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The Chemistry of Developers and the Development Processe - Development Processes, Physical Development, Using a Metal Ion Developing Agent, Using an Organic Developing Agent

silver image color solution

SCOTT WILLIAMS, PH.D.
Rochester Institute of Technology

Photography was moved from the artisan into the consumer’s hands by 20th century innovations in film and paper improvements. Important events were also imaged and preserved in color, for the first time during this century, with realistic sharpness and vibrancy. Most photographic media innovations, whether on film or paper, relied on the same key element for image creation—silver. As a result, the development and fixing processes developed in the 19th century were largely carried over into the 20th century and remained, for the most part, unaltered. This essay will highlight the general schemes that were applied to the specific formulations used during this important time period of photographic history.

The fundamental photographic objective can simply be illustrated by the chemical reaction shown below.

Ag + + 1e – ? Ag 0

Silver ion to silver metal conversion, by the addition of one electron, is inherent in every silver halide grain. This presented a unique challenge. To create an image-wise silver distribution, the silver formation rate of the light-exposed grain must exceed that of the non-exposed grain. Chemical development methods have been used to exploit the differential development of latent image-containing grains over non-exposed silver halide grains. Once the image is resolved, chemical solutions must be applied to remove the unwanted silver halide in the non-image areas, thus preserving the image for a useful period of time. These solutions, known as fixing solutions or “fixers,” would “fix” the image, imparting the archival properties observed with these modern materials. Washing steps would then complete the photographic development process by the removal of by-products produced in the prior steps.

Development Processes

Two general developer classes were used throughout the latter part of the 19th century through the 20th century. Developing solutions, defined as chemical compositions or solutions capable of resolving a latent image, were placed into one of the two classes based on the source of the metal ion used for image amplification. Silver was the metal of choice, but not exclusively. Other metal ion sources will be discussed below. In physical development, the metal ion source was external to the silver halide grain containing the latent image. Silver ion, for example, would be added to the developing solution when silver was to comprise the image mark. Direct, or chemical, development was reserved for those processes that would use the metal ion reserve, typically silver, present in the latent image-containing grains. Economic and stability considerations would weigh in favor of direct development as the predominant choice in commercial photographic applications.

Regardless of the developer type, developer solutions were required to possess several attributes to be commercially successful. These solutions all had to be able to reduce a metal ion to form a stable metal. The metal reduction must be selective for the light-exposed area of the medium. In the case of color photography, metal reduction was immediately followed by a color dye forming reaction, which likewise, had to result in a dye product that was stable to chemical decomposition. The developer components had to be sufficiently soluble and chemically stable with time and environmental factors such as the presence of oxygen. Finally, in all cases, the reaction by-products, that would promote time-dependent image decomposition, had to be preferably colorless and soluble to facilitate prompt removal in the finishing steps of development. Increasingly, as the art matured, there was a desire for developers to have minimal toxicity and environmental impacts.

The root reaction mechanism is also common to both developer classes as the end game is the same—metal reduction to a visible image mark. Both direct and physical development can be characterized by an electrochemical cell model involving a chemical reaction class known as redox, or reduction-oxidation, reaction. These reactions are thermodynamically selected to favor metal reduction.

A minimum of two reactants must participate in the image forming process, an oxidizing agent and a reducing agent. The metal ion, which will be reduced to a visible metal speck, is known as the oxidizing agent. Silver ion, for example, is the agent that causes developer oxidation. The reducing agent is the developing agent, or developer, which will become oxidized in this process. Although confusing at first, the main concept is that silver ion is converted to silver metal by the addition of an electron from the developer. Photographic chemists keep the term twists in order by noting that the oxidizing agent gains an electron on its way to reduction (gain of an electron is a reduction reaction, GeRR). The developer agent loses an electron in the process (loss of electron is an oxidization reaction, LeO). These reaction agents must always work together. One cannot have LeO without the GeRR, or GeRR without LeO, based on a word play off the king of beasts. The generalized reaction, shown below, illustrates the transfer of the electron that results in a stable image metal speck.

