The Collaborative International Dictionary of English v.0.48:

Overglaze \O"ver*glaze`\, a. (Ceramics)
   (a) Applied over the glaze; -- said of enamel paintings,
       which sometimes are seen to project from the surface of
       the ware.
   (b) Suitable for applying upon the glaze; -- said of
       vitrifiable colors used in ceramic decoration.
       [Webster 1913 Suppl.]