The silver image in motion picture film is formed as a result of exposure of the silver halide by light and its subsequent development. In a developed image the silver forms as thin tangled filaments only a few molecules in diameter, fig 5.3.
Silver is a reactive metal and is readily attacked by oxidising agents and other pollutants, such as acids or sulfurs. The silver metal reacts with these in a variety of ways.
Oxidising agents will form silver oxides which are mobile through the emulsion. The effect can be noticed when the silver oxides reach the surface and are reduced back to metallic silver, giving the darker parts of the image a metallic sheen when viewed by reflected light. This is known as 'silvering out’ (fig 5.4).
Acids, notably nitric acid formed during the decomposition of nitrate film, will attack the silver to form silver compounds that may be coloured or nearly transparent causing bleaching, (Figure 5.5a). However, acetic acid is considered a weak acid and will not actively affect the silver in the image.
Sulfur compounds will also readily react with the silver to form yellowish or brownish compounds.
Washing during processing is an important step in ensuring the stability of the silver in the image. Residual processing chemicals contain sulfur compounds and, if these are not removed by washing, will rapidly deteriorate the image. Testing the level of residual fixers is carried out by the Methylene Blue Test. This test, however, must be carried out within two weeks of the film having been processed.
The Methylene Blue Test is described in ANSI/ISO 417-1993.1 The standard does not prescribe a specific level of residual chemical that is acceptable for long term storage as the effect of residual chemicals can differ with differing products. From Annexe A Appraisal of keeping characteristics of the standard:
'It is not possible to establish a universally applicable level of residual chemicals that will result in the longest life for all products because of differences among types of products, differences between products of the same type and variations in the combinations of chemicals, even when they contain the same level of residual chemicals.’
Ironically, a certain quantity of residual thiosulfate is required to form a passivating layer over the silver. If all the thiosulfates are removed after processing, the silver is more susceptible to attack from oxidising agents.
1 ANSI/ISO 417-1993 Standard, Methylene Blue Test