INTRODUCTION
The following pages of chemical formulas consist of various general and special purpose developing compounds, fixers, toners and the like. Some of these formulas are currently being used with contemporary photographic materials, while others are old formulas and it ought to be emphasized; they may or may not produce desirable results when used with current materials. Some of site formulas could be considered obsolete, however, they are included because many photographers are interested in old compounds and processes. Pre-packaged, commercially available formulas offer many advantages not the least of which is convenience. Also, all other factors being equal, the end result is consistent and dependable. However, to the photographer inclined toward a better understanding of the photographic process or experimentation concocting one's own formulary can be a rewarding experience. A few words of caution:
PHOTOGRAPHIC FORMULAS
When a photographic emulsion is exposed to light, the silver salt (silver bromide, chloride or iodide) which the light reaches, undergoes, a definite though invisible change to form what is known as the latent image. It is not yet definitely known just how this change takes place, but it is believed that the exposed parts of the emulsion gain a certain "activation" that makes this susceptible to the reducing action of a developer. When placed in a developing solution the exposed, "activated" particles of silver salt are reduced chemically to black metallic silver, leaving the unexposed particles of silver salt unchanged. Reduction in this sense does, not have the meaning commonly thought of in the photographic field, namely, the lessening of density in a film negative. This chemical reduction is a conversion of the silver salts to free silver and for the reaction one or more reducing agents-which photographers call "developers" are necessary.
THE DEVELOPER'S BASIC COMPONENT
The Reducing Agent
There are many chemicals which are reducing agents, but most of them are too powerful to be used for developing because they reduce all the silver salt in the emulsion without regard to the latent image which exposure in the camera has produced, therefore a reducing agent must be selected which is satisfactory as a developer and which confines its action to the exposed particles of silver salt, leaving the remaining unaffected. Of the reducing agents that are satisfactory for photographic use, metol and hydroquinone are most commonly used, and there are in addition other developing agents such as glycin, amidol, pyro, phenidone and rodinal frequently employed. There are several developing agents on the market under different names from metol (such as etol), but which are basically ehemical-mono-methylpara-amino-phenol-sulfate.
As has been indicated, the chemical action of these developing agents is fundamentally the same, The photographic effect, however, depends to a large extent on the particular developing agents, and one formula may have, for example, a high percentage of hydroquinone to produce brilliant photographic images while another formula may use a larger ratio of metol to produce softer results.
It is obvious, therefore, that great care should be taken in the preparation of developing solutions, for a slight error in the type or amount of the developing agents (or the other constituents too, for that matter) may have a serious effect on the behavior of the developer.
Metol
(N-Methyl-p-aminophenol sulfate) is a
readily water-soluble, non-staining agent which, with the addition of a small
amount of bromide, is very clean working and has excellent keeping qualities.
It is relatively unaffected by low temperatures, but will not dissolve in
alkaline solutions. In carbonate solutions it gives rapid development times
which may be extended by dilution. With sulfite alone, or mild alkalis such as
borax or
sodium metaborate, it produces a slow, fine grain developer for films.
Metol-only developers with carbonates, such as
Kodak Selectol Soft and
Ansco 120, are very soft working paper developers.
Metol-
hydroquinone developers are considerably more contrasty than
metol-only, and are the most widely used
developers for both film and paper. This agent can cause skin rashes in some
people.
Metol by itself is a soft-working, low-fog agent, and with the low alkalinity of the sodium sulfite plus its anti-oxidant qualities, D-23 is a formula that is almost guaranteed not to block up high values even when long development times are required. This is the developer Ansel Adams chose to develop his " Moonrise, Hernandez, New Mexico" negative in, because he was concerned that the moon would come out overdeveloped and not show any detail. He later said if he had known how dark the foreground area was he would have given the whole negative another stop of exposure--but under normal circumstances, where correct exposure is given, D-23 renders excellent shadow detail. (Adams couldn't find his exposure meter in his hurry to make the shot before the sun set behind him. He later used a silver-based intensifier to enhance the foreground of this famous negative.)
