I have some older 8x10 WT paper that I use for work prints. It was processed alongside prints made on the new 11x14 stock. The prints made on the older paper don't show any split toning.
I'll agree that there are a whole lot of variables that can affect toning results. But that new and old paper tone differently when processed side by side (OK, there was an hour between them ...) points to a difference in paper.
Ilford's emulsion consistency is exemplary. Without it DA's whole exposure control system [or RH's for that matter] wouldn't work. I work on a print with smaller paper and then crank the head up, adjust the exposure and get an identical prints from boxes of paper purchased years apart. And when untoned the two prints were identical, as expected. The difference only appeared when the larger print was toned.
Maybe I just need to age paper a few years before using it [insert smileys if you need them].
I'll mix up fresh chems next week and try again. I have to clean the place up so the housekeeper can find the floor to vacuum it, and then it is off to John Power's get-together.
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Split toning is caused by the two (or three) emulsions that make up VC paper reacting to selenium in a different fashion. Where in the tonal range the split occurs and the severity of the split changes with changes in contrast filtration. Modern papers that are sold as 'fixed grade' are often crypto-VC papers made with two emulsions with different fixed sensitivities. The "black circle/white box" Oriental paper had a noticeable steep spot in the HD curve where the curves of the constituent emulsions overlapped. See the application note on the DA web site
http://www.darkroomautomation.com/support/appnotevcworkings.pdf for more on VC paper.
The late Forte Polywarmtone paper had the most outrageous split toning. Dark tones went a chocolate brown while light tones were completely unaffected. Although chemistry and phases of the moon can affect split toning, the paper seems to play a large role. Apart from pathologies like Forte, I have never had split toning problems to this degree before.