Recent advances in dry silver color

A totally photographic process was developed by 3M in 1964 called dry silver.This process creates high quality, continuous tone images on paper and film byexposing to light and developing with heat (Fig. 1). The silver process isbased on the light sensitivity of silver bromide and the heat developability ofsilver organic salts. The silver bromide that is utilized in silver issimilar to that used in classical photography but with some significant differ-ences. Importantly, only about 1/100 of the silver bromide that is used in wetsilver is needed, and it is not consumed in forming the silver image. The silverbromide remains chemically unchanged during the process that forms metallicsilver; that silver comes from a totally non-light sensitive silver source such assilver behenates. Other organic silver sources can be used; however, the use ofsilver behenate is preferred because it can be prepared in very small particles,dispersed in solvents or water, developed with heat in the presence of developingagents, and can be photo catalytically sensitized. It has the additional featureof being very stable to light. The shape of the silver behenate crystals ormicelles is quite unique. They are approximately 1 micron in length, 1/10 micronin width, and 1/100 micron in thickness. This could best be described as appearinglike a tongue depressor, and the uniformity of the crystals that can be achievedis excellent (Fig. 2).The process of sensitizing a non-light sensitive silver source is done, as men-tioned above, by silver bromide or other silver halides. This is a criticalaspect of the technology and must be controlled carefully. Traditionally, silverbromide is formed in situ by reacting silver behenate with bromide ions or otherhalide ions. The result of this process is the formation of very fine grains ofsilver bromide, less than 100 angstroms in diameter, and in catalytic proximity tothe silver behenate (Fig. 3).The probable explanation of this effect is suggested by the fact that the moleculesof silver soap are disposed in small particles with their silver ions associatedtogether in distinct planes of layers. The hydrophilic, polar silver ions areoriented toward each other, and the planes of silver ions are separated from otheradjacent planes by the intervening extended organic radicals of the molecules.Conversion of some few of the silver ions of the silver soap from each particle tovery small crystals of silver halide then occurs. The resulting silver halidecrystals are in synergistic association with the silver ions of the remainingsilver soap. Exposure to light causes photolytic reduction at the silver halidecrystal and provides a silver nucleus in position to permit electron migration tothe remaining silver ions under the influence of heat. These silver nuclei thencatalyze the reduction of the organic silver soap by an organic reducing agent atthe elevated temperature employed, to produce a visible change. The nature of thesilver halide crystal, its close proximity to the silver soap, and the presence of