Put a drop on a glazed surface, underside of a saucer will do if you don't have any loose tiles (v. useful BTW), let it dry naturally and see what you get.
Just in relation to the idea of using thickened turpentine in making varnishes, this would be to make varnish. As in from scratch using raw resins etc. as the other ingredients. In addition to the hassle (and sometimes hazards) of making such varnishes they just aren't that great by modern standards. Now some traditionalists strongly disagree with this, but they're using their own standards for "great" which won't necessarily match yours. I can virtually guarantee that your requirements for a wood finish will be very different to theirs.