Frits

"Frits can be used in any glaze and can also be used as fluxes in bodies, slips and engobes. When originating a recipe which involves a frit, the best way is to consider the frit as a flux only and not as a glass-former. In this way one will not be satisfied with the result simply because it melts. A frit will always melt, but it is not a suitable glaze alone. It requires other alumino-silicates."
-The Potter's Dictionary of Materials and Techniques