and the best results generally come when they are used with fibre-based paper.

Perhaps the most obvious example of toning is the classic sepia tone. This darkroom process uses a bleach made of potassium ferricyanide to convert the silver in the print back to silver halide. The print is then washed to clean the potassium ferricyanide from the print and then sodium sulphide is used to tone the print by turning the silver halides to silver sulphate.

Other metals, including gold and even platinum, can also be used to tone prints. As well as giving images a different appearance, the aim of toning is to help preserve images for archival purposes. The most popular toner for this purpose is selenium, which adds a warm tint to prints.

You may also see cyanotype listed in digital image effects. Rather than being a type of toning, a cyanotype is actually a photographic print process. It was discovered by Sir John Herschel in 1842, and uses ammonium iron (III) citrate and potassium ferricyanide to create a solution that is sensitive to UV light. The resulting prints are blue, hence the ‘cyan’ reference in the name. It is also where the word ‘blueprint’ comes from.

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