Borax Hydrous

Boron does most of the melting in low temperature glazes. It forms glass far below the temperature silica needs, it lowers the maturing temperature of a glaze, and at normal glaze levels it does this without raising thermal expansion, where soda and potash both do. Borax carries that boron, but it dissolves in water, and that causes a problem in a glaze. As a freshly glazed piece dries, the water soaks into the bisque and takes the dissolved boron with it, so the glaze ends up short of boron and the clay underneath gets more than it needs. For that reason most boron is added to a glaze as a frit, a pre melted glass that barely dissolves. Borax itself is used where being soluble is an advantage. Dissolved in warm water and brushed thinly over bisque it makes a simple flux wash. It is close to half water by weight. It loses some of that water in warm dry air and cakes in damp air, so an opened bag drifts from its stated weight.