Charles Eicher is likely correct in guessing that the lithograph was dry
mounted, but his advice will create more problems than it solves.
 
Do not lift the litho from the mounting board; that will only damage the
print.
 
Most modern mounting boards are built up as layers, with a sheet of paper
on either side.
 
If you can, lay the print face down on a clean sheet of paper
and split the mounting board in half. Then split it again, trying to get
to the layer of paper with the dry mount tissue/litho on the other side.
At the lab I use a combination of bamboo and steel knives to do this, but
you may be able to get by with a butter knife....
 
Apply heat to this layer of paper and pull it away if it will come.  Just
don't pull on the litho.
 
Even if the separation is successful there will be some residual adhesive
on the back of the litho.  This can be reduced with solvents, but that is
not something you should use at home.
 
It would be best to take the litho to a conservator.  Failing that, a
frame shop may be able to help you.
 
Framers have a product available to them which is called <Adhesive
Release>; it is a mixture of solvents and it will allow them to reverse
most dry mount tissues.
 
However, it is not a simple process and I have had to re-restore quite a
few pieces over the years which were started by framers.
 
Good luck!
 
Jack C. Thompson
Thompson Conservation Lab
Portland, OR
 
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