By 'borax', do you mean 'borax frit', or some raw form of borax? The former is more usual for glaze use, as the latter (raw borax) is soluble in water, which is going to give problems in most cases. You might like to note that it is an alkali, which can cause skin problems, and certainly will cause eye problems, if splashed about and not treated with due caution.
There are lots of recipes using borax frit as a flux, either alone or more usually in combination with other Frits; some Raku, some not, but mostly low temperature stuff. As an alkali, it can combine nicely with copper to give rather lovely blues in oxidation.
Raw borax can be used in producing Egyptian Paste, but that's another story altogether.