Photographic processing is the chemical means by which
photographic filmPhotographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
and
paperThis article is about light-sensitive photographic media; for digital printing media please see Photo printer, photo paper and inkjet paper.Photographic paper is paper coated with light-sensitive chemicals, used for making photographic prints....
is treated after photographic exposure to produce a negative or positive
imageAn image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person....
. Photographic processing transforms the
latent imageA latent image on photographic film is an invisible image produced by the exposure of the film to light. When the film is developed, the area that was exposed darkens and forms a visible image...
into a visible image, makes this permanent and renders it insensitive to light.
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
PolaroidPolaroid Corporation is a multinational consumer electronics and eyewear company, originally founded in 1937 by Edwin H. Land. It is most famous for its instant film cameras, which reached the market in 1948, and continued to be the company's flagship product line until the February 2008 decision...
and thermally developed films.
KodachromeKodachrome is the trademarked brand name of a type of color reversal film that was manufactured by Eastman Kodak from 1935 to 2009. Kodachrome was the first successfully mass-marketed color still film using a subtractive method, in contrast to earlier additive "screenplate" methods such as...
requires Kodak's proprietary K-14 process, and can only be processed by a single facility, located in
KansasKansas is a state located in the Midwestern United States. It is named after the Kansas River which flows through it, which in turn was named after the Kansa tribe, who inhabited the area. The tribe's name is often said to mean "people of the wind" or "people of the south wind," although this was...
, USA.
IlfochromeIlfochrome, is a dye destruction positive-to-positive photographic process used for the reproduction of slides on photographic paper. The prints are made on a dimensionally stable tri-acetate polyester base, essentially a plastic base opposed to traditional paper base...
materials use the
dye destructionDye destruction or dye bleach is a photographic printing process, in which dyes embedded in the paper are bleached in processing. Because the dyes are fully-formed in the paper prior to processing, they may be formulated with few constraints, compared with the complex dye couplers that must react...
process.
Common processes
All film and paper is treated in a series of chemical baths, which are closely monitored and maintained at a specific temperature and treatment time. Developer baths are most sensitive to deviations from the standard time and temperature of treatment; other baths are less sensitive.
Black and white negative materials
- The film may be soaked in water to swell the gelatin
Gelatin is a translucent, colorless, odorless, brittle, nearly tasteless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning...
layer.
- The developer
In the processing of photographic films, plates or papers, the photographic 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 elemental silver in the gelatine matrix...
converts the latent image to metallic silver.
- A stop bath
Stop bath is a chemical bath usually used in processing traditional black-and-white photographic films, plates, and paper used after the material has finished developing. The purpose of the stop bath is to halt the development of the film, plate, or paper by either washing off the developing...
, typically a dilute solution of acetic acidAcetic acid, CH
3COOH, also known as ethanoic acid, is an organic acid which gives vinegar its sour taste and pungent smell. It is a weak acid, in that it is only partially dissociated acid in aqueous solution...
or citric acidCitric acid is a weak organic acid, and it is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of virtually all living things...
, halts the action of the developer. A rinse with clean waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
may be substituted.
In modern automatic processing machines, this step 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.
- The fixer
Photographic fixer is a chemical used in the final step in the photographic processing of film or paper. The fixer stabilises the image, removing the unexposed silver halide remaining on the Photographic film or photographic paper, leaving behind the reduced metallic silver that forms the image,...
makes the image permanent and light-resistant by dissolving any remaining silver halideA silver halide is one of the compounds formed between silver and one of the halogens — silver bromide , chloride , iodide , and two forms of silver fluorides. As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX...
salts. Fixer is sometimes called "hypo," a misnomer originating from casually shortened form of the alchemist's name hyposulphite. Neither "hyposulphite," "hyposulfite" nor "hypo" is used to mean thiosulfate in modern chemistry.
- Washing in clean water removes any remaining fixer. Residual fixer can corrode the silver image, leading to discolouration, staining and fading. The washing time can be reduced and the fixer more completely removed if a hypo clearing agent is used after the fixer.
- 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
Hard water is water that has high mineral content . Hard water minerals primarily consist of calcium , and magnesium metal cations, and sometimes other dissolved compounds such as bicarbonates and sulfates...
. (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.
