Solarization: Reversal Effects Arising from Variation of Developed-Grain Size with Exposure

With heavy exposures, effects akin to solarization and re-reversal may be encountered because of variations in developed grain size and shape. At ordinary maximum density, developed grains are of irregular spiky shape: with increase of exposure they develop in a more compact manner, so that the shapes of developed grains are very similar to those of undeveloped grains. With still heavier exposures, the shape of the developed grain is again irregular.

It is shown that these effects are due to variations in the number of developable surface latent image specks. Development is "compact" when initiated by a large number of latent image specks scattered over the surface of a grain, while irregular development results from initiation by one, or a few, specks. Thus the fall of density with increasing exposure beyond that required for maximum density is due to increase in the number of surface specks, while the rise at heavier exposures is due to reduction in speck number owing to the onset of true solarization. In conformity with this interpretation the effects are more pronounced with short than with long exposure times.

The relationship between the above effects and true solarization and re-reversal is illustrated for the emulsion which prompted this study, and is also discussed briefly in general terms.