ALBOT'S CALOTYPES faced another problem though, that of permanence. In an effort make photographs that would not fade over time, Talbot aggressively pursued photogravure, contributing two new developments to the process.
The first, which he patented in 1852, was his discovery that gelatin treated with potassium bichromate hardened when exposed to light. In subsequent refinements, this material was used as the acid resist in place of Niépce's bitumen. The second, was his recognition that some sort of screen was needed to break up the image area. Niépce's portrait already had a linear structure because it was produced from an engraving, but Talbot's images came directly from nature, and thus required a network of lines, so large etched areas of the plate would hold ink. At first he introduced a gauze mesh (his "photographic veil") that gave the plate a screen pattern over which he laid objects such as fern leaves, before exposing them to light. However, in 1858 he patented an improved technique of dusting the plate with a copal resin powder to give the image a finer and more even screen tint. At this time, he also began using waxed paper positives of his camera images to make prints that he called photoglyphic engravings.
|William Henry Fox Talbot |
For Talbot, photogravure had been the logical evolution of his original invention of photography; transforming nature's sketches into permanent and beautiful printer's ink.