A dilute metol developer restrains highlight development more than shadow development.
Hydroquinone
(p-Dihydroxybenzene) is a
clean-working, non-staining agent that is readily soluble in warm water
(somewhat less so in cold), and is extremely sensitive to the effects of low
temperature and bromides. In highly alkaline solutions it produces a high
speed, high contrast developer suitable for process work. In combination with
metol or
phenidone, and carbonate, it produces a virtual universal developer which
can be adjusted for almost any use.
Phenidone
(1-phenyl-3-pyrazolidone) is a
clean-working, non-staining, highly active agent that is only moderately
soluble in hot water. Due to its high activity it requires considerable
restrainer-- usually benzotriazole in
addition to potassium bromide. In
carbonate solution it produces a fast but soft working developer. With
borax or
sodium metaborate it makes a fine grain developer. In combination with
hydroquinone it becomes a normal
contrast all purpose developer with excellent keeping properties. It has taken
the place of metol in many formulas because it
is much less likely to cause skin rashes. FX 37 is an
outstanding PQ non-solvent film developer, particularly with T-grain films,
that utilizes phenidone's
speed-enhancing properties.
p-Aminophenol hydrochloride
p-Aminophenol hydrochloride is much better known by its trade name Rodinal.
It is a fast-working, fog-free, stain-free agent that keeps extremely well and
produces negatives of very high acutance. It is used primarily in highly
concentrated pre-mixed solutions, and is rarely found in home darkrooms because
pre-mixed Rodinal remains cheap and readily available.
Rodinal produces a unique gradation between that of D-76 and the extreme
sharpness of FX-1. Rodinal 1:50 with Agfa APX 400 makes a good general
combination. Rodinal at 1:100 with Agfa 25 is highly recommended for landscapes.
Amidol
(2,4-Diaminophenol hydrochloride) is a low-fog agent which dissolves easily
in water and sulfite solutions, and is moderately sensitive to bromides. It is
toxic, stains the fingers with oxidation by-products, and does not keep well,
though its keeping properties may be improved by the addition of lactic or
citric acid. Amidol was the paper developer of choice of Edward and Brett
Weston, and is still cherished by many fine-art photographers. It was famous
fifty years ago for the blue-black tones it produces on bromide papers. Amidol
possesses the unusual quality of being an active developing agent in a sulfite
only solution, without the addition of carbonate, and even remains active in
slightly acidic environments. It is also said to begin its development in the
depth of the emulsion rather than on the surface as do most agents.
The mystique of amidol endures, despite the difficulties involved in its use. The chemical is quite poisonous, doesn't keep, stains fingers and clothing, and is the most expensive developing agent still in use. Adams never used it much, stating that he felt he could get as good or better results with other formulas, though Edward and Brett Weston used it all their lives and a number of fine art photographers continue to maintain that it has certain unique but almost undefinable qualities that make it the most desireable of developing agents for papers.
Earlier this century Amidol was prized for the blue-black tones it produced on pure bromide papers. Most modern papers are mixed chloro-bromide emulsions, each one differing slightly in composition, and only experimentation will reveal what sort of tones amidol may produce on each one. Typically, however, it gives a rather greenish hue on chloride and chloro-bromide papers, particularly if restrained with potassium bromide. Selenium toning will transform this to a warm purple-brown. Agfa Brovira is said to be the only pure bromide paper still in production, and amidol formulas do indeed give cold blue-black hues with this paper. [Brovira has been removed from the market since this was written.]
Amidol has also long been famous for the depth of its blacks. Modern tests indicate that there are several formulas that produce blacks that are equal to or greater in density than those produced by Amidol formulas, but true afficionados of amidol know that subjective affect is much more important than scientific proof when it comes to art. In truth, prints made in amidol give an impression of greater depth, and have remarkably good tonal separation.
