The Quest For The Glaze Source

The glaze version of finding the source of the Nile, Where does it begin? What are the first steps
of transforming glaze slops to glass? Mixed ground up rocks, none of which melt at any temperature to which
a kiln is fired chemically react together, and make a glass. Here, what might the first alteration be?

I'm looking at small systems with few components at relatively low temperatures.

I coat a tile with salt, and fire to a temperature a bit below the melting point of Na2O. That melting point
is given in Wikipedia and various other web sources as 2070 deg F. I'm presuming that the disassociated Na+ in
the water oxidizes as the glaze dries.

The result is a deep purple-y brown surface coating with small glossy black inclusions. Already, I am seeing
something just a bit different from the usual. Those shiny black bits are not seen in salt glazing at a higher
temperature. I expect the sodium has captured iron from the underlying clay body. There are Na2O-Fe2O3-SiO2
compositions that melt at 1472 deg F. Figure 520 in "Phase Diagrams for Ceramicists", Levin, Robbins, McMurdie,
The American Ceramic Society shows this.