Borax is a boro-silicate it brings in the element Boron (look on periodic chart) which is one of the fluxes for glass and ceramic glazes.
The Borax mine is located in southern California, mined for the ceramics and Glass industry and sold retail as 20 mule team Borax soap. A very common flux containing sodium as stated. In solution with water, the Boron. when decanted can be used for healing devitrification leaving alumnia and silica behind in the container. Colemanite is another borate from the same area of California.
I don’t know anything about fusing glass but, used to batch glass for glassblowing.