Almost every manufacturer of chemicals and paper has produced an amidol formula at one time or another, and the vast majority of them give very similar results. One modern formula stands out from the rest--that of Samuel Fein. Mr. Fein chose to use benzotriazole as an anti-foggant rather than potassium bromide, and to add a significant quantity of citric acid. The citric acid prevents premature oxidation of the developer and eliminates unwanted stains, while the benzotriazole gives a much colder hue, even with chloro-bromide papers. This is definitely a developer worth experimenting with.
Amidol solutions do not keep and must be mixed just prior to use. However, everything but the developing agent may be pre-mixed. Amidol in its pure form is a white, fluffy crystalline powder. As it ages it turns grey, then black, and becomes increasingly difficult to dissolve fully.
Gloves must always be worn when working with amidol solutions, and the developing agent should never be added to a hot solution, as it gives off noxious fumes. It is wise to leave a half-inch to an inch margin around the print because tongs or gloves which are dipped in amidol developer and then exposed to air tend to stain the print.
Pyrocatechin
(o-Dihydroxybenzene), also known as
Catechol, is a toxic, staining, low-fog agent that is only slightly
sensitive to bromide in its most common solution with
sodium hydroxide. It is considerably
more susceptible to the influence of bromide in carbonate solutions.
Pyrocatechin oxidizes very readily,
particularly in the absence of a sulfite preservative; the oxidation
byproducts have a tanning effect on the gelatine of the photographic emulsion,
which tends to harden and stain it. This tanning effect is proportional to the
amount of silver reduced and is therefore greatest in the highlight areas which
have received the greatest exposure. Local tanning and hardening tends to
inhibit further developing action in the area, making
pyrocatechin an ideal compensating
developer. The tanning effect also prevents development below the surface of
the emulsion. User's report that T-Max 100, Agfa 100, and Agfa 25 do not
show as stain as other films.
Pyrocatechin seems to be the preferred name for this agent, but catechol is sometimes more convenient as it prevents confusion with pyrogallol. Because of the tanning effect of catechol on emulsion gelatine, it has many of the properties of pyrogallol, including high acutance and low fog. Both are essentially surface developers: the hardened emulsion prevents development of halides below the surface, thereby reducing halation effects. Halation is caused by stray light bouncing around inside the gelatine layer and exposing halide crystals that would not otherwise receive exposure, but halation takes place almost exclusively in the depths of the emulsion and so its effects are largely negated by surface developers.
Catechol is a much underutilized agent, and formulas for it are rarely found. Virtually its only use is as a compensating developer for subjects with extreme scales in the ten to fifteen stop range. Such extreme exposure value ranges are actually quite common in the Southwest. Ansel Adams gives an excellent two-solution catechol formula in his book The Negative. He states that it reduces emulsion speed considerably in the low values, so that it is necessary to give twice the normal exposure.
The staining effect of catechol is somewhat unpredictable, depending upon the age of the dry chemical, the age of the mixed solution, and numerous environmental factors. The stain also makes it difficult to determine correct exposure of the negative, short of actual printing. Bracketing of exposures is recommended for those inexperienced with the formula, as is development by inspection. Pyrocat-HD is a semi-compensating, high-definition developer, formulated by Sandy King as an alternative to PMK.
Pyrogallol
(1,2,3-Trihydroxybenzene), also known as
Pyro, is a staining agent that is quite toxic, produces low fog, and is
moderately sensitive to the addition of bromide. Used by itself it is very soft
working, but with the addition of metol it
produces normal contrast negatives. It oxidizes readily and the oxidation
byproducts tan the gelatine of the emulsion, producing a distinct yellow-green
stain in direct proportion to the amount of silver halide reduced. This has the
effect of somewhat masking image grain and improving detail in the high values.
