The Collaborative International Dictionary of English v.0.48:

Pyroboric \Pyr`o*bo"ric\, a. [Pyro- + boric.] (Chem.)
   Pertaining to derived from, or designating, an acid, H2B4O7
   (called also tetraboric acid), which is the acid ingredient
   of ordinary borax, and is obtained by heating boric acid.
   [1913 Webster]