BOHY

Borax hydrous is a water-soluble source of boron, the oxide that gets low-fire glazes melting. Because it dissolves, it is normally brushed on as a wash rather than mixed into a glaze.

Boron is what gets most low-temperature glazes to melt. It forms glass far below the temperature silica needs, it brings a glaze's maturing temperature down with it, and at the levels a glaze uses it does that without raising thermal expansion, which soda and potash both do. Borax carries that boron, but it dissolves in water, and that is the whole difficulty with it. As a freshly glazed piece dries, water is drawn into the bisque and takes the dissolved boron with it, leaving the glaze short of boron and the body underneath with more than it needs. That is why most boron reaches a glaze as a frit instead: a frit is a pre-melted glass, and it holds the boron in a form that barely dissolves. Where borax itself is used, the solubility is the point — dissolved in warm water and brushed thinly over bisque, it makes a simple flux wash. It is close to half water by weight, and it gives some of that up in warm dry air while damp air cakes it, so an opened bag drifts off its stated weight.