But I don't think that any of the UV sensitive reducable ferric salt is going to be bound to water. After all, once you coat a piece of paper you let the layer dry before exposure. So the molecules of sensitizer will be different than the molecules of the structural support.
Of course, I could be wrong!!
Clearly this is going to take some research.
*Decades ago a technician's knowledge of Portland cement's properties in presence of various contaminants was important in my work. But the vast majority of that knowledge is long gone now.