Back in the day we used a commercial product which contained, I imagine, either ethelyne or propolyne glycol. Beware, ethelyne glycol, a constituent of auto antifreeze, is poisonous, causing kidney failure! Its cousin, propolyne glycol, isn't. The prints were soaked in this stuff after the usual wash. As I recall it helped, at least some. I think the glycols are hygroscopic; their presence in the print's fiber base would encourage them to retain some moisture and thus not curl, at least to some extent. Perhaps this will jog another member's memory....