Metal Ion () + Developer (e-) ? Stable Metal (e-) + Oxidized Developer ()

Physical Development

The physical development method involves a metal addition reaction. In other words, the metal ion that will be used to resolve the image will be added by the photographer’s careful skill. Image quality attributes can all be modulated by the principle variables in photographic development—time, temperature and agitation. In addition, for silver halide processes, physical development affords an additional control element on image quality by the choice of the metal ions selected, and whether the process would be carried out in the presence or absence of the exposed silver halide grain. The former process is known as pre-fixation physical development, and the latter, post-fixation physical development. Physical development can also be subdivided into two main areas of practice categorized by the selection of the developing agent. The most common 20th century practice was the addition of silver nitrate in the developing solution containing an organic developer, like pyrogallol; or, the addition of a silver halide solvent that will dissolve the grain providing a bath of available silver in solution, less common, but widely used for esthetic effects, are the processes that use a metal ion as the developing agent.

Using a Metal Ion Developing Agent

The generalized physical development reaction scheme is shown below.

         M + + M n+ ? M 0 + M (n+1)

Example 1: Ag + + M (n)+ ? Ag 0 + M (n+1)+

Example 2: Ag + + Fe +2 ? Ag 0 + Fe +3

There are two requirements for successful image creation using physical development. First, the developing agent must only lose one electron in the exchange; and second, the oxidized developer must be stable and soluble in the solution for proper removal. Iron is selected most often when using this method of development.

The second requirement can be satisfied by the addition of a developing compound known as a chelating agent. The primary role of the chelating agent is to aid in the solubility of the metal components. Oxalate, tartrate, citrate, and EDTA salts were often added to aid in the metal ion transport in and out of the image support. When silver was the metal added to create the image, removal of the metal-based developer agent was critical for image permanence. Implicit in the electrochemical cell model, which will be described in more detail below, the silver can be just as easily oxidized by the same developer agent that caused the silver to be reduced. Removing the oxidized developer, therefore, was one primary role for the chelating agent.

The best example of a physical development process, using a metal ion developing agent, was largely practiced by the artisans in the 20th century. Such processes were the iron-based methods developed in the 19th century by William Herschel, which involved image formation through the addition of silver or a “noble” metal such as platinum, palladium, or gold. In these processes, the image was captured in light-sensitive iron, which was used as the developing agent to create an image with a second metal. As illustrated by the platinum-palladium process, all of the necessary components that define a successful developer work to produce an image that began in one metal and was fixed in another.

PtCl 4 -2 + PdCl 4 -2 + xFe(C 2 O 4 ) 2 -2 + x(C 2 O 4 ) -2 ? Pt/Pd 0 + xFe(C 2 O 4 ) 3 -3 + 8Cl -1

Using an Organic Developing Agent

Physical development, using a familiar organic developer and used mainly with silver halide image materials, was more common throughout the 1900s. Developing solutions were prepared either by the addition of silver nitrate or a silver halide solvent salt, such as sodium sulfite. Developers containing a significant concentration of silver halide solvent were often categorized as fine-grain developers. Two common examples of each type mentioned are listed in Table 1.

The mechanism of its action is similar to that described below using direct developers with the difference being the supply of reducible silver ion. As mentioned, the reducible silver ion, in the case of a physical developer, comes from a silver solution bath. In other words, a silver speck developed by the physical development process would comprise silver that was originally part of some other grain or prepared initially by the photographer. In direct development, the silver speck is produced from the silver content that was originally part of the same latent image grain. In general, silver-added physical developer solutions are unstable and are often prepared by adding the silver nitrate component just prior to use.

