John, When I worked for the Amish company that built cabinetry,
the old fellow that did the finish work told me that _varnish_
wasn't used commercially until the very late 1920s and early '30s.
I have definitely noticed that there was a definite change in the
characteristics (generally for the cheaper) in finishes and glues
in that period after 1927.

When varnish gets old it is even less resilient than shellac.  The
slightest exposure to light and moisture will cause the varnish to
craze and flake off.

Brian Thornton - Short Mountain Music Works
Woodbury, TN 37190
http://www.mindspring.com/~goatboy/smmw.htm