Boron oxide melts expansivity

Boron oxide often behaves as a flux. Boron softens glass for easier melting, but unlike alkahes, boron oxide increases expansion only slightly. This is the basis of the easily melted, low expansion commercial glasses known as borosihcates. 

The major use of boric acid is as the starting material for its anhydride, boron oxide, B203. Because it melts (at 450°C) to a liquid that dissolves many metal oxides, boron oxide (often as the add) is used as a flux, a substance that cleans metals as they are soldered or welded. Boron oxide is also used to make fiberglass and borosilicate glass, a glass with a very low thermal expansion, such as Pyrex (see Section 14.21). 

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