NESY
An alkali rock ground fine — nepheline with feldspars, no free quartz · Melts earlier than a feldspar: more alkali and alumina, less silica · The raw flux behind most cone 6 glazes · Raises thermal expansion, so a glaze built heavily on it can craze
$0
Nepheline syenite is an alkali rock ground fine, and it melts lower than a feldspar does. It is the raw flux most cone 6 glazes are built on.
What it is, mineralogically, is nepheline with feldspars alongside it and no free quartz at all. Against a feldspar it carries more alkali and more alumina and less silica, and that combination is why it melts earlier — roughly a quarter more feldspar would be needed to do the same fluxing. It is not a straight swap for one, though: trading feldspar for it takes silica out of the recipe, and that silica has to go back in somewhere. The alkali also raises thermal expansion, more than a feldspar would, so a glaze that relies on it heavily can craze unless something else in the recipe pulls the other way. The finer grind melts more than the coarse.
