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         In "Basic Chemistry" we discussed the normal sequence of development steps -- developer, stop bath, fix (one or two fixers), and wash.  Here we address the subject of what these chemicals actually are.
         Developers are generally comprised of two developing agents, an "accelerator," and a preservative.  The reason for using two active developing agents is that most developers are "superadditive."  For example, a developer made purely of metol (mono-methyl para-amino phenol sulfate) or purely of hydroquinone is often an order of magnitude less active than one that contains a little of both.  Metol and hydroquinone are by far the most two common developing agents in use today, and probably well over 90 percent of the commercially available black-and-white developers available today are metol-hydroquinone or "M-Q" based.
         It's also not at all uncommon in a developer with two active agents for one to begin working sooner than the other, and for the other to contribute more to development at the end of that stage of the process.  For example, metol is a fairly slow-working developer, but its action begins almost immediately upon contact with exposed film.  Hydroquinone, on the other hand, is an extremely aggressive developing agent (more so than metol); but its developing action usually starts later than metol.  For this reason, extremely short development times with M-Q developers are not recommended.  Further, controlling the lightness or darkness of a final print by removing it from the developer tray when the desired density is reached is a similarly bad idea, since virtually all paper developer formulae are M-Q based.  (Amidol paper developer are among the rare exceptions.)  In almost all cases, photographic paper *exposure* should be adjusted to control print density and the print should be allowed to continue to develop until the process grinds to a halt on its own.  Doing things any other way tends to result in loss of contrast or "splotchiness" in the final print.
         Developers also contain an "accelerator," which raises the pH of the solution enough that the developers themselves become active.  Most developers do not show significant chemical activity (if any at all) until their pH is as high as 9 or so.
         Accelerators tend to be "buffered" alkali agents for the reason that as development proceeds, the accelerator tends to be depleted (H+ ions are released as a development by-product).  An accelerator that is buffered is at least partially self-replenishing.  Those parts of the original accelerator that did not create OH- ions when the developer is first mixed will frequently do so when the pH drops -- pushing the pH back up in the process and reestablishing equilibrium.   By maintaining a stable pH level in solution, the level of activity of the developing agent becomes considerably more predictable, and time-temperature curves for a developer solution can be drawn with some hope of repeatability.
         For the reason that maintaining stable pH is so important, something like sodium metaborate (which is a buffered alkali that can readily push a solution to a pH of 10.8) is a very popular accelerator.  Sodium carbonate (pH 11.5) is another popular accelerator choice.  Strong bases like sodium hydroxide (which are unbuffered and dissociate quickly -- and fully -- to as many OH- ions as will ever appear in solution) tend not to be anywhere near as widely used as accelerators.  Probably the only time that something like sodium hydroxide is used in a developer is when pH needs to be extremely high and development by inspection is a viable option.
         A table of common developing agents and the pH at which they become active appears below:
         Frequently, developers contain a restrainer or "anti-foggant" of some kind, and the purpose of a restrainer is to inhibit developing action on any grains that might not have been exposed to light.  Some of these will develop to black grains simply as a consequence of mechanical stresses in the film emulsion or electrical charges carried by an adjacent, possibly exposed grain.  Because the distribution of these grains in the film is usually fairly random, they tend not to greatly distort image information.  But they do add to negative density in ways that can lower overall contrast or make the negative harder to print.  They can also lower image acutance.
         Finally, many developers generally contain some sort of preservative to prevent the oxygen in the atmosphere to lead to their spoilage.  The common preservatives are sodium sulfite (often used in the preservation of wines as well) and EDTA disodium salt (a common food preservative).
         D-76 is the classic film developer, and it has been formulated and reformulated a dozen or so times over the three-quarters of a century in which it has seen common use.  One of the mainstream D-76 formulae appears below:
         It's worth noting that D-76 will work with just about any film for the fairly simple reason that it has stood for so long as the "reference" developer -- and no film manufacturer will dare release a film for common use that *won't* work with D-76.
         Development times will shorten as temperature increases and constituent chemicals in the developer become more active.  So long as pH can be held reasonably stable with a good accelerant, this increase in activity can be fairly reliably predicted within a narrow temperature band.
         For example, the "activity vs. temperature coefficient" of metol around 70 degrees F ( degrees C) is about 1.3 for every 10 degree F increase.  That is to say a purely metol developer that takes 13 minutes to develop film at 70F would probably only require 10 minutes at 80F.  And because (at least within a limited bands of times and temperatures) the relationship remains fairly linear, one can interpolate times and temperatures in between.  One can also take averages of coefficients for developers that contain more than one active agent (and most do).  For example, the coefficient for metol (as noted) is 1.3 and that of hydroquinone is 2.5.  The coefficient of a 50-50 M-Q developer is, as one would expect, in the vicinity of 1.9.

         Films and papers are comprised of light-sensitive grains deposited on some substrate.  In the case of film the substrate is usually acetate or polyester, and in the case of paper the substrate is most commonly paper -- or in some cases a white polyester sheet.  In the early days of photography "paper" substrates could be materials as unusual as tin (hence the term "tintype"), and film substrates were either glass plates or some nitrate-based material.  Nitrate films were highly flammable, and the early history of photography and cinema is replete with tales of spectacular studio fires.  Combine flammable film with carbon arc lighting or the semi-explosive phosphorous compounds used for early "flash" photography, and you have a formula for disaster.  Much of Ansel Adams early work, for example, burned in a studio fire in Yosemite in 19xx, and the value today of the few surviving prints made from those negatives is understandably very high.
