[rhettyeakley]

If there is a way of removing the thickness of the cardboard slowly from the back side working towards the card without damaging the card itself…that is the way to go. Essentially reducing the cardboard to a thin sheet of paper attached to the back of the card then applying water directly to the back of the card to allow for the paste to be soluble in the water and breaking the bond by turning the solid paste back into a liquid phase would be the way to do it.

This will be very time consuming and difficult for even a trained/experienced restorer but is doable (depending on the paste/glue used)