BOAC

Boric acid is a source of boron that carries no sodium with it, unlike borax. It is mostly used for melting your own frit or low-fire glaze base rather than going into a glaze directly.

Boric acid is boron and water, with no sodium or potassium in it. Boron brings a glaze's melting point down without pushing up its thermal expansion, at the levels a glaze uses it — where an alkali arriving alongside the boron would push that expansion straight back up. So where a recipe cannot afford the soda that comes with borax, this is the boron source that fits. It dissolves in water, though, and dissolved boron does not stay where it is put: the water in a drying glaze layer soaks into the bisque and carries the boron in with it, so the glaze comes out under-melted and the clay beneath it over-fluxed. That is why boron usually reaches a glaze as a frit, a pre-melted glass that holds the boron in a form which barely dissolves, and boric acid is what frit makers melt to get there. In a studio it is the material for making your own frit or your own low-fire glaze base. For ordinary glaze work a boron frit is the more dependable route.