Bill,(posted 8656 days ago)You should take a look at Carl Weese's article on Pyro in PhotoTechniques printed last summer. He explains the relationship between the type of process being used and the light wavelength required to print it. I'm afraid that I don't have the information in front of me to give a better answer. I belive Hutchings says somewhere in his book that Pyro stain replaces the silver which is why it doesn't become too dense to print under an enlarger light source. The stain, however, is more absorptive of UV rays required for alternative processes, resulting in greater density.