Image Quality Attributes

While physical development is used primarily for black and white film, it is more often seen in paper development processes. Grain size, morphology, and metal purity directly impact the image quality attributes with these materials. Tone and resolution are influenced by the size and shape of the image speck. Light scatter plays the largest role in determining image tonality. How the exposed grain develops into an image metal speck is largely governed by whether the physical development process begins with the latent image still embedded with the silver halide grain or existing as a free nucleation site. Pre-fixation physical development results in silver filament growth due to the interfacial nature of the latent image and the limited access of fresh developer blocked by the silver halide grain. Image speck morphology is more varied with post-fixation. In the presence of slow time-extended development, the silver image specks can adopt a hexagonal shape. A slightly faster development rate results in rounded image silver. An elliptical image silver shape results when the development rate is increased through concentration, time, temperature, or agitation. These varying shapes will contribute to image tonality; small/rounded shapes tend toward cooler tones compared to larger/multi-sided specks, which exhibit a more neutral tonality appearance in reflective prints. As a result, physical developers are often used as tone modifiers.

Direct, or Chemical, Development

Indisputably, the predominant development method practiced in photography is the direct (or chemical) development process. In direct development, the image-forming silver originates from the silver halide grain containing the latent image site. The chemical reduction of silver ion proceeds at one or more points on a grain until the majority of the silver reserve on the grain is converted to silver metal. With direct development, grain morphology is more uniform with variations in tone mostly resulting from the extent of physical development in proportion to the grain solvent content of the developer. The greater the relative rate of direct versus physical development, the more filamentous and neutral the image tonality. Of course, the control of time, temperature, and agitation also has an impact on tone.

Kodak DK-50 is used to illustrate the typical components that comprise a photographic direct developer.

The largest chemical component is the solvent, which is typically water. There is much debate as to whether the water should be deionized or straight out of the tap, and one could reconcile any position on this matter. Tap sources are cheap and readily available. If the water is too hard, however, metal-developer complexes could form resulting in staining reactions. Other experts support a view that deionized water may not be the best source because it could provoke an adverse osmotic gradient to form between the developing environment and the image gelatin. This latter position may be less likely since developers can contain as much or more salt per unit volume than that contained in the gelatin layer.

The developing, or reducing agent, is the key ingredient in a developer. Table 3 lists the names and general properties of the most popular developing agents used in chemical development processes.

Upon inspection, there are common structural properties that lend themselves to an effective developer. These common properties are governed by the Kendall-Pelz rule, which states that the general structure for a developer can be represented by the structure

General Developer Structure: a – (A = C) n – b

where for A = C , a carbon, the developer is referred to as a Kendall developer; when A = N , a nitrogen, the developer is called a Pelz developer. The integer value of n can be from zero to any practical integer number limited largely by solubility. In most practical cases, the Kendall developer is used whereby n = 3, a benzene ring of six carbons. The lowercase a and b can be any combination of hydroxyl or amine moiety. Hydroquinone is a Kendall developer, for example, while 1-phenyl-3-pyrazolidone is a Pelz developer. Hydrazine would be an example of a hybrid n = 0 Kendall-Pelz developer, but as this compound is the active ingredient in the Space Shuttle solid rocket propellant, this molecule may not be the wisest choice for the photographic darkroom. Overall, the propensity of a developer to reduce silver ion to silver metal rest more in the side groups (a, b) rather than the fine-tuning that the heterocyclic moiety (A) places on the chemical activity. The Kendall-Pelz rule specifies that the side groups can be in either the para or ortho position on the benzene ring (the typical backbone), but not the meta position. Hydroquinone is an example of two hydroxyls substituted para to each other. Catechol illustrates an ortho example. Resorcinol, 1,3-dihydroxybenzene would seem to be a poor choice under the Kendall-Pelz rule, but in fact, this reducing agent has been used in photographic development. As a side note, by applying a generalized understanding to developer structure, any organic reducing agent that has the general developer structure may satisfy the requirement. Developers prepared from coffee and mint leaves have been successfully applied as a method of image development.

Following the addition of the developing agent to water, a preservative is added. When the preservative is sodium sulfite, which is the most common choice, two functions are performed. First, sodium sulfite preserves the solution by scavenging oxidizing agents, such as dissolved oxygen, which can degrade developer activity. Secondly, with its intrinsic metal chelating ability, sodium sulfite performs a grain solvent function in proportion to its concentration. Fine grain developers may contain as much as 15% weight per volume of sodium sulfite making it the major added component. To serve as a preservative, only a few percent would be required.

