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Black And White Photography Chemistry

Black And White Photography Chemistry Black And White Photography Chemistry

The light-sensitive film (or paper) contains crystals of silver halide, which is light sensitive. At this point, the film is opaque grey. When light hits the film, the silver halide crystal splits into a silver ion and a bromine atom.

The purpose of developer is to amplify the latent image. The chemical composition of developer varies and is complicated so I won’t go into it here – other than to say that it promotes silver crystal growth where the small silver crystals already exist.

† In modern automatic processing machines, the stop bath is replaced by mechanical squeegee or pinching rollers. These treatments remove much of the carried-over alkaline developer, and the acid, when used, neutralizes the alkalinity to reduce the contamination of the fixing bath with the developer.

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The washing time can be reduced and the fixer more completely removed if a hypo clearing agent is used after the fixer.

Chromogenic materials use dye couplers to form colour images. Modern colour negative film is developed with the C-41 process and colour negative print materials with the RA-4 process. These processes are very similar, with differences in the first chemical developer.

Film may be rinsed in a dilute solution of a non-ionic wetting agent to assist uniform drying, which eliminates drying marks caused by hard water. (In very hard water areas, a pre-rinse in distilled water may be required – otherwise the final rinse wetting agent can cause residual ionic calcium on the film to drop out of solution, causing spotting on the negative.

) Film is then dried in a dust-free environment, cut and placed into protective sleeves.

In selenium toning, the image silver is changed to silver selenide; in sepia toning, the image is converted to silver sulphide. These chemicals are more resistant to atmospheric oxidising agents than silver.

Even after taking the film out of the developing solution, it continues to develop (your hands are still wet after taking them out of the sink, right?) so a stop bath is used to halt development. Developing requires an alkaline environment to work, so stop bath is simply a weak acid – usually acetic acid.

The negative may now be printed; the negative is placed in an enlarger and projected onto a sheet of photographic paper. Many different techniques can be used during the enlargement process. Two examples of enlargement techniques are dodging and burning.

After development, the latent image has been converted to an actual image, made of metallic silver crystals. It appears black, although the film itself is still opaque.

External links[edit] Kodak Processing manuals The Massive Dev Chart – film development times The Comprehensive Development Times Chart – Manufacturer’s film development times database Ilford guide to processing black & white film

So far, we have ended up with either a film or a print which has an image made from metallic silver. If there’s one thing we know about silver, it’s that it tarnishes. Depending on the storage conditions, silver prints may degrade with time. Toning the image serves two purposes: it improves the longevity of the image, and it can produce the colourful sepia effects.

Sheet films can be processed in trays, in hangers (which are used in deep tanks), or rotary processing drums. Each sheet can be developed individually for special requirements. Stand development, long development in dilute developer without agitation, is occasionally used.

Key stages in production of Ag-based photographs. Two silver halide particles, one of which is impinged with light (hν) resulting in the formation of a latent image (step 1). The latent image is amplified using photographic developers, converting the silver halide crystal to an opaque particle of silver metal (step 2).

Finally, the remaining silver halide is removed by fixing (step 3).

Black and white negative processing is the chemical means by which photographic film and paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light.

In commercial processing, the film is removed automatically or by an operator handling the film in a light proof bag from which it is fed into the processing machine. The processing machinery is generally run on a continuous basis with films spliced together in a continuous line. All the processing steps are carried out within a single processing machine with automatically controlled time, temperature and solution replenishment rate. The film or prints emerge washed and dry and ready to be cut by hand. Some modern machines also cut films and prints automatically, sometimes resulting in negatives cut across the middle of the frame where the space between frames is very thin or the frame edge is indistinct, as in an image taken in low light.

Tags: Bath, Chemistry, developer, fix, Photography, sepia, stop, tonerCategories: Photography, Science

Once the film is processed, it is then referred to as a negative.

All photographic processing use a series of chemical baths. Processing, especially the development stages, requires very close control of temperature, agitation and time.

Then, the silver ion recombines with the free electron to give an atom of metallic silver.

Transparency films, except Kodachrome, are developed using the E-6 process, which has the following stages:

Jonathan Gazeley A journal of photography, computing and audio

References http://www.cheresources.com/photochem.shtml http://en.wikipedia.org/wiki/Photographic_developer http://en.wikipedia.org/wiki/Photographic_print_toning

Photographic processing or development is the chemical means by which photographic film or paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light.[1]

The film may be soaked in water to swell the gelatin layer, facilitating the action of the subsequent chemical treatments. The developer converts the latent image to macroscopic particles of metallic silver.

