If other materials
containing fluxes are mixed with silica, in the correct proportions
and within certain limits, the melting point of the mixture will
be lower than that of its constituent parts.
when a mixture of silica and fluxes melt it very quickly becomes
fluid like water and would run off anything other than horizontal
This very small
range of temperature within which the silica + fluxes mixture melts
presents another problem as temperatures in pottery kilns often
vary considerably from one place in the kiln to another.