Donald Miller said:
I would be very surprised if this would work. The VC materials have two emulsions whereas Azo is a single emulsion material. In other words it either exposes or it doesn't...
The degree of variation from two light sources to expose Azo through a stained negative are best accomplished with other means. For instance Amidol with water bath would be far more effective to decrease contrast. It is virtually impossible to increase a papers exposure scale past the emulsion characteristics.
Well, you might be surprised, but Sandy has said that he can get two different contrasts out of the Azo by using different light sources. I'm taking his word on that observation. So it only goes to follow that if one varied the light sources, one could control the contrast range of the paper. We don't have to change the contrast of the paper at all, since we are controlling the contrast with the filtration that is inherent in the stained negative.
And yes, VC paper's have two emulsions, each with different intrinsic contrasts. In fact, Ilford MG IV claims to have 3 emulsions to accomplish the VC properties.
Sandy's measurements have shown that the PyroCat density increases with decreasing wavelength, i.e. PyroCat has a much greater density in the UV than in the blue or green wavelengths. That density probably increases fairly continuously as the wavelength decreases. Combine this with the spectral characteristic of the Azo paper, and that is what would give us the VC properties of this technique. See this for the spectral properties of Azo.
http://www.kodak.com/global/en/professional/support/techPubs/g10/f009_0106ac.gif
So we accomplish this either of two ways here -
1) Pick a light source that is "tunable" so that we can vary the wavelength of the light that goes through the negative, and the negative will have a different contrast range based on the wavelength we "tune" to, or
2) Vary the amount if light coming from each end of the spectral sensitivity spectrum of the Azo paper. By changing the ratio of these colors of light, we would be making 2 exposures on to the same paper with a negative that had 2 different "contrasts". The combination of those exposures, when properly balanced, could create an exposure of either contrast extreme, or somewhere in the middle.
I'm not saying it would be easy to do, but it should work...
You may be right, water baths may work better. But then how do you increase the contrast with a water bath?
Donald, I don't mean to say there aren't better ways to control the contrast, I'm just pointing out that here's another way that probably hasn't been tried yet.