beeswing

noun2

1. •
   A filmy, translucent crust found in port and other old wines which have been bottled-aged for a long time.

   • The three glasses were grouped together, all of them tinged with wine, and one of them containing some dregs of beeswing. — 1904
2. •
   Cream of tartar; potassium bitartrate; the residual salt of tartaric acid.