Varnish is used by potters as an adhesive to hold colors and other decorating materials on to already glazed work. Cover the glazed surface with a thin layer of ordinary, colorless wood varnish. When dry the colors adhere to it and when fired the varnish burns away.
THE FORMULA FOR VARNISH
For on-glazing you mix 1 part varnish with 3 parts pure turpentine, this is then mixed with a color and can be painted directly onto an on-glaze surface.
Read more about: On-Glaze