Photographic developer
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In film developing, photographic developer (or just developer) is a chemical that makes the latent image on the film or print visible. It does this by reducing the silver halides that have been exposed to light to metals of elemental silver in the gelatine matrix. As a generalisation, the longer a developer is allowed to work, the greater the degree of reduction of the silver halide crystals to silver and therefore the darker the image.
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[edit] Formulae
For black and white photography, the developer is often a mixture of Metol, Phenidone or Dimezone and hydroquinone. These are made up in aqueous solution with a suitable alkaline agent such as sodium carbonate, borax, etc. to create the appropriately high pH and with sodium sulfite to delay oxidation of the developing agents by atmospheric oxygen. Hydroquinone is superadditive with metol, meaning that it acts to "recharge" the metol after it has been oxidised in the process of reducing silver in the emulsion. Sulfite in a developer not only acts to prevent aerial oxidation of the developing agents in solution, it also facilitates the regeneration of metol by hydroquinone (reducing compensation and adjacency effects) and in high enough concentrations acts as a silver halide solvent.
Most developers also contain small amounts of potassium bromide to modify and restrain the action of the developer to suppress chemical fogging. Developers for high contrast work have higher concentrations of hydroquinone and lower concentrations of metol and tend to use strong alkalis such as sodium hydroxide to push the pH up to around pH 11 to 12.
Because Metol is difficult to dissolve in solutions of high salt content, instructions for mixing developer formulae almost always list Metol first. It is important to dissolve chemicals in the order in which they are listed. Some photographers add a pinch of sodium sulfite before dissolving the Metol to prevent oxidation, but large amounts of sulfite in solution will make it very slow for Metol to dissolve.
Because Metol is relatively toxic and can cause skin sensitisation, modern commercial developers often use Phenidone or Dimezone S instead. Hydroquinone can also be toxic to the human operator as well as environment; some modern developers replace it with ascorbic acid, or vitamin C. This, however, suffers from poor stability. Ascorbate developers may have the advantage of being compensating and sharpness-enhancing, as oxidation by-products formed during development are acidic, meaning they retard development in and adjacent to areas of high activity. This also explains why ascorbate developers have poor keeping properties, as oxidised ascorbate is both ineffective as a developing agent and lowers the pH of the solution, making the remaining developing agents less active. Recently, practical methods to improve the stability of ascorbate developers have been worked out by Ryuji Suzuki. Some of his formulae are published on his website [1]], and the latest formulae are available in ready-to-use products from Digitaltruth Photo ([2]).
Other developing agents in use are p-aminophenol, glycin (N-(4-hydroxyphenyl)glycine), pyrogallol and catechol. When used in low sulfite developer composition, the latter two compounds cause gelatin to harden and stain in the vicinity of developing grains. Generally, the optical density of the stain increases in the heavily exposed (and heavily developed) area. This is a property that is highly sought after by some photographers because it increases negative contrast in relation to density, meaning that highlight detail can be captured without "blocking" (reaching high enough density that detail and tonality are severely compromised). Hydroquinone shares this property. However, the staining effect only appears in solutions with very little sulfite, and most hydroquinone developers contain substantial quantities of sulfite.
In the early days of photography, a wide range of developing agents were used, including chlorohydroquinone, ferrous oxalate, hydroxylamine, ferrous lactate, ferrous citrate, Eikonogen, atchecin, antipyrin , acetanilid and Amidol (which unusually required midly acidic conditions).
Developers also contain water softening agent to prevent calcium scum formation (e.g., EDTA salts, sodium tripolyphosphate, NTA salts, etc.).
Modern lith developers contain hydrazine compounds, tetrazolium compounds and other amine contrast boosters to increase contrast without relying on the classic hydroquinone-only lith developer formulation. The modern formulae are very similar to rapid access developers (except for those additives) and therefore they enjoy long tray life. However, classic lith developers using hydroquinone alone suffers very poor tray life and inconsistent results.
[edit] Development
The developer reduces the silver halides in the latent image in the exposed photograph into reduced, opaque, black silver metal. The image is then fixed using photographic fixer.