Black & white reversal processing
This process has three additional stages:
- 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
Fogging in photography is the deterioration in the quality of the image caused either by extraneous light or the effects of a processing chemical.-Taxonomy of fogging:...
, 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
ChromogenicChromogenic refers to color photographic processes in which a traditional silver image is first formed, and then later replaced with a colored dye image.- Description :...
materials use
dye couplersDye coupler is present in chromogenic film and paper used in photography, primarily color photography. When color developer develops exposed silver-halide crystals, the developing agent molecules become oxidized, and the oxidized developer molecules react with dye coupler molecules to form dye in...
to form colour images.
Modern colour negative film is developed with the
C-41 processC-41 is a color print film developing process. C-41, also known as CN-16 by Fuji, CNK-4 by Konica, and AP-70 by AGFA, is the most popular film process in use, with most photofinishing labs devoting at least one machine to this development process....
and colour negative print materials with the
RA-4 processRA-4 is Kodak's proprietary name for the chemical process most commonly used to make color photographic prints. It is used for both digital printers of the types most common today in photo labs and drug stores, and for prints made with older-type optical enlargers and manual processing...
. These processes are very similar, with differences in the first chemical developer.
The C-41 and RA-4 processes consist of the following steps:
- 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.
In the RA-4 process, the bleach and fix are combined. This is optional, and reduces the number of processing steps.
Transparency films, except
KodachromeKodachrome is the trademarked brand name of a type of color reversal film that was manufactured by Eastman Kodak from 1935 to 2009. Kodachrome was the first successfully mass-marketed color still film using a subtractive method, in contrast to earlier additive "screenplate" methods such as...
, are developed using the
E-6 processThe E-6 process is a photographic process for developing Ektachrome, Fujichrome, and other color reversal photographic film....
, which has the following stages:
- 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 exhausted developing agents couple with the dye couplers
Dye coupler is present in chromogenic film and paper used in photography, primarily color photography. When color developer develops exposed silver-halide crystals, the developing agent molecules become oxidized, and the oxidized developer molecules react with dye coupler molecules to form dye in...
in each layer.
- The film is bleached, fixed, stabilised and dried as described above.
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
formaldehydeFormaldehyde is a chemical compound with the formula CH2O. It is the simplest aldehyde. Formaldehyde also exists as the cyclic trimer trioxane and the polymer paraformaldehyde. It exists in water as the hydrate H2C2. Aqueous solutions of formaldehyde are referred...
and
glutaraldehydeGlutaraldehyde is an organic compound with the formulat CH22. This pungent colorless oily liquid is used to disinfect medical and dental equipment...
. In modern processing, these hardening steps are unnecessary because the film emulsion is sufficiently hardened to withstand the processing chemicals.
Further processing
Black and white emulsions both negative and positive, may be further processed. The image silver may be reacted with elements such as
seleniumSelenium is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature....
or
sulphurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
to increase image permanence and for
aestheticAesthetics is commonly known as the study of sensory or sensori-emotional values, sometimes called judgments of sentiment and taste...
reasons. This process is known as
toningthumb | right | 200px | A sepia-toned image taken and created in 2009 In photography, toning means changing the color of black-and-white photographs. In analog photography, toning is the result of several chemical processes carried out on silver-based photographic prints...
.
In selenium toning, the image silver is changed to silver selenide; in
sepia-Marine life:* Sepia , a genus of cuttlefish* Sepia , a subgenus of cuttlefish* The ink of a cuttlefish-Media and entertainment:* Sepia * "Sepia" , a song by the Manic Street Preachers...
toning, the image is converted to silver sulphide. These chemicals are more resistant to atmospheric
oxidising agentsAn oxidizing agent can be defined as either:#a chemical compound that readily transfers oxygen atoms, or#a substance that gains electrons in a redox chemical reaction...
than silver.
If colour negative film is processed in conventional black and white developer, and fixed and then bleached with a bath containing
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
and
potassium dichromatePotassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is potentially harmful to health and must be handled and...
solution, the resultant film, once exposed to light, can be redeveloped in colour developer to produce an unusual
pastelPastel is an art medium in the form of a stick, consisting of pure powdered pigment and a binder. The pigments used in pastels are the same as those used to produce all colored art media, including oil paints; the binder is of a neutral hue and low saturation.The noun "pastel" gives rise to:*...
colour effect.
