I agree that Aggies discovery is beautiful, having just received the print of Slot Canyon #3 in a print exchange, the actual print goes far beyond the viewing of an internet image.
I had an interesting toning experience a couple of days ago with some old emulsion Azo (probably over 20 years old) The grade three paper of that era has the most incredible split toning capability in selenium at a 1-20 dilution. The print tonal values zones IV and below go to an almost metalic reddish tone that is most apparent at an oblique angle whereas the rest of print does not tone markedly at all.
I contacted Michael Smith about this and he indicated that this was a property of Azo at one time and that some people were quite taken with this attribute. He went on to say that the new emulsion grade two may possess the same tendencies. I assume that this is possibly why Michael and Paula tone at a 1-128 dilution on Azo (for archival considerations primarily). I have not tried the new grade two emulsion to determine if this is also present in that paper.