The mechanism by which this reduction occurs preferentially on those halide grains containing the silver atoms of the latent image is complex. The developer molecule (typical a relatively simple benzene-ring molecule) may act as an 'electron bridge'. (See www.kodak.com for more detailed discussion).
The time over which development takes place, and the type of developer, affect the relationship between the density of silver in the developed image and the quantity of light. This study is called sensitometry and was pioneered by F Hurter & V C Driffield in the late 1800s.
[edit] Colour development
In colour and chromogenic black and white photography, a similar development process is used except that the reducing silver catalyses the production of dye-stuffs in the emulsion. There are three distinct processes used here. The C-41 process is used for almost all colour negative films and in this process dye couplers in the emulsion react with dye formers in the developer to generate the visible dyes. An almost identical process is then used to produce colour prints from films. The developing agents used are derivates of paraphenylene diamine.
[edit] Reversal film development
In many colour transparencies, the film is first processed in an MQ (Metol-Hydroquinone) black and white developer for 6:00 at 100.4°F (38°C), with more time yielding "push" processing to increase the apparent film speed. The first developer is the most critical step in Process E-6. The solution is essentially a black-and-white film developer, because it forms only a negative silver image in each layer of the film; no dye images are formed. Then, the film goes directly into the first wash for 2:00 at 100°F, which acts as a controlled stop bath. Next, the film goes into the reversal bath. This step prepares the film for the colour-developer step. In the reversal bath, a chemical reversal agent is absorbed into the emulsion. No chemical reaction takes place until the film enters the colour developer. The reversal process can also be carried out using 800 footcandle-seconds of light, which is used by engineers to troubleshoot reversal bath problems.
Next, the film is developed to completion in a colour developer bath. When film enters the colour developer, the reversal agent absorbed by the emulsion in the reversal bath chemically exposes the unexposed silver halide. The colour developing agent then reacts with the silver halide to form metallic silver. (The metallic silver image formed in the first developer is not a part of the reaction that takes place in this step.) The colour developer acts on the chemically exposed silver halide to form a positive silver image. As the metallic silver image is formed, the oxidised colour developing agent reacts with the colour couplers in each of the three layers of the film to form coloured dyes. The dyes form only at the sites where the image is being converted to metallic silver. Next, the film goes into the pre-bleach (formerly conditioner) bath, which has formaldehyde and EDTA to "kick off" the bleach. Next, the film goes into a bleach solution. The bleach converts metallic silver into silver bromide, which is converted to soluble silver compounds in the fixer. During bleaching, iron (III) EDTA is changed to iron (II) EDTA (Fe+++ EDTA + Ag° + Br−→ Fe++ EDTA + AgBr) before fixing, and final wash. The most common processing chemistry for such films is E6, derived from a long line of developers produced for the Ektachrome range of films.
Standard black and white stock can also be reversal processed to give black and white slides. After 'first development', the initial silver image is then removed (e.g. using a potassium bichromate/sulfuric acid bleach). The unfixed film is then fogged (physically or chemically) and 'second-developed'. (See say M J Langford Advanced Photography Focal Press (1980) p345 for materials details; kits containing the necessary fogging and bleach agents are also available commercially). However the process works best with slow films such as Ilford Pan-F processed to give a high gamma.
[edit] Proprietary methods
The Kodachrome process is a proprietary process in which all the dyes are added to the emulsion during development.
In colour print development, the Cibachrome process also uses a print material with the dye-stuffs present and which are bleached out in an appropriate places during developing. The chemistry involved here is wholly different from C41 chemistry; (it uses azo-dyes which are much more resistant to fading in sunlight).
[edit] Primitive processes
Primitive processes (i.e. ferrotype, wet plate, ambrotype) use ferro sulphate to oxidise silver. Most silver-based emulsions need to oxidise the free silver in it to form an image.
The chemical process can be described as ferro sulfate splitting into iron and sulfurous acid leaving a radical oxygen molecule that combines with the silver forming silver oxide.