Once widely used (notably by Edward and Brett Weston, among many others),
pyro formulas fell out of vogue due to the
difficulty of controlling the staining effect.
Gordon Hutchings'
PMK (pyro-
metol-
kodalk) formula gives very good control over the staining action and has
brought pyro into widespread use once again,
particularly by fine art photographers.
Gallic acid was used by Fox Talbot as early as 1840 to sensitize as well as develop his paper negatives, but it proved to be an extremely slow developing agent. By 1838 it had already been discovered that when gallic acid is heated to 200-250°F it becomes more active. In early chemistry substances were often heated to see what new properties they might acquire, and these heated substances were then prefixed with the Greek word pyro, meaning fire. So heated gallic acid was referred to (incorrectly) as pyrogallic acid. Actually, heating gallic acid liberates carbon dioxide, leaving 1-2-3 trihydroxybenzene or pyrogallol. Pyrogallol found use as early as 1851 in the collodion wet plate process, and continued to be used for the rest of the 19th century and well into the 20th in both wet and dry processes. Gordon Hutchings goes so far as to say that pyrogallol was the most popular developing agent of the 19th century, despite the many difficulties associated with its use. Though it was famous for its tonal gradation and rendering of fine detail, it was also known for its tendency to oxidize rapidly and cause mottling, streaks, and stains.
With the discovery of hydroquinone in 1880, metol in 1891, and the subsequent invention of metol- hydroquinone formulas, pyrogallol began to fall into disfavor. While it was discovered that metol could be added to pyrogallol to increase film speed and contrast, inconsistent staining of the negative by oxidation byproducts continued to be a major problem; and, though non-staining formulas were developed, it was quickly discovered that many of the advantages of pyro over other developing agents resulted from the very stain that had been eliminated. Pyro continued to be used by a few fine art photographers and portraitists who appreciated its unique qualities, and by a few press photographers who required the speed of development delivered by certain pyro-metol formulas, but for popular use with miniature films (i.e., 35mm) pyro was completely supplanted by metol- hydroquinone.
Gordon Hutchings appears to have done more research on pyro than anyone in recent history, and his The Book of Pyro is definitive. Hutchings has developed a formula that maximizes the stain, but allows it to take place in a controlled manner, within the emulsion and after development. His PMK formula (which stands for pyro-metol -kodalk), is now justly famous and continues to gain adherents among fine art photographers.
The advantages offered by pyro are many, and far outweigh the disadvantages. The yellow-green color of pyro's stain adds effective density to the negative (which can only be accurately measured with a color densitometer), but unlike mere overdevelopment, which tends to increase graininess and mask fine detail, the stain serves to fill the gaps between silver grains, thereby partially masking the grain and providing a uniquely smooth tonal gradation. PMK is unequalled in its ability to effectively render fog and mist. Due to the density added by the stain, development can be reduced somewhat to allow for translucent negative highlight values, thereby producing detail in the high values that cannot be acheived with any other developer. Reduced development in turn restricts the possibility of contagious development and virtually eliminates fog. Pyro's tanning effect on gelatine helps to reduce grain migration, and also inhibits the diffusion of bromide out of the emulsion. This gives the pyro-metol form enhanced adjacency effects and extremely high acutance-the detail it renders sometimes seems almost unreal. Because, with PMK, the staining action takes place within the emulsion rather than as a result of oxidation in the developer, it is directly proportional to the amount of silver reduced and the unwanted, random stain of older pyro formulas is eliminated. The stain and tanning of the emulsion allow extended inspection times.
Pyro formulas oxidize rapidly and must be mixed just prior to use, but PMK is a simple two-solution formula that keeps very well. Pyro is quite toxic-long term exposure can cause kidney disease and is probably carcinogenic. It is possible that Edward Weston's early death was the result of long-term exposure to pyro and amidol--there is a photograph of him by Ansel Adams that shows his fingernails black with chemical stains. But it is easy to over-react: simple kitchen gloves and eye-wear are adequate protection when handling pyro solutions. An occasional few drops of dilute solution on the skin can be flushed with water and are no cause for alarm. Long-term exposure is another matter, and direct ingestion of even a gram of the pure chemical would certainly be fatal. For those who don't wish to handle crystalline pyrogallol, PMK is now available pre-mixed from Photographer's Formulary.
