The approach uses the property that certain chemicals
will maintain a constant humidity above the surface of
their saturated solution. I could envision a sealed box
with screen shelves. On the bottom of the box would be
a tray containing the saturated chemical solution.
Place the papers on the screens and close the box.
After a time, the papers will be at the same humidity
as the box environment, which is controlled by the
saturate solution. Of course, one would want to avoid
getting the solution and papers in contact.
For example: Calcium chloride (saturated)....31-32%
Calcium nitrate (saturated).....51%
Potassium bromide (saturated)...84%
For additional listings refer to the CRC Handbook of
Chemistry and Physics, "E" section under Constant
Humidity.
Usual disclaimers about chemical safety, handling and
disposal, etc.
Paul A. Lehman