Processing apparatus
Before processing, the film must be removed from the camera and from its cassette, spool or holder in a light-proof room or container.
Small scale processing
In amateur processing, the film is removed from the
camerathumb |right|Cameras from Large to Small, Film to Digital A camera is a device that records images, either as a still photograph or as moving images known as videos or movies...
and wound onto a
reelA developing tank is a light-tight container used for developing film. A developing tank allows photographic film to be developed in a daylight environment. This is useful because most film is panchromatic and therefore can not be exposed to any light during processing. A developing tank can hold...
in complete darkness (usually inside a
darkroomA darkroom is a room that can be made completely dark to allow the processing of light sensitive photographic materials, including photographic film and photographic paper. Darkrooms have been created and used since the inception of photography in the early 1800s...
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.
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.
Commercial processing
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.
Environmental and safety issues
Many photographic solutions have high
chemicalIn environmental chemistry, the chemical oxygen demand test is commonly used to indirectly measure the amount of organic compounds in water. Most applications of COD determine the amount of organic pollutants found in surface water , making COD a useful measure of water quality...
and
biologicalBiochemical oxygen demand or BOD is a chemical procedure for determining the rate of uptake of dissolved oxygen by the rate biological organisms in a body of water use up oxygen. It is not a precise quantitative test, although it is widely used as an indication of the quality of water.BOD can be...
oxygen demand (COD and BOD). These chemical wastes are often treated with
ozoneOzone or trioxygen is a simple triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O
2. Ground-level ozone is an air pollutant with harmful effects on the respiratory systems of animals...
,
peroxideA peroxide is a compound containing an oxygen-oxygen single bond. The simplest stable peroxide is hydrogen peroxide. Superoxides, dioxygenyls, ozones and ozonides compound are considered separately.- Organic chemistry :...
or
aerationAeration is the process by which air is circulated through, mixed with or dissolved in a liquid or substance.-Methods of aeration of liquids:Aeration of liquids is achieved by:...
to reduce the COD in commercial laboratories.
Exhausted fixer and to some extent rinse water contain silver
thiosulfateThiosulfate , sometimes spelled Thiosulphate, is an oxyanion of sulfur produced by the reaction of sulfite ions with elemental sulfur in boiling water. Thiosulfate occurs naturally in hot springs and geysers, and is produced by certain biochemical processes...
complex ions. They are far less toxic than free silver ion, and they become
silver sulfideSilver sulfide , Ag2S, is a black sulfide of silver. It is black and constitutes the tarnish that forms on silver when exposed to the hydrogen sulfide of the atmosphere. It is found in nature as relatively low temperature mineral acanthite...
sludge in the sewer pipes or treatment plant. However, the maximum silver concentration in discharge is very often tightly regulated. Silver is also a somewhat precious resource. Therefore, in most large scale processing establishments, exhausted fixer is collected for silver recovery and disposal.
Many photographic chemicals use non-biodegradable compounds, such as
EDTAEDTA is a widely used initialism for the chemical compound ethylenediaminetetraacetic acid . EDTA is a polyamino carboxylic acid with the formula [CH
2N
2]
2. This colourless, water-soluble solid is widely used to dissolve scale...
,
DTPADiethylene triamine pentaacetic acid is a polyamino carboxylic acid consisting of a diethylenetriamine backbone modified with five carboxymethyl groups. The molecule can be viewed as an expanded version of EDTA. DTPA is used as its conjugate base, often undefined, which has a high affinity for...
,
NTANitrilotriacetic acid , C
6H
9NO
6, is a polyamino carboxylic acid and is used as a chelating agent which forms coordination compounds with metal ions such as Ca
2+, Cu
2+ or Fe
3+. In 1999 about 20,000 tons of NTA were used in...
and
borateBorates in chemistry are chemical compounds containing boron oxoanions, with boron in oxidation state +3. The simplest borate ion is the trigonal planar, BO33−, although many others are known. BO33− forms salts with metallic elements. Boron found in...
