Not varnish, study says.  Also not oils, or proteins or minerals or anything else that can be found.  So what makes a Stradivarius better?  You get to the last paragraph of the article to find the probable unscientific truth.  Maybe the instrument is just more beautiful because Stradivari (says Jean-Philippe Échard, a chemist at the Museum of Music in Paris) " took a painterly approach to finishing his instruments. The pigmented top coat, he said, may have been applied much the way Rembrandt or Titian applied glazes to soften flesh tones.  Perhaps that, Mr. Échard suggested half-jokingly, is what makes some of Stradivari's violins special. 'Maybe a player, when seeing a beautiful instrument, he plays better,' he said. 'Maybe this is the secret.'"  The chemist is now a philosopher.  Good.