With some developers, particularly both developing agents listed above for Kodak DK-50, sulfites are known to undergo a substitution reaction that inhibits the formation of quinones; thereby, providing yet another preservative function. In early Technicolor applications, quinone formation was desired to promote tanning reactions. The formation of quinone and semi-quinone intermediates is also a key requirement in modern chromogenic color process. In black and white applications, this formation is undesirable as quinones self-react to form polymeric products that may result in brown stain formation. Sulfites do find their way into chromogenic formulas, but they are typically in concentrations that primarily serve the scavenger function. Other sulfite salts are selected such as sodium metabisulfite as well as the potassium form of these additives.

A buffering agent, also known as an accelerator, is typically added next. The accelerator function is self-explanatory. The developing agents are most active in their anionic state, in the case of those developers comprising a hydroxyl group; and in a neutrally charged state for the amine class. In either case, the preferred state is a form of the developing agent that raises the propensity for the developer to deliver its electrons for silver reduction. The ionization sweet spot corresponds to the developer’s ionization constant or pKa (negative log of the acid dissociation constant). A simple experiment can be conducted whereby strips of black and white film, all similarly exposed, are developed in a series of developing solutions of increasing pH (negative log of the hydronium ion concentration) from a pH of 5 to 14. For hydroquinone, there will be two pH values whereby a dramatic increase in activity results. The two regions roughly correspond to the two pKas (pKa 1 = 10 and pKa 2 = 11.6, one for each hydroxyl group) that exist for the molecule. Therefore, by modulating the pH, developer activity can be varied by pH control. A popular choice for a buffering, or accelerating, agent is sodium tetraborate (Borax). Borax, however, has a tendency to cake upon storage. This led Kodak to develop its Kodalk alternative which is sodium metaborate. Sodium carbonate may also be used, but this choice may promote blistering processes to occur through the formation of carbon dioxide release with variations in temperature. Phosphate salts, although excellent buffering agents, provide a rather healthy medium for biological activity that would reduce the shelf-life of the developer.

A final ingredient falls into a class of compounds known as restrainers. As the name suggests, restrainers serve to impart fine control over the development process. Their effect is greatest in the highlight region. A typical restrainer is potassium bromide, but organic restrainers (also called anti-foggants) have been used, such as benzotriazole. Organic restrainers inhibit development through an absorption process that impedes developer access to the exposed latent image site. Organic restrainers are very efficient and used in sparingly low concentrations in the developer formula. Potassium bromide works by a mass action process that favors the formation of silver bromide as opposed to silver metal development. Potassium bromide is cheaper to use, soluble, and has a better health and environmental impact perspective than its organic alternatives.

Mechanism of Action

Electrochemical and adsorption catalysis models are the two theories that detail the mechanism of photographic development. The electrochemical model is the most likely model from a kinetic and thermodynamic viewpoint (see Figure 7).

In the electrochemical model, photographic development can be viewed in the same way a battery or galvanic works. On the anode, or negative pole, a silver metal pole is suspended into a solution of photographic developer. The silver metal pole would represent the silver latent image site on a silver halide grain. The developer adsorbs onto the latent image site and becomes oxidized via electron transfer to the silver metal pole. The electron travels down the “wire,” the bulk of the latent image site, until it encounters a silver metal ion. Silver mobility is facilitated by the migration of halide ion into the developer solution. The silver metal ion, in close proximity to the silver latent image site, receives the electron, thereby, becoming reduced. The macroscopic effect would be a growth of silver on the surface of the cathode, the positive pole. Silve would continue to collect onto the cathode silver until either all the silver ions are used up or all of the developer becomes depleted. These two conditions are satisfied in the microscopic example of a silver halide grain during development.