Glycin
(p-Hydroxyphenyl glycine) is a
slow-working, low-contrast, long-lasting agent that is quite sensitive to the
addition of bromide. It held a brief vogue as a fine grain film developer, but
has primarily been used as a warm-tone paper developer in combination with
metol and
hydroquinone. It is rarely used today, but still offers beautiful gradation
and unique print color.
Glycin is a less expensive developing agent than it seems at first glance because it is very long-lasting in solution. Ansco 130 will keep for many months and can be used again and again. As it ages, the solution oxidizes somewhat and turns darker. It is still perfectly functional as a developer, though it tends to produce less contrast as it ages, and prints should always be developed face up to prevent unwanted stains. The hydroquinone in the formula breaks down and ceases to produce its superadditive effect with the metol, but the metol and glycin continue to function long after the hydroquinone is exhausted. Ansel Adams probably got the idea for his variant from using Ansco 130 well beyond the effective life of the hydroquinone.
Chlorquinol, p-Phenylenediamine, and Phenylene-diamine
There are other well known developing agents, including Chlorquinol
(Chlorhydro-quinone), p-Phenylenediamine (1,4-Diaminobenzene), and its variant
o-Phenylene- diamine--these agents are virtually obsolete for home darkroom
use, though they still find commercial applications. There is also a chemical
of the naphthalene series (a double benzene ring) known as Eikonogen
(1-amino-2-naphthol-6-sulphonic acid) which has long been obsolete as a
developing agent but which may occasionally be encountered in very old books.
The function and importance of the reducing agent in the developer have both been mentioned-but there are three other components which also play an important role in any developing solution. The first of these is the alkali-which is ordinarily essential for development. Most of the reducing agents are neutral or slightly acid in their normal state, and in this condition give little or no developing action. However, when an alkaline salt like sodium carbonate is introduced into the solution containing the reducing agent, a very interesting change takes place. The reducing agent forms what is called an alkaline salt which in a photographic sense is a more active material, and it is this alkaline salt of the reducing agent that actually reduces the exposed grains of silver salt to metallic silver. The alkali has a secondary effect in the developing solution which is also important. It helps the gelatin emulsion to swell and thus facilitates the penetration of the developing solution throughout the network layer of the emulsion.
It is obvious that the alkali is an important component of the developing solution and is likewise evident that care must be exercised in using the right kind and correct amount of alkali. Sodium carbonate is normally recommended though potassium carbonate is sometimes used in its place. A developer containing a large amount of carbonate can cause carbonic gas bubbles to form in the emulsion of film or paper when trasferred to an acetic stop bath. The caustic alkalis, sodium hydroxide and potassium hydroxide, should not be substituted unless definitely specified, as they are much stronger and can easily cause fog. Normally they are used only in special-purpose developers giving high contrast. Borax and similar alkalis which are less energetic are often specified for fine-grain development in which grain size must be controlled by softer development. Another alkali used for photographic work is sodium metaborate which is helpful in reducing blister formation where it is difficult to control the temperature of processing solutions during hot weather.
The amount of alkali should be weighed accurately to the amount specified, as too much may cause fog in developed negatives; too little may result in slow, soft development. It is important to remember when using carbonate that the potassium salt is generally available only in the anhydrous form, while the more generally used sodium salt can be obtained as (1) the anhydrous (desiccated) salt containing about 2% water, (2) the monohydrated salt containing about 1% water, or (3) in crystal form containing about 63% water. The anhydrous and crystalline forms are both unstable at ordinary conditions of temperature and humidity, and must be kept in tightly sealed containers and used with great care to prevent considerable absorption of water from the atmosphere by the anhydrous salt, or loss of water by the crystalline form. The monohydrated form of sodium carbonate is stable and therefore preferred by most photographers for accurate preparation of developing solutions.