[3] A stop bath,† typically a dilute solution of acetic acid or citric acid, halts the action of the developer. A rinse with clean water may be substituted. The fixer makes the image permanent and light-resistant by dissolving remaining silver halide.

A common fixer is hypo, specifically ammonium thiosulfate.[4] Washing in clean water removes any remaining fixer. Residual fixer can corrode the silver image, leading to discolouration, staining and fading.


In some old processes, the film emulsion was hardened during the process, typically before the bleach. Such a hardening bath often used aldehydes, such as formaldehyde and glutaraldehyde. In modern processing, these hardening steps are unnecessary because the film emulsion is sufficiently hardened to withstand the processing chemicals.

Before processing, the film must be removed from the camera and from its cassette, spool or holder in a light-proof room or container.

All processes based upon the gelatin-silver process are similar, regardless of the film or paper’s manufacturer. Exceptional variations include instant films such as those made by Polaroid and thermally developed films. Kodachrome required Kodak’s proprietary K-14 process. Kodachrome film production ceased in 2009, and K-14 processing is no longer available as of December 30, 2010.[2] Ilfochrome materials use the dye destruction process.

Alternatively (or as well), the negative may be scanned for digital printing or web viewing after adjustment, retouching, and/or manipulation.

If colour negative film is processed in conventional black and white developer, and fixed and then bleached with a bath containing hydrochloric acid and potassium dichromate solution, the resultant film, once exposed to light, can be redeveloped in colour developer to produce an unusual pastel colour effect.[citation needed]

1 Common processes 1.1 Black and white negative processing 1.2 Black and white reversal processing 1.3 Colour processing 2 Further processing 3 Processing apparatus 3.1 Small scale processing 3.2 Commercial processing 4 See also 5 References 6 External links

Following the stop bath, the film is bleached to remove the developed negative image. The film then contains a latent positive image formed from unexposed and undeveloped silver halide salts. The film is fogged, either chemically or by exposure to light.

The remaining silver halide salts are developed in the second developer, converting them into a positive image. Finally, the film is fixed, washed, dried and cut.[6] Colour processing[edit]

A black and white developer develops the silver in each image layer. Development is stopped with a rinse or a stop bath. The film is fogged in the reversal step. The fogged silver halides are developed and oxidized developing agents couple with the dye couplers in each layer.

The film is bleached, fixed, stabilised and dried as described above.[7]

See also[edit] List of photographic processes Fogging Darkroom Cross processing Caffenol References[edit]

Wall, E.J. (1890). Dictionary of Photography. London: Hassel, Watson and Viney Ltd.  The British Journal (1956). Photographic Almanac. London: Henry Greenwood and Co Ltd. 

Although we have now developed the film and ended up with a black image in metallic silver, the areas of the film that were not exposed to light are still opaque, and still sensitive to light. Bathing the film in fixer dissolves the unexposed silver halide, leaving a near-transparent film backing that is not sensitive to light. At this stage, you can take the film out of the developing tank and look at it in daylight.

I’ve been practising traditional silver-based black & white photography for a couple of years but today it occurred to me that I don’t really know what is going on with the various chemicals. It’s just a process of remembering which bottle is which. I had a vague idea of what was going on, but I decided to look it up – and summarise it here.

After exposure, there is an image on the film made from a tiny quantity of metallic silver. This is known as the latent image. It would be invisible to the eye and the film is still dull and opaque. For an individual grain of the silver halide emulsion to count as “exposed”, at least two photons must have interacted with it, to form small silver crystals consisting of two or more silver atoms.

In the RA-4 process, the bleach and fix are combined. This is optional, and reduces the number of processing steps.[8]

The colour developer develops the silver negative image, and byproducts activate the dye couplers to form the colour dyes in each emulsion layer. A rehalogenising bleach converts the developed silver image into silver halides.

A fixer removes the silver salts. The film is washed, stabilised, dried and cut.[7]

Various toners exist, but they all work in the same way. They react with the silver to produce silver salts, such as silver sulphide which is more stable then pure silver. It is also slightly brown in colour, hence the sepia tone.

In amateur processing, the film is removed from the camera and wound onto a reel in complete darkness (usually inside a darkroom with the safelight turned off or a lightproof bag with arm holes). The reel holds the film in a spiral shape, with space between each successive loop so the chemicals may flow freely across the film’s surfaces. The reel is placed in a specially designed light-proof tank (called daylight processing tank or a light-trap tank) where it is retained until final washing is complete.

A cut-away illustration of a typical light-trap tank used in small scale developing.

Black and white emulsions both negative and positive, may be further processed. The image silver may be reacted with elements such as selenium or sulphur to increase image permanence and for aesthetic reasons. This process is known as toning.

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