The less likely model, for kinetic reasons, is the adsorption catalysis theory (see Figure 8). The process and end result are not that much different than the electrochemical model except that there is a point in the process whereby a transient developer-silver ion complex forms on the surface of the latent image site. The silver metal catalyzes the concerted transformation from silver ion to silver metal. Concerted reactions are very rare in nature, but plausible.

In either case, the reaction is believed to involve the adsorption of a developer molecule to the latent image surface. In this process, a thin layer of developer molecules are oxidized and diffused away or are made anew by the oxidation of a fresh developer that happens to move into close proximity to the oxidized developer lying directly in contact with the latent image site. The activity difference between Metol or phenidone compared to hydroquinone is largely due to the surfactant properties of each developing agent. Even though hydroquinone is superior to Metol as an electron donor, Metol adsorbs onto silver metal more strongly than hydroquinone. As a result, Metol is said to be an excellent shadow developer (in reference to the characteristic curve). Notice in the formulae, however, that these two developing agents are used and work better together than the sum of their separate activities. This is known as superadditive development, and is the reason why developing agent combinations are common. One developing agent is superior at latent image adsorption while the other serves to keep the adsorbed developer replenished with a supply of electrons. Once the silver image speck grows large enough for the stronger electron donor to take over, the development rate dramatically increases.

Color Development

Color development has been practiced primarily using two different schemes. The largest application involved the color photographic process known as chromogenic development. Chromogenic development may be further categorized into color negative and color positive processes. The primary alternative process was known as silver-dye bleach which will be discussed in a separate section. Early and fine art color prints were also developed using color separation dye transfer processes, but these will not be discussed in this section. Instant photographic methods, which rely on dye migration, will also be featured in the section on Color Photography.

Chromogenic color development takes advantage of the by-products of silver halide chemical development and may involve as many as 14 to 16 different chemical solution or exposure applications. The first step(s) in chromogenic development requires a black and white development function. Consider, as an example, the making of an image of a black and white chessboard. If the capture medium was a color negative material, the first step would be a black and white development in proportion to the amount of exposure in each of the spectrally sensitized color subtractive layers. In the case of the chessboard, the white squares would be rendered by equal neutral densities in each of the cyan, magenta, and yellow layers. There would be no silver density development in the black square regions, hence a negative material. Chromogenic color develops alongside silver development by the formation of a color dye via a reaction between the oxidized developing agent, typically derivatives based on the Kendall developer para-phenylenediamine, and an incorporated color coupler. Incorporated color coupler refers to a pre-dye that is added in the emulsion upon manufacture and is only activated upon reaction with oxidized developer in the proximity of a silver image speck. For example, a cyan dye is resolved by a reaction between the oxidized developer, known as a quinonediamine, and the incorporated pre-dye 1-napthanol. The development is then stopped by an acid bath application. The silver is then removed by a bleach-fix (BLIX) step that involves the conversion of the silver back to silver halide followed by a fixing, or removal of the silver halide. In the case of the chessboard, the visual density on the negative material would appear neutral. By measuring the color densities, one would find about equal red, green, and blue color densities corresponding to equal formations of cyan, magenta, and yellow dye, respectively, in each of the color layers of the medium. The commercial applications of this type of color negative film development are usually referred to as C-41 (Kodak), CN-16 (Fuji), and AP-70 (Agfa). Color print development applications are referred to by the names RA-4 (Kodak) and MP45 AC (Fuji), for example.