THE PRESERVATIVE
It is characteristic of many photographic reducing agents in alkaline solutions to combine freely and easily with oxygen, Because of this "hunger" for oxygen, alkaline solutions of the developing agents spoil very quickly when exposed to air. To increase their useful life, to allow the developing agent to do its work on the exposed silver halide as desired, and to prevent the occurrence of stains, a preservative must be added to the developing solution,
Sodium sulfite is ordinarily used as the preservative, though in developers prepared for stock in two solutions, preservatives which are slightly acid in solution such as sodium bisulfite and potassium metabisulfite are preferred. Because developing agents keep better in acid solution than in one which is alkaline, it is common practice to use one of these acid sulfites as the preservative in the developer part of the stock solution. There is no practical difference between sodium bisulfite and sodium metabisulfite in most photographic formulas. In single-solution developers, sodium bisulfite is never used alone as a preservative, since it neutralizes some of the alkali in the solution and would result in softer development. One other interesting point about preservatives is that in some cases the preservative performs a secondary function in the developer, In some fine-grain developers, for instance, a large amount of sodium sulfite is used to aid in keeping grain size at a minimum. T-grain films appear to perform best when the working solution contains less than 35 g/L of sodium sulfite.
THE RESTRAINER
The fourth and final important component of the typical developing solution is the restrainer, potassium bromide. This necessary constituent of the developing solution acts as a "brake" on the chemical reaction of development and keeps the operation under control. The action of the restrainer is such that an increase in the concentration of potassium bromide in the developer tends to slow down or "restrain" the development of the photographic image. The concentration of potassium bromide in the solution is obviously important, for too much may retard development excessively and indicate an apparent loss of speed, while too little may permit development of fog. Bromide is commonly used in neutral to warm tone paper with more bromide for a warmer tone.
Benzotriazole has greater antifogging action, particularly in high pH developers. It is often used when blue-black tones are desired. Benzotriazole is often used in medium to high alkiline developers, where one-tenth the amount of specified bromide is substituted.
Potassium Iodide as a restrainer has not been thoroughly investigated. It may prove to be superior to bromide. If you wish to experiment, generally 1/10 to 1/00 the weight of bromide is used. Iodide is sometimes used in combination with bromide.
DEVELOPER EXHAUSTION
The chemical reaction of development results in a depletion of certain constituents of the developing solution so that with continued use the developer becomes less efficient. This "exhaustion" of the developer is characterized by a loss in effective speed and gradation of the photographic emulsion (of importance in both film and paper development) and by a change in tone of the developed image (of special importance in making prints). In consequence of this condition, it is standard practice to use fresh developing solution whenever possible, as it is good insurance of obtaining uniformly optimum results with photographic films and papers.
There are, however, occasions when a rather large quantity of developer must be put in use, as in the tank development of films, and in such circumstances it becomes desirable, for reasons of economy, to prolong the usefulness of the developer by the addition of a "replenisher" solution which replaces solution carried away on developed films and helps restore the balance of active ingredients in the solution.
By the occasional addition of such replenishers to maintain a constant volume of solution in the developing tank, the useful life of the developer can be prolonged three to four times without seriously degrading the quality of developed negatives. If large amounts of replenisher are to be added at any one time, the activity of the solution may be so increased that developing time will have to be shortened excessively, unless the replenisher is diluted somewhat with water.
The exhaustion characteristics of a developer depends largely on the three following factors which should be considered when interpreting data on exhaustion characteristics.
