|Monthly Tech-Tip |
This is pure soda feldspar (Minspar 200) fired like a glaze at cone 4, 5, 6 and 7 on porcelainous stoneware tiles. The bottom samples are balls that have melted down at cone 7 and 8 (although they appear to be well melted they exhibit no movement in a melt fluidity test. Notice there is no melting at all at cone 4. Also, serious crazing is highlighted with ink on the cone 6 sample (it is also happening at cone 5 and 7). Feldspars have high KNaO, that means they have high thermal expansions. That is why high-feldspar glazes almost always craze. Since feldspar is barely melting at cone 6 it is not possible to make a functional cone 6 glaze that only uses feldspar as the flux (boron, lithia or zinc are also needed).
Feldspar is a natural mineral that, by itself, is the most similar to a high temperature stoneware glaze. Thus it is common to see alot of it in glaze recipes. Actually, too much.
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt.