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Specific Problems of Silver Dye-Bleach Photography - Unsatisfactory Color Reproduction, Silver Sludge Formation in the Developer

blue layer yellow development

Unsatisfactory Color Reproduction

The color reproduction of the simple six layers of silver dye-bleach material is insufficient for some applications; mainly in colors located in the blue tints. The reason is the unwanted side absorptions of all image dyes: the high absorption of magenta in the blue spectral range between 400 and 500 nm and of cyan in the green and blue range.

A pure and saturated blue reproduced on such a simple silver dye-bleach material (dashed line) is quite dark, because it has a considerable light absorption (between 400 and 500 nm) due to the side absorption of the unbleached magenta and cyan. If this dark blue is copied a second time on the same material, the blue shifts even more toward black, because the transparency for blue light is further reduced. Every copy thereafter results in a supplemental shift toward black.

A color masking system based on the diffusion transfer of silver thiosulfate is a reasonable solution. An automatic correction of this problem, a so-called self-masking system to increase the color reproduction, needs to depend on the blue speed on the green and red exposure. The blue speed has to be inversely proportional to the green and red exposure. This interdependence can be managed by chemical means.

As mentioned above, the yellow filter layer of a self-masking silver dye-bleach material contains a small amount of colloidal silver metal. The blue-sensitive emulsion in the yellow layer is pure silver bromide, whereas the green- and red-sensitive emulsion contains a certain amount of silver iodide.

A small concentration of sodium thiosulfate is added to the developer. Thiosulfate is a strong complexing agent for silver. A minor part of the AgBr is dissolved during development and diffuses down as a silver-thiosulfate complex in the intermediate layer containing some colloidal silver. There the complex is reduced to metallic silver under the catalytic influence of the colloidal silver nuclei (known as physical development). The supplemental silver formed in the yellow filter layer contributes to yellow bleaching in the following bleach bath and increases the blue speed of the system. It is important to remember that distant bleaching is blocked by an empty gelatin interlayer but is possible into a direct neighboring layer.

Iodide is liberated during the development of the magenta layer, and these ions diffuse up in the yellow filter layer, where they efficiently block the colloidal silver nuclei and stop their catalytic action of the physical development of silver.

The silver diffusion transfer from the blue-sensitive yellow layer into the yellow filter layer is possible when the magenta layer remains unexposed and, consequently, there is no development and no liberation of iodide ions. Increasing exposure of the magenta layer producing increasing iodide concentration on development slows down the physical development and reduces the blue speed.

Figure 49 demonstrates the following basic facts:

  1. The yellow contrast for pure blue exposure is enhanced, giving more brilliant saturated blues. (The dashed line shows the contrast for non-masking processing.)
  2. The yellow density for full green exposure is enhanced, giving yellowish, warmer green hues.

The influence of the development of the cyan layer on the silver diffusion transfer is comparable but weaker.

One negative consequence of masking by silver diffusion transfer is a sluggish silver bleaching process. The addition of thiosulfate to the developer to get physical development in the masking layer provokes a more compact form of all developed silver. Another negative consequence is the probability of a formation of silver sulfide traces at the surface of the silver grains. This occurs because of the instability of silver thiosulfate in acid solution. Both effects reduce the reactivity of silver.

The bleaching is delayed further by the formation of very insoluble silver iodide on the surface of the silver metal grains. Silver iodide is one of the products of the bleach reaction. Deposed at the surface, it hinders the access of the bleach catalyst. Filamentous silver remains more reactive; more compact silver particles are strongly inhibited. Overall the bleach reaction is slowed down considerably and has to be accelerated by the addition of a bleach accelerator.

Silver Sludge Formation in the Developer

Sodium thiosulfate in the developer dissolves some silver bromide from the silver dye-bleach material. The silver thiosulfate complex is unstable in the developer and is reduced by hydroquinone in the bulk of the developer solution to metallic silver. The silver precipitate as a black sludge intolerably contaminates the processing machinery and the surface of the photographic material.

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