. EDTA, DTPA, and NTA are very often used as
chelating agentChelation is the formation or presence of two or more separate bindings between apolydentate ligand and a single central atom.Usually these ligands are organic compounds, and are called chelants, chelators, chelating agents, or sequestering agents....
s in all processing solutions, particularly in developers and washing aid solutions. EDTA and other polyamine polycarboxylic acids are used as iron ligands in color bleach solutions. These are relatively nontoxic, and in particular EDTA is approved as a food additive. However, due to poor biodegradability, these chelating agents are found in alarmingly high concentrations in some water sources from which municipal tap water is taken. Water containing these chelating agents can leach metal from water treatment equipment as well as pipes. This is becoming an issue in Europe and some parts of the world .
Another non-biodegradable compound in common use is
surfactantSurfactants are wetting agents that lower the surface tension of a liquid, allowing easier spreading, and lower the interfacial tension between two liquids.- Etymology :The term surfactant is a blend of surface active agent...
. A common wetting agent for even drying of processed film uses Union Carbide/Dow Triton X-100 or octylphenol ethoxylate. This surfactant is also found to have estrogenic effect and possibly other harms to organisms including mammals .
Development of more biodegradable alternatives to the EDTA and other bleaching agent constituents were sought by major manufacturers, until the industry became less profitable when the digital era began.
In most amateur darkrooms, a popular bleach is potassium hexacyanoferrate (III) (common name
potassium ferricyanidePotassium ferricyanide is the chemical compound with the formula K
3[Fe
6]. This bright red salt consists of the coordination compound [Fe
6]
3−...
). This compound decomposes in the waste water stream to liberate
cyanideA cyanide is any chemical compound that contains the cyano group , which consists of a carbon atom triple-bonded to a nitrogen atom. Inorganic cyanides are hydrogen cyanide salts in which cyanide is generally the anion CN
-. Organic compounds that have a -C≡N functional group bonded to...
gas . Other popular bleach solutions use potassium dichromate (a
hexavalent chromiumHexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via conversion to sodium dichromate. Approximately of hexavalent chromium were produced in 1985. Other hexavalent chromium compounds are chromium...
) or
permanganateA permanganate is the general name for a chemical compound containing the manganate ion, . Because manganese is in the +7 oxidation state, the manganate ion is a strong oxidizing agent...
. Both ferricyanide and dichromate are tightly regulated for sewer disposal from commercial premises in some areas.
BorateBorates in chemistry are chemical compounds containing boron oxoanions, with boron in oxidation state +3. The simplest borate ion is the trigonal planar, BO33−, although many others are known. BO33− forms salts with metallic elements. Boron found in...
s, such as
boraxBorax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...
(sodium tetraborate),
boric acidBoric acid, also called boracic acid or orthoboric acid or Acidum Boricum, is a weak acid often used as an antiseptic, insecticide, flame retardant, in nuclear power plants to control the fission rate of uranium, and as a precursor of other chemical compounds. It exists in the form of colorless...
and sodium metaborate, are toxic to plants, even at a concentration of 100 ppm. Many film developers and fixers contain 1 to 20 g/L of these compounds at working strength. Most non-hardening fixers from major manufacturers are now borate-free, but many film developers still use borate as the buffering agent. Also, some, but not all, alkaline fixer formulae and products contain a large amount of borate. New products should phase out borates, because for most photographic purposes, except in acid hardening fixers, borates can be substituted with a suitable biodegradable compound.
Developing agents are commonly
hydroxylatedHydroxylation is any chemical process that introduces one or more hydroxyl groups into a compound thereby oxidizing it. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases....
benzeneBenzene, or benzol, is an organic chemical compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H. Benzene is a colorless and highly flammable liquid with a sweet smell and a relatively high melting point...
compounds or aminated benzene compounds, and they are harmful to humans and experimental animals. Some are
mutagenIn biology, a mutagen is a physical or chemical agent that changes the genetic material of an organism and thus increases the frequency of mutations above the natural background level. As many mutations cause cancer, mutagens are typically also carcinogens...
s. They also have a large chemical oxygen demand (COD).
Ascorbic acidAscorbic acid is a sugar acid with antioxidant properties. Its appearance is white to light-yellow crystals or powder, and it is water-soluble. One form of ascorbic acid is commonly known as vitamin C. The name is derived from a- and scorbutus , the disease caused by a deficiency of vitamin C...
and its isomers, and other similar sugar derived reductone reducing agents are a viable substitute for many developing agents. Developers using these compounds were actively patented in the US, Europe and Japan, until 1990s but the number of such patents is very low since late-1990s, when the digital era began.
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