a cao containing silicate glass composition which can further contain small amounts of lead oxide and barium oxide is generally useful for hermetically sealing the electrical in - leads of various electrical devices . the present glass has a composition range in percentages by weight of : ______________________________________ constituents weight percent______________________________________sio 2 65 - 75na 2 o 9 - 13k 2 o 3 - 6al 2 o 3 1 - 4cao 4 - 8bao 0 - 4pbo 0 - 6li 2 o 0 - 2______________________________________ along with minor amounts of incidental impurities , residual fluxes and refining agents .