Quote Originally Posted by Michael R 1974 View Post
Variable contrast paper basically has two emulsions, a low contrast layer sensitive to green light, and a high contrast layer sensitive to blue light. When light hits the paper, the more blue there is in the light relative to green, the more the paper's high contrast emulsion is exposed relative to the low contrast emulsion. This produces a higher contrast image. The reverse is true when there is more green light relative to blue light.
Michael, while I follow your statement, I wonder if you would agree that there is a more detailed explanation of how VC papers work? I would humbly observe that it seems, based on their characteristic curves, that the component emulsions are differently sensitised to the blue and green light and that it is their additive result that creates the actual, observed contrast. Further, modern VC papers, like Ilford (or like Polymax used to) seem to have three emulsions, with the third one being sensitive to both green and blue.

Unfortunately, this can also lead to some odd behaviours at extreme low grades, such as 00. See Nicholas Lindan short paper: "The Workings of Variable Contrast Papers and Local Gamma". For that reason, split-grade technique can be a little easier to use with filters a little harder, such as 1 and 5, rather than 00 and 5. I believe that is what Bob Carnie practices. The effect will be the same, but the observed changes will seem more logical when using 1 rather than 00 for certain mid-tones.