Feldspar is a rock that melts. In ceramics, non-soluble sodium and potassium is sourced almost exclusively by feldspars. Feldspars begin to soften starting at cone 6 depending on chemistry. This page is about feldspar powder used in glazes and clay bodies. For the less common feldspar sand / pebbles used to make clay bodies crunchy, see Granular Feldspar.
Nepheline Syenite is NOT a feldspar for geologist reasons, but it's useful to group with the feldspars from a glaze chemistry perspective. It does the same thing (source of sodium and potassium at mid to high fire temperatures). The main difference is that it is low in silica (and thus relatively high in alumina).
The ideal sodium feldspar is made of pure albite (NaAlSi3O8) and the ideal potassium feldspar is pure orthoclase (KAlSi3O8). However, most feldspars are a mix of the two minerals. Feldspars are labeled with the element they contain more of but in my experience sodium and potassium are identical in their effects in glazes.
The main thing to be careful of is the total KNaO content in a feldspar. A low KNaO content (less than 10wt% total) suggests silica and kaolin contamination, which means less melt.
These glazes are primarily feldspar
Additionally, some celadon glazes are high in feldspar (> 40%) for high gloss and crazing.
These are commercial feldspars commonly available to potters in North America.
See Which common American/Canadian feldspars can substitute for each other? - Digital Fire for a melt test comparison.
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