         Today, with almost all films being manufactured on a nonflammable polyester substrate (acetate is considerably less common), studio fires are by and large a thing of the past.  Film may melt in a fire, but it won't contribute to the conflagration; and in large measure it hasn't since the introduction of "safety films" in the 1930's.  To this day, many films still carry the designation "safety film," and even the most cursory glance at the edge of a piece of film may reveal those words.
         The active "salts" or light sensitive grains on film are usually some combination of silver and a halide, or "group VII element" in the periodic table.  (Platinum and palladium can be used in paper and are regarded by many as the superior compounds, but their use is considerably less common than the silver compounds).  The most commonly used salts are silver bromide, silver chloride, and silver iodide, all of which have a "simple cubic" crystal structure.  Of the three compounds, silver chloride is probably the least light sensitive, naturally forming smaller grains than the other two compounds and being largely blind to infrared light.  Silver iodide is probably the most sensitive, with an IR sensitivity extending well beyond that of AgCl and a propensity to naturally form larger grains (which present larger targets to an incoming photon of light).  The sensitivity of AgBr is somewhere in between in both regards.
         Below is a table of IR sensitivities of the three common silver salts.
         Note that the addition of AgI grains to an emulsion is a relatively recent (read: "mid-20th century") technical development in the history of photography.  Before the addition of AgI, a great many films were termed "red blind" due to their lack of infrared (and in some cases red) sensitivity.  Such films also earned the moniker of "orthochromatic."
         "Panchromatic" films make use of silver iodide, and their sensitivity usually extends to the edge of the infrared -- and beyond in the cases of those films designed specifically to be "infrared" films.
         All these films (red-blind, red-sensitive, and infrared) have their uses, and the great images have been made on all three.  Red-blind (orthochromatic) films are particularly good for portraiture because they tend to hide blemishes (which are usually redder than the surrounding skin).  Alas, few manufacturers make much of an effort to develop or sell orthochromatic films any more, so the common portraiture technique in this day and age is to roughly approximate their effect in B&W by photographing through a green filter or by using green gels with studio lighting.
         Panchromatic films see the widest use, being the films that most nearly approximate the spectral sensitivity of the human eye.  Infrared films, once used primarily for scientific photography, are now finding a niche in fine art photography, largely for the unusual (at least to human eyes) tonal response they exhibit.  Infrared film is a medium that can capture some truly brilliant, radiant nudes, since the body itself radiates in the infrared.
         Much has been made of the virtues of keeping grain size consistent in a film.  (Witness all the hype that accompanies T-grain films.)  And there's no question that the response of a T-grain film to light is considerably more linear than that of a film with more uneven grain size.  What the T-grain proponents never tell you is that the uniformity of grain size also produces higher contrast -- not all of which is desirable.  T-grain films are particularly bad for portraiture (T-grain based color films are an exception, and we deal with that issue in the "Advanced Chemistry" section of this website) because the exposure zones of interest are clustered fairly tightly around Zone VI.  The loss of subtle contrast information in the region between zone IV and zone VIII is highly detrimental to most portraits.  Aspect ratio of the T-grain seems also to have an impact on the suitability of the film in question to portraiture.  Ilford "Delta" films (with a 1:6 grain aspect ratio) seem to be capable of creating decent portraits with tightly controlled studio lighting and "N-minus" development, although it's worth noting that a lot depends on the developer chosen and the complexion and coloring of the subject.  But the "Delta" are unsuitable for all but the flattest of ambient light when shot on location.  Kodak "T-Max" films, with their greater (1:9) aspect ratio are just about hopeless for portraiture under all shooting conditions, though they are excellent for certain kinds of reproduction work and black-and-white reversal chemistries.  I just count myself among the vast majority of professional photographers who won't shoot an original negative onto T-Max.
         Aside from silver halide composition, another matter that distinguishes films from one another is the gelatin content of the emulsion.  Since it's recognized that gelatin has the capacity to distort the image, in recent times it's become fashionable to lower gelatin contents as much as possible.  (It also lowers the lowers the manufacturing cost as well.)  There is however, a down side to lowering gelatin content if the film is developed in a "tanning" developer such as pyrogallol or pyrocatechin.  The highly desirable yellowish-green stain on the negative that is the hallmark of pyro depends largely on having enough gelatin in the emulsion to accept the tannins in the developer.  Without gelatin, the stain is almost completely lost.
         Throughout the history of photography, certain films have become all the rage or "cult classics."  For example, I've heard the tale of a photographer who convinced Kodak to do "one last run" of the cult film "Super Double-X" by dangling a $10,000 order under their nose.  I understand he now has a house whose square footage is divided neatly in half -- between real living space and bathrooms and utility closets full of Super XX piled floor-to-ceiling.
         The honest truth is that there is no holy grail of film.  There are a lot of good films out there that when carefully characterized (see "Characterizing Film") to determine their true speeds and sensitivities (and not the one the manufacturer would like you to believe) can deliver exceptional performance.  It's all a matter of doing your homework, choosing your film carefully, and matching it to a suitable developer and development process.  The likelihood that I'm going to pile up a mountain of anything like Super XX any time soon is pretty low.