Developing times listed with formulas as well as time temperature compensation methods previously described, are based on effective agitation of the film in the developing solution. Effective agitation can be considered to be any method which provides a continual flow of solution across the surface of the film, but for practical considerations an intermittent form of agitation can be employed which will adequately remove development byproducts and supply fresh developing solution to the film emulsion. Such a method requires actual movement of the film in the developer, or developer over the film for 5 seconds out of every minute, and can be achieved by rocking the tray in tray development, or by agitation of the film in the solution when tank development is employed. The important point is that a repeatable method of getting effective agitation should be established if uniformly excellent results are to be obtained in film development.
THE IMPORTANCE OF A SHORT STOP
As negatives or prints are removed from the developing solution, they carry with them considerable amounts of alkali and other chemicals which can contaminate the fixing bath and interfere with its action. Used-up developer carried along with negatives and prints can also cause troublesome stains if some method is not used to stop development instantly and completely. The best and most reliable way of doing this is the well-known short-stop bath of dilute acetic acid which neutralizes any alkali remaining on negatives or prints and prevents contamination of the fixing solution. Yet it is surprising how many photographers still try to get along without this intermediate bath between development and fixation, it is true that an acid fixing bath will give satisfactory results without the use of a preliminary short-stop bath, but it's useful life is severely limited when a short-stop is not used.
Photographers frequently ask why acetic acid is used for the short-stop bath and fixing bath instead of other common acids like hydrochloric or sulfuric. The answer lies in the fact that a relatively large amount of acid must be available but the solution must not be too strongly acid. Consequently a compound is used which is weak in acidity but which has available a high reserve of acid to neutralize alkali. A correspondingly larger amount of the weak acetic acid may therefore be used than could be used of a strong acid.
COMPOSITION AND FUNCTION OF THE FIXING BATH
The procedure of fixation is relatively simple but is should be carried out with considerable care, as it can be the source of much trouble when improperly handled. The photographic film negative upon removal from the developing solution is still sensitive to light, as it contains undeveloped silver salt in the shadow portions of the image. To make the negative image permanent by removing this undeveloped silver salt, as well as to make it clear and transparent for printing, the action of the fixing bath must be employed. The principal constitute of the fixing solutions is sodium thiosulfate, more commonly known as ''hypo'' (for its older name of sodium hyposulfite), for in solution this useful chemical has the property of dissolving light-sensitive silver salts. The method by which the silver salt is removed is generally considered as, first, a conversion to a soluble double salt by the hypo, and second, the washing out of this soluble salt with water.
The conventional fixing solution generally contains other chemicals in addition to the hypo. Acetic acid is often included to aid in regulating the acidity of the fixing solution and to prevent stains. However, a hypo solution containing much acid is apt to precipitate sulfur, so another chemical, sodium sulfite, is added to prevent this unwanted reaction.
An additional component of the usual fixing bath is the hardening agent which prevents frilling and softening of tire gelatin emulsion. White potassium alum (potassium aluminum sulfate) is usually employed for this purpose, though some photographers prefer potassium chrome alum used with a small amount of sulfuric acid. Care must be used with chrome alum as the hardener rapidly loses its strength and is only truly effective when a fresh solution is used. The fixing bath will seldom, if ever, give trouble when properly prepared from pure chemicals, if a bath turns milky after preparation, it indicates that sulfur is precipitating because of too much or too strong an acid, too little sulfite, too high a temperature of the solution, or improper mixing. A milky appearance of the bath during use is due to the presence of excess alkali and indicates that the bath should be replaced, it is important not to overwork the fixing bath, because a nearly exhausted fixing solution will not completely remove the silver salts, and prints or negatives will turn yellow or stain on aging.
A gallon of standard strength fixing bath should fix 100 8x10" double-weight prints or their equivalent. Between 100 and 120 rolls of 120 film (or equivalent) may be fixed in one gallon of standard-strength fixing bath if the films have previously been rinsed in a short-stop bath or plain water. When the bath froths or foams, it should be replaced. Many photographers have found that a convenient, certain and economical method of insuring complete fixation lies in the use of two fixing solutions. Fixing is carried out first in the more used of the two baths and finally in the fresher solution. When the older bath becomes exhausted, the partly used solution takes its place and a fresh fixing bath is prepared for the second solution.
