But it is also a tanning agent, and negatives developed in pyro form an image not only from silver reduced to its metallic state, but also from an accompanying proportional stain, and a hardened, dimensional relief image in the gelatin itself. Many of the early variations in the pyro formulation were attempts to gain control over the staining action, which tended to be variable and hard to predict. Another approach used by some photographers sought to minimize the stain, leaving only the high-acutance silver image. This worked well with large negatives, giving high resolution and fine rendering of detail, but resulted in too much grain for enlarging. With the introduction of metol and hydroquinone developer formulas, and the general movement of photographers to small negatives printed by enlargement, the use of pyro all but disappeared.
Highlights have more silver, and proportionally more stain.
When printed, both components of the image affect the response of the paper, resulting in a variety of printing characteristics that can't be matched by a negative consisting of silver alone. Pyro may have been considered outmoded by the 1920s and '30s, but Weston insisted on staying with it. The pyro myth and legend can partly be attributed to his discussions of it in his Daybooks. One of its special aspects is revealed in the Mexico Daybooks where it is clear that Weston expected to be able to print his pyro negatives in either platinum or silver. The next big step in the pyro saga came in the 1980s, when Gordon Hutchings began intensive research into pyro formulations because of his dissatisfaction with standard developers.