UltraStable: Sensitizers and Sensitometry
Although some of this has been discussed here before, it bears repeating:
1. Light hardened gelatin layers (carbon, Fresson), unlike gelatin hardened by AgX-tanning developers (carbro and dye-transfer) have a "soft" and diffuse boundary between soluble and insoluble which are sensitive to slight differences in processing . Most apparent in the highlights and in neutral color balance, these processing variations make it difficult to produce two similar continuous tone color carbon prints. Although prized by artists and hobbyists, these one-of-a-kind- print characteristics make the con-tone carbon process unsuitable for most commercial applications. Furthermore, the absence of shoulder in the (straight line) characteristic curve of carbon tissue impairs the printing of highlights (e.g. the details of a white-on-white wedding dress) necessary for accurate photographic reproduction.
2. The UltraStable sensitizer (Diazidostilbene Sodium Sulfonate) is similar to dichromate in its ability to create a variable thickness image in response to actinic light exposure. It is also subject to the same problems of highlight loss and repeatability that affect dichromated gelatin layers. By using dot area rather than negative density (half-tone-vs-con-tone), pigment films can be processed with a high degree of repeatability and no highlight loss.
3. UltraStable pigment films are capable of producing a four-color neutral gray scale (using screened separation negatives) with densities ranging from from .02 to 2.50+ with a base fog density of less than .01 (when printed on a white PE base). By comparison, dichromate sensitized carbon tissue, has a high base fog density (typically .10+) and when used with continuous tone separation negatives, has no visible highlight contrast until reaching a .20-.30 density level.
4. UltraStable films are balanced so that the exposure times are the same for all four colors. Once the time to produce the recommended Cyan solid (non-screen) density of 1.39 is determined, identical exposures of the other color layers will produce Solid Densities of (approximately) M=1.34, Y=.96 and K=1.24, which, when combined, will result in a neutral (4/C) gray scale.