B₂O₃ in the glass industry

     The glass industry in Western Europe and the US accounts for about half the B₂O₃ consumed . Fused B₂O₃ dissolves metal oxides to give metal borates. Fusion with Na₂O or K₂O results in a viscous molten phase, rapid cooling of which produces a glass; fusion with appropriate metal oxides leads to coloured metal borate glasses. Borosilicate glass is of particular commercial importance. It is formed by fusing B₂O₃ and SiO₂ together; a metal oxide component may sometimes be added.

     Borosilicate glasses include Pyrex which is used to manufacture most laboratory glassware as well as kitchenware. It contains a high proportion of SiO₂ and exhibits a low linear coefficient of expansion. Pyrex glass can be heated and cooled rapidly without breaking, and is resistant to attack by alkalis or acids. The refractive index of Pyrex is 1.47, and if a piece of clean Pyrex glassware is immersed in a mixture of MeOH/C₆H₆, 16/84 by weight, it seems to ‘disappear’; this gives a quick way of testing if a piece of glassware is made from Pyrex. Although the linear coefficient of expansion of silica glass is lower than that of Pyrex glass (0.8 versus 3.3), the major advantage of borosilicate over silica glass is its workability. The softening point (i.e. the temperature at which the glass can be worked and blown) of fused silica glass is 1983 K, while that of Pyrex is 1093 K. Fibreglass falls into two categories: textile fibres and insulation fibreglass. Of the textile fibres, aluminoborosilicate glass has the most widespread applications. The fibres possess high tensile strength and low thermal expansion, and are used in reinforced plastics. Insulation fibreglass includes glass wool which contains  ≈ 55–60% SiO₂, ≈ 3% Al₂O₃, ≈10–14% Na₂O, 3–6% B₂O₃ plus other  components such as CaO, MgO and ZrO₂.