Solution Physical Development - An Experimenter’s Delight
Harvey W. Yurow, Ph.D.
In black and white photographic processing, direct development may be thought of as the reliable work horse, while solution physical development is best considered as the exhibition horse kept for special events. Most development procedures are combinations of the two, with direct development usually overshadowing its showy cousin. However, on certain occasions, solution physical development can predominate, with quite remarkable results. The following sections are designed to give the reader a basic understanding of solution physical development, allowing exploration with confidence of certain unfamiliar pathways in photography.
A. Mechanisms Of Direct Development And Solution Physical Development.
In direct development, silver is formed from silver ion supplied directly from the silver halide grain. By contrast, with solution physical development, a silver ion complexing agent, such as thiourea, which dissolves silver halide from the grains, can be the major source of silver ion for reduction. The principal uses of solution physical development are for production of warm tone or colored images, and for fine grain negatives.
In the table below are listed a number of development techniques that operate to a significant degree by solution physical development.
For the Waterhouse developer, the phenylthiourea-silver bromide complex formed decomposes in unexposed areas where no direct development has occurred, to give silver sulfide development centers, which undergo both direct and solution physical development.
The equidensity developer operates on the same principal as the Waterhouse developer. However, the presence of dichlorohydroquinone impurities in chlorohydroquinone give a considerably greater degree of solution physical development and a brown positive and blue negative equidensity image, along with a border effect.
Mees’s and Carrell’s thiourea developers appear to operate mostly in the solution physical mode, because of the relatively high concentrations of complexing agents. (Milner) Wellington’s developer is similar, but without thiourea, so that only red and brown tones are obtained.
The Burki and Jenny formula (Glafkides) contains ammonium sulfate, so that solution physical development is favored, while with Agfa 123, this is accomplished by the relatively high potassium bromide concentration.
For Jolly’s duotone Sabatier process, a normal first developer gives black tones, while a second developer, following exposure, contains excess potassium bromide and gives brown tones.
Sease #3, and Kodak DK-20 and D-76 were introduced in the early days of miniature camera films as fine grain developers to permit a maximum degree of enlargement.
The Russian hologram developer (Bjelkhagen) uses a relatively low concentration of developing agent, to allow for a slow rate of development, while the discontinued Agfacontour had a silver chloride emulsion and a high sodium sulfite concentration developer.
Solution physical development is the basis of both Rott and Weyde’s silver diffusion process and Land’s Polaroid technique. In essence, undeveloped silver halide in the exposed negative layer is dissolved by thiosulfate and migrates to a positive layer, where it is reduced on silver or silver sulfide development specks.
Silver chloride emulsions tend to give warmer tones than silver bromide emulsions in photographic papers, because the former salt is more readily complexed by sodium sulfite in the developer than is the latter. Mixtures of the two, e .g. chlorobromide papers, give increasingly warm tones as the percent of chloride increases. By contrast, Current found that processing of chlorobromide papers with developer containing 20-80g/l of potassium bromide for extended periods gave colder tones. This resulted from partial conversion of silver chloride to silver bromide prior to reduction. As regards film, coarse grain silver bromide emulsions are less amenable to complexation and resulting warm tones, as compared to fine grain silver bromide emulsions. Fine grain emulsions tend to produce fewer filaments, which favors warmer tones. Coarse grain emulsions are found amoung the very high speed negative films, while those with fine grain include high resolution process films and lithographic films. Fine grain silver bromide negative emulsions have particle size range 0.2-0.6 nm, while positive silver bromide emulsions have particle size range of 0.05-0.15 nm. (John and Field)
C. Developing Agent Characteristics And Image Tones.
In the table below are listed the common developing agents and relevant properties. In general, warm tones are given by “lower activity” developing agents (Lowe), i.e., those with both relatively low bromide potential values (Nietz) and relatively low Watkins factors (Neblette).
Bromide potential refers to the amount of potassium bromide that must be added to developer to produce a specified decrease in activity of a developing agent. Watkins factor, when multiplied by the time of first appearance of image with a developer, gives the time required for the image to build up to normal contrast.
In this connection, Nietze and Huse found that the most satisfactory developing agent to produce sepia tones on a contemporary chloride paper was chlorohydroquinone.
D. Complexing Agents For Silver Ions In Developers.
The following complexing agents are used in developers to promote solution physical development. (James) For a given complexing agent, the smaller the dissociation constant, the lower the concentration required to dissolve silver halide at a given rate.
James, and Rott and Weyde have noted that solution physical development depends upon the concentration of uncomplexed silver ion in the developer (pAg+), not complexed silver species.
E. Development Conditions Affecting Solution Physical Development.
The rate of solution physical development is determined by the rate at which silver halide dissolves. (James) This rate in turn is dependent upon 1) concentration of complexing agent, 2) its silver stability constant, 3) total surface area occupied by silver halide grains. (Mason, Rott and Weyde) Area can be replaced by number of grains x average surface area of each grain. Fine grain, thin emulsions would be expected to give the greatest amount of solution physical development, all else being equal. Complexation occurs to the greatest extent in unexposed or lightly exposed areas. However, James indicated that silver bromide grains in the immediate vicinity of developed silver show more partial dissolution than corresponding grains in areas which did not contain developed silver.
F. Silver Structure And Image Tone.
Mie calculated that the finest spherical particles appeared yellow (absorption at 425 nm) and with larger particles, peaks shifted to longer wavelengths, so that the sequence was yellow, yellow-orange, orange, orange red. According to Koerber, sizes have the strongest influence on color in the range 10-50 nm. Gans extended Mie’s calculations to ellipsoidal particles, which should give a peak near 400 nm, and a second peak at a longer wavelength. As the particles become more elongated, the peak near 400 nm becomes weaker, and the second peak increases in intensity, and to longer wavelengths. Because of the wide variety of shapes and sizes of silver particles, a wide variety of colors could be obtained. (James, James and Vanselow, Skillman and Berry, Baker and Davidson, Rott and Weyde) The following table due to Wiegel for silver hydrosols is illustrative.
This color sequence is observed with the Mees thiourea developer (Glover), probably indicating that color change is due to increase in size of spherical silver particles with increased development time.
In gelatin emulsions, individual smaller silver particles can congregate together to give aggregates, which have increased size and concurrent change in color. James indicated that in fine grain emulsions, which tend to form very few filaments per grain, large increases in the size of the developed grain can result from solution physical development of silver from unexposed grains onto developed grains. Tone may also depend to some extent on the surface structure of the silver filaments or on adsorbed substances present. Warmest tones correspond to a very loose structure of the mass of filaments, and become more black as the filamentary structure becomes more compact . As the amount of solution physical development occurring along with direct development increases, filaments become thicker and shorter.
In the Land Polaroid silver diffusion process, (Neblette) early sepia prints were composed of single crystals of silver in the range 50-100 nm, spaced at least as far apart as the crystal diameter, or loosely packed aggregates of very fine crystals. Later images were produced as black when crystals were greater than about 100 nm (e.g., 150-300 nm), or when crystals or aggregates of crystals were tightly packed together.
The growth of compact arrays of large aggregates (100 nm or more) accounts for the spectral neutrality of the image , which requires that the fine particles within each aggregate be in contact or close proximity, acting together as a single unit of appropriate size and conductivity. Acting individually, 10-30 nm particles would produce yellow or orange images.
In summation, color of deposited silver can vary with:
Developers Exhibiting Significant Solution Physical Development
LIST OF REFERENCES
“Agfacontour Professional in Photographics”, Agfa, Leverkusen, West Germany, n.d.