I've had soap from the 20s-50s that were dry but fine. I have also had soap from the same date range that turned into powder when attempting to do anything, and some soaps turned into a hard brown substance that was not usable. Most were ok but there were some pucks that were not ok at all.
What I think is that it's not just whether or not the water is gone, or not. I suspect that what also matters are storage conditions such as heat and humidity along with rapid changes in those factors.
What I think is that it's not just whether or not the water is gone, or not. I suspect that what also matters are storage conditions such as heat and humidity along with rapid changes in those factors.