Borosilicate glass: in this boric anhydride is used in place of lime. This glass generally contains 75-81% silica, 10-13.5% B2O3, 4.5-6% Na2O and 2-5% Al2O3. Sometimes Zno may be added to improve the properties. Its two outstanding proper is are high chemical resistance and low coefficient of thermal expansion which make it most suitable for laboratory and medical ware, oven and table ware and industries glass ware. Borosilicate glass may contain a small quantity of fluorine which is used to lower the viscosity. Calcium phosphate may also be included in borosilicate composition to produce optimization during forming of the glass. Pyrex is an example of such glasses known for their low coefficient of expansion. These can be made by fusing a mixture of sand, lime, borax and alkali metal carbonates.