Feldspar is a group of common minerals that melt at high temperatures and act as a flux in clay bodies and glazes. It helps stoneware and porcelain fuse into a dense body and is a key ingredient in many high-fire glazes.
What Is Feldspar?
Feldspars contain silica and alumina along with fluxing oxides of potassium, sodium or calcium. On their own they melt into a stiff glass at high temperatures, which is why they appear in so many stoneware and porcelain recipes. Potash feldspar, soda feldspar and nepheline syenite are the versions most potters meet.
In clay bodies, feldspar is the main reason stoneware and porcelain become dense and vitreous. Digitalfire notes that the amount of feldspar largely sets the temperature at which a porcelain matures, and also how readily it slumps, which is why porcelains are more prone to warping.
In glazes, feldspar supplies flux, silica and alumina in one material. Glazes very high in feldspar often craze, because the sodium and potassium it contains expand a lot with heat. Swapping one feldspar for another changes a glaze, so it is worth testing whenever a supplier changes the feldspar they stock. Keep a few test tiles of your favourite glazes on hand so any change is easy to spot.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.