Chromogenic color positive development, such as that used to produce color slides, involves a similar development scheme to color negative development except that a reversal strategy is applied. Consider, again, the image making of a black and white chessboard using a color positive material, such as Ektachrome (Kodak) or Velvia (Fuji). These materials possess similar incorporated color pre-dye stuffs, in each of the emulsions layers, similar to those found in color negative materials lying in wait for an oxidized developer. After exposure, a black and white developer, using developing agents that will not react with the color coupler component such as hydroquinone and phenidone, is applied and used to fully develop the material. The development is then stopped and rinsed leaving a black and white negative image of the chessboard. At this stage, the non-image silver halide is still present on the medium. The color positive material is then uniformly exposed to white light exposing only the remaining non-image silver halide. This uniform exposure is then resolved by development using similar color developing agents, for example, derivatives of para-phenylenediamine, capable of reacting with the incorporated pre-dye stuffs. The steps following color development are similar to those described for the color negative processes. What remains, after washing, is a color positive. In this case, neutral densities corresponding to the black squares comprised of equal spectral contributions of cyan, magenta, and yellow dyestuffs in their incorporated layers. The most common commercial application of this type of development is usually referred to as an E-6 process. The Kodachrome process is similar to E-6, but with two main variations on the theme. One, the pre-dyes are not incorporated into the sensitized emulsions which requires that they be added during development, thereby two, creating many more steps than is typically practiced in E-6 development. Kodachrome processing may involve as many as 14 steps requiring carefully controlled, expert applications for a successful image.

At the heart of chromogenic development is the reaction between oxidized developer and a pre-dye creating a colored product. Formation of the oxidized form of the color developer, usually a derivative of quinonediamine, requires the reduction of two silver atoms. Depending on the pre-dye selected, the reaction of the oxidized developer with the pre-dye is immediately followed by the intramolecular conversion to the visible dye form. The pre-dye that promotes this type of color formation is known as a two-equivalent coupler. There is, however, a class of couplers that, once the reaction between the oxidized developer and pre-dye occurs, forms a colorless or “leuco” dye. To convert the leuco dye to a color dye, a reaction between the leuco dye and another oxidized developer agent is required. Since two oxidized developer agents, each formed from the reduction of two silver ions, are required per color dye formed, those couplers are referred to as four-equivalent color couplers.

These nuances, associated with the color couplers, relate more to technology incorporated into the color photographic medium and not part of the developer formulation. Other development regulating strategies are also built into the film or paper base such as development inhibition releasing (DIR) couplers or agents. DIR couplers are molecular systems that release a silver development inhibitor upon photographic development and serve to regulate the rate of oxidized developer-color coupler reaction by scavenging the oxidized developer in proportion to the development rate. These strategies have been used to fine-tune the color-forming process in these materials.

Post-Development Processes

Stop Bath

To arrest the photographic development process, one must adjust the one key condition making development possible, pH. A stop bath is a simple solution of acidic pH. In the presence of an acidic pH (less than seven), the propensity for electron transfer by the developing agents is greatly reduced, and the gelatin returns to a pre-development, non-swollen state slowing solution permeation. These two changes serve to immediately stop development, as well as prepare the photographic medium for the fixing step. Another name for this solution is short stop. Stop bath is typically a 3 percent solution of acetic acid in water. The vinegar smell in the darkroom may be attributed to this photographic solution. The reported acetic acid concentrations in water have ranged from 1 to 5 percent, but the exact concentration is not critical. What is critical is the pH maintenance in the region of pKa for acetic acid, which is 4.75. To monitor pH condition, an indicator stop bath is commonly selected. Indicator stop bath is an aqueous acetic acid solution with the addition of less than one-tenth of a percent of bromocresol purple indicator dye. This dye turns from a pink color, in acidic conditions, to an increasingly purple hue in alkaline conditions.

Fixation

Fixation is the step whereby the unused silver salts are removed for image permanence. The active ingredient in these solutions are silver salt solvents that work by forming water-soluble chelating complexes. The active ingredients, therefore, are typically thiosulfate salts, but other ingredients such as urea, ammonia, and cyanide salts have been used. A typical fixer solution comprised of sodium thiosulfate (hypo) is listed below. Sulfite addition, which is a grain solvent function, produces a slightly acidic solution pH. As shown in the formula, the hypo concentration can be as high as 20 to 30 percent.

There are several popular fixer solution variations that are selected for permanence reasons.

When aggressive development strategies are used with high temperature or pH, a hardening fixer may be selected. These fixer compositions often comprise organic or inorganic hardening agents to help maintain gelatin emulsion layer structural integrity. The potassium alum fixer composition is one such fixer composition (see Table 5).

