Digital BW, The Print

Paul,

I dont know if I'll ever know which varnish Da Vinci used. My
knowlegde of chemistry is very limited. Here's what I've been able to
  get. Hope this is of any help.

Varnish is made up of binders (resins, oils, gums), solvants such as
alcohol and eventually diluants or other additives like colorants. But
you probably know that.

Da Vinci probably used varnish made from natural resins. 

But, what seems just as important, if not more, is the method of
applying the varnish. When not mixed with the pigments, it was applied
in several coats. When done correctly it will last. If not, bubbles
and/or premature cracking will appear. The different varnish coatings
can be distinguished under UV lights.

Even if applied coorectly, cracks will eventually appear in the
varnish from natural ageing or from stress of shocks. Mona Lisa was
painted on a board and in 1951 a perimetric frame made of oak was
installed on the back side. I wonder how cracking of the varnish could
be prevented on a photograph which by nature has a flexible support.

The other problem is yellowing. In 1999-2000 a grand was given by the
C2RMF to the Université Blaise Pascal,France  for research on the
prevention of yellowing of varnish, but I don't know if any progress
was made.

I wonder how long the Mona Lisa painting would have lasted if it had
been subjected to todays airborne pollutants and UV levels from day
one. It is now displayed behind glass in a controlled atmosphere.

And finally, museums do have conservationist which are able to prolong
the life of artworks which would have deteriorated a long time ago.

HTH,
Andre Moreau