Not Happy With Hahnemuhle Platinum Rag

Mike Ware said:

The "dossier" that Robert Poole kindly refers to has just been uploaded to my website:
https://www.mikeware.co.uk/mikeware/Paper_Additives.html
I'd be interested to hear from anyone with knowledge of "retention aids" and their use in papermaking.
For siderotypes, I think they are disastrous.

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I don't know if this has any relevance to your findings, but I recollect reading fairly recently of a blotching problem that had apparently afflicted Kodak's dye transfer paper in about the time period that you define as marking the changeover in papermaking methods. It may be completely unrelated, but I was struck by the correlation in time period.