Borax is a complex compound and has odd chemical characteristics. It is able to act both as a glass-former (acid) as well as a flux (alkaline). Chemically, it is written as Na2B4O7.1OH2O. Since for ceramic purposes the water is inconsequential, potters often prefer to work with the formula of borax as Na2O.2B2O3.1OH2O when doing glaze calculation.
Warning: All boron compounds are toxic. Use proper precautions when handling.