Minspar Feldspar

Minspar Feldspar begins to soften around cone 4 and fluxes from there up through the top of the stoneware range. Below cone 4 it stays unmelted and acts as a filler, so low fire glazes take their melt from boron instead. A soda feldspar melts earlier and runs thinner than a potash one, so a glaze built on it melts sooner and over a narrower band, and a body vitrifies at a lower temperature but slumps more readily if it is overfired. It is used as the main flux in glazes and as the body flux in clay and porcelain bodies. Because about a tenth of it is free quartz, a recipe using it contains more silica than the feldspar line alone suggests. It contains no clay, so a glaze made largely from it settles hard in the bucket unless a clay or a suspender is added.