Edmund Osthaus Cleaning a print

Went to an estate sale this morning and George found this Edmund Osthaus print. We own two others and really like his dogs. This appears to be glued to a board and is not valuable but we like it. It is really yellowed, maybe from age and maybe from smoke. Any suggestions on how to clean it.
Osthaus lived in Toledo for many years and was a principal of the Toledo Academy of Fine Art.

 

I don't know how to clean paper glued to board (if it is printed on paper?). That is the kind of thing I think a professional should do. But since it is not valuable you may not want to spend money on that sort of thing.

 

It's not valuable except to us. After looking at it more carefully, it looks as if it has been shellacked. It has that yellowish color that old shellac gets. I used a damp microfiber cloth and wiped it down so it's not so dirty, but is still yellow. I have seen others of his glued to board so it must have been a fairly common practice at that time. We will have to live with it since we like the subject. Thanks Fig.