MIXING PHOTOGRAPHIC SOLUTIONS
Distilled water is recommended for film developers, as even minute quantities of impurities can cause negative defects. It can be heated on the stove to the necessary mixing temperature, and small quantities can be kept refrigerated for adjusting solution temperatures downward. Tap water is generally sufficient for mixing paper developers.
Some chemicals mix perfectly well at room temperature, but many require hot water to dissolve completely, so most developer formulas call for water at 125°F. Generally one begins with water at 75% of the liquid volume desired, so if one liter of solution is to be mixed, one begins with 750 milliliters of water. After the chemicals are added, the solution is topped up to the desired volume with cold water.
Chemicals should be added to the water slowly, one at a time, while stirring constantly. Each chemical must be thoroughly dissolved before the addition of the next. Tiny bubbles are formed as chemical reactions take place during the mixing process, and the liquid should be stirred until these bubbles dissipate and the solution clears before addition of the next chemical. This requires a great deal of stirring and patience.
Mix all components of a solution in the order listed in the formula. This is extremely important and lack of attention to this point can easily result in the formation of precipitates which will not dissolve in the solution. A worthwhile corollary to this rule is to wait until each chemical is thoroughly dissolved before adding the next component of the solution. In most single-solution developers the preservative sodium sulfite is usually added immediately after the developing agent but before the hydroquinone, if this chemical is used. When two developing agents such as metol and hydroquinone are used, the addition is generally made in the order metol, sulfite, hydroquinone. However, with developing agents like glycin, the sulfite and carbonate are dissolved first, as the glycin dissolves with greater difficulty otherwise.
Developer chemicals are usually mixed in the following order:
The primary exception to the above rule is metol, which is always mixed first because it does not dissolve well in alkaline solutions. Most formularies recommend adding a pinch of sulfite before mixing the metol to retard oxidation. Always mix chemicals in the order given in the formula.
A third important rule for any photographer is to use the purest obtainable, innumerable troubles in developing and fixing have been traced to impurities present in the water. Many photographers find it a wise decision to use distilled water for all stock solutions, adding tap water for dilution.
Store developer solutions in brown glass or plastic bottles, with little or no air trapped in them. Several small bottles are preferable to one large one-the less air a developer is exposed to, the longer it will keep.
Most of the developing agents are somewhat toxic and should not be ingested or breathed. They are all crystalline, but some dust is inevitable, particularly when hygroscopic agents become caked up and have to be scraped from their containers. It cannot be emphasized enough that dust from developing agents should not be breathed. Darkroom workers should make every effort to minimize the production of dust, and should aways wear a mask when mixing developers.
Concentrated acids and caustic alkalis should always be handled with the greatest care. Though such chemicals are rarely used in the modern darkroom, they are not unknown-acid proof gloves are strongly recommended. Generally speaking, water should not be added to concentrated acids because they tend to boil and splatter acid everywhere, though glacial acetic acid is an exception to this rule. Similarly, care should be taken when adding caustic alkalis to water, particularly hot water. Strong acids and bases should not be combined except under expert supervision.
Pyrogallol, pyrocatechin, amidol, and glycin are highly toxic and readily absorbed through the skin--a few drops now and then will not be dangerous, but gloves and eye protection should always be worn when working with solutions of these agents. Stock solutions of pyro-gallol should be mixed under a chemical vent hood or out-of-doors to prevent the concentration of toxic fumes in the darkroom. Amidol should never be mixed into a solution much above 80°F for the same reason.
REFERENCE
The Film Developing Cookbook,
Stephen G. Anchell and Bill Troop.
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