Silver halides make for the key component of the emulsion. Without them, no light sensitivity. These crystals are scattered in a gelatin substrate providing a buffered environment as well as stabilizing some chemical reactions. A cozy place of sorts.
If you want to understand the chemical reactions at work during the processing of a film without going back to your school bench:
We rapidly end-up with a small gathering of metallic Silver atoms which correspond to a development seed. All crystals in the emulsion containing a development seed, form together what is called the latent image.
From a light sensitive black and white film, how do we obtain a negative which is no more light sensitive and can be observed in broad daylight ?
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.
This is where the developer agent function comes handy: it amplifies the difference between exposed crystals and non exposed ones by transforming the entire exposed crystal into metallic Silver just from the seed presence. The development seed acts as catalyst of the transformation.
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
Alternatively (or as well), the negative may be scanned for digital printing or web viewing after adjustment, retouching, and/or manipulation.
During the second phase of the latent image forming process, the kicked out electron, which has too much energy to be captured by the Silver ion, goes on to roam around the entire crystal. And so until being trapped by an imperfection, a local defect in the crystal structure, or by an impurity in the form of a foreign atom or molecule.
First during the exposure i.e. when the shutter opens up to let a certain quantity of light or photons go through. It’s the moment when the latent image is being formed in the film emulsion. Then during the development when the latent image is “amplified” by the developing agent Lastly this developed latent image is fixed, it’s the fixing process, to become insensitive to light.
It is the well known negative obtained as a result of a film being entirely processed.
Light sensitive Silver Halides used in film photography are mostly based on Chlorine [Cl] and Bromide [Br]. Silver Bromide [AgBr] being used often, this is the one we will use our examples.
Jonathan Gazeley A journal of photography, computing and audio
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.
(*) This remains true if we don’t let the emulsion in contact with the fixer agent for too much time, because in the long run, eventually the fixer will also attack and dissolve metallic Silver. The control of chemical reactions timings during processing is therefore important in many respects.
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.
 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. Washing in clean water removes any remaining fixer. Residual fixer can corrode the silver image, leading to discolouration, staining and fading.
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. Colour processing
In the third step, this local accumulation of electrons generate a negative electric field that can attract towards itself the positive Silver ions not well set in the crystal structure. Each positive silver ion combining with an electron gives birth to a metallic Silver atom.
† 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.
Silver ion already reduced to metallic Silver are not affected by the action of the fixer agent (*). We are therefore only getting rid of the emulsion components that did not received light. The Film base being transparent, emulsion parts having not received any light appear as transparent also because they are now devoid of unexposed Silver Halide which have been washed out. Whereas parts that did received light are loaded with metallic Silver, hence appearing opaque.
In 3 steps: (1) Exposure of the light sensitive emulsion followed by its processing, which are (2) the action of the developer agent and (3) the fixer agent. I will present below the essential procedures that cannot be omitted. Along these 3 majors steps a lot of other intermediary processes of secondary importance can be carried out, but for the sake of clarity I will leave them out.
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.
References http://www.cheresources.com/photochem.shtml http://en.wikipedia.org/wiki/Photographic_developer http://en.wikipedia.org/wiki/Photographic_print_toning
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.
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.
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).
The importance of these imperfections is quite astounding as they are the key to the image forming process on film.
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.
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.
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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.
Clearly one needs to wait until the end of the fixing process before bringing the film in broad daylight.
This is the reason why the negative holds its name, and gives a representation of the scene by transmitted light, through its opaque parts matching the light parts of the scene, and through its transparent parts matching the dark parts of the scene. The negative is actually a real photograph. We prefer to inverse it during the printing process for obvious common esthetic reasons, but it just remains an act of preference…
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.
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.
The first step of the latent image forming process is the exposure. It’s the moment when a photon of light strikes a Silver Halide crystal. In doing so it kicks out the supplementary electron of the large Bromide ion.
In this last illustration above, we can observe that only the exposed parts are covered with metallic Silver. But also that the unexposed parts of the film have been thoroughly cleaned by the fixer, and contain no more Silver Halides, nor do they contain any metallic Silver which could not have been formed in those areas.
See also List of photographic processes Fogging Darkroom Cross processing Caffenol References
In the RA-4 process, the bleach and fix are combined. This is optional, and reduces the number of processing steps.
It’s worth noting the Bromide ion has a bigger volume than the Silver ion. And more importantly that real life crystals, made of well ordered stacks of Silver Bromide pairs, are not perfect stacks. They contain imperfections, cracks, shears but also impurities made of foreign atoms or molecules.
The thought that the developer agent acts only on the Silver Halide crystal containing a development seed is only true during a certain time. In reality if we leave the emulsion in contact with the developer agent long enough, it will transform all crystals in the emulsion. The developer acts more on the difference in the reduction speed of Silver contained in the exposed crystals versus Silver contained in the unexposed ones.
Then, the silver ion recombines with the free electron to give an atom of metallic silver.
At macroscopic scale this image is not sufficiently contrasted to be usable as is. After all there is very little difference between an exposed crystal that contains a development seed and an unexposed crystal that does not contain such a seed i.e. a crystal not struck by light (or a photon).
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.
The washing time can be reduced and the fixer more completely removed if a hypo clearing agent is used after the fixer.
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.
Now we need to make the emulsion non sensitive to light anymore. The metallic Silver is already desensitized, but the Silver Halide crystals which were not exposed (the one corresponding to the dark part of the scene) are still light sensitive. The most obvious thing to do is to get rid of those i.e. flush them out of the emulsion. This is the function of the fixer agent, it dissolves the Silver Halides into a solution. By this mean, now being soluble, they can be flushed out with the fixer agent.
More down to earth, let us take the example below, a simple dark scene: a lit desk lamp on a dark desk, in a dark room.
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.
Once the film exposed, the latent image is similar to the illustration below. The dark part of the scene matches the unexposed Silver Halides since no light struck them. The light part of the scene: the lamp, matches crystals dotted by a black point (representing the development seed) on the film emulsion.
A cut-away illustration of a typical light-trap tank used in small scale developing.
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.
Transparency films, except Kodachrome, are developed using the E-6 process, which has the following stages:
External links 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.
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.
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. Ilfochrome materials use the dye destruction process.
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.
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.
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.
The personal reason that initially pushed me to understand the chemical reactions occurring all along the processing of a photographic film was to know at which stage I could open my developing tank. Or in other words at which stage I could expose my processed film to light without deteriorating the image.
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.
First, let us look at a typical photographic film cross section in the following illustration (not to scale):
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.
Before processing, the film must be removed from the camera and from its cassette, spool or holder in a light-proof room or container.
Then after several photons have stricken the crystal and kicked out electrons of their Bromide ion, an accumulation of those electrons is created around this local imperfection or impurity.
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.
Then, the latent image is kind of intensified by the developer agent action and all exposed Silver Halide crystals will be entirely transformed into metallic Silver. The latent image is theretofore revealed or developed and its macroscopic density is now sufficient to be of some use.
Once the film is processed, it is then referred to as a negative.
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.
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All photographic processing use a series of chemical baths. Processing, especially the development stages, requires very close control of temperature, agitation and time.
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.
Let us see how the latent image is formed in the following series of illustrations. The admitted theory reckons 3 steps in the forming process of this very distinctive latent image.
Tags: Bath, Chemistry, developer, fix, Photography, sepia, stop, tonerCategories: Photography, Science