Potassium alum is the dodecahydrate salt of potassium aluminum sulfate (KAl(SO 4 ) 2 · 12(H 2 O)) and works by forming intermolecular cross-links with neighboring gelatin chains, thereby providing enhanced molecular-level support.

Rapid fixer is the most common fixer composition, and takes advantage of the solvation properties of thiosulfate with those of ammonium ion. This buffered solution is shown below with ammonium thiosulfate as a key ingredient (see Table 6).

Rapid fixer is more efficient, and therefore faster (more rapid), at removing silver salts than its hypo variation.

Photographic emulsion exposure to fixer solutions must be well monitored. Since sulfur compounds are found in every one of these fixer formulations, they may impart some silver sulfide toning quality to the image. Fixer solution contact with acidic conditions also may cause the liberation of hydrogen sulfide (the rotten egg smell) gas presenting the need for adequate darkroom ventilation. Excessive exposure to fixer may also cause silver bleaching to occur in the image highlight regions.

The mechanism of action for the hypo-fixing process is shown below.

Soluble argentothiosulfate complexes are formed and are subsequently removed from the emulsion. When the silver concentration becomes too large, through repeated use of the fixer solution, the reaction no longer favors complex formation, which results in a longer clearing time. Clearing time refers to the time required for a strip of photographic film to be dipped into the fixer solution, and the time required for the base to become transparent or visible. Generally, when the required time to clear a piece of film doubles, the solution should be discarded by one of the several silver reclamation procedures. Spent fixer must never enter the environmental waste stream. A 5 percent potassium iodide solution test is also conducted whereby 1ml of the potassium iodide solution is mixed with 25ml of fixer solution. If the resulting cloudy solution, newly formed silver iodide, does not re-dissolve upon shaking the mixture, the solution should be replaced.

Washing

Washing the photographic media in a rinsing solution, or water, is important to ensure the removal of development by-products. Archival image quality may directly depend on the efficiency of the removal of deleterious chemicals. The washing solutions, washer configurations, and washing time will be selected based on the need for archival products and the type of medium washed. The table below illustrates the recommended length of washing times for various photographic media.

Washing

Hypo-clearing agents are used to improve the efficiency of thiosulfate removal. Hypo-clearing agents include 2 percent sodium sulfite as well as low concentration citrate and sulfate salts. Acidified silver nitrate solutions may be prepared to test for the presence of residual thiosulfate, remaining on a print, through the formation of brown silver sulfide stains. The stain density may be related to the efficiency of the washing step. The acidified solution is typically 7g of silver nitrate dissolved in a 100 ml of 3 percent acetic acid aqueous solution.

Black and white print tone may be varied through the application of a toning procedure. Toning procedures are typically conducted on a black and white fiber-based print after careful and complete washing; however, there are toning baths that may be added at the development or pre-fixation stage. Toning may be segregated into four main categories: metal replacement, silver salt toning, metal ferricyanide toning, and dye toning.

Gold and mercury toning would be a metal replacement example where some silver is oxidized in favor of the reduction of some other metal such as gold. The gold tonality would be added by the coexistence of the two metals within the same image speck. The more popular methods of toning involve the conversion of the silver image into a stable silver salt. Sepia and selenium toning are two examples of the silver converting into a stable silver sulfide or selenide salt, respectively. Metal ferricyanide toning processes are performed by the conversion of silver into a silver ferricyanide complex followed by subsequent conversion to a ferricyanide salt of a different metal such as copper, uranium, and iron.

Dye toning methods involve the formation of a dye cloud in proximate juxtaposition with the silver image spec. These methods use a procedure similar to the dye coupler strategies discussed with color development such as the K-14 Kodachrome process. A black and white print would be bleached, under room light, in a re-halogenating bleach. Redevelopment would then occur using a color developer solution, such as that used for the Kodak RA-4 process, with the drop-wise addition of a color coupler dissolved in a volatile solvent such as acetone. The silver would remain to contribute to the image tone.

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What are the hazzours of the developer and fixer and silver recovery unit to the boby