Infant's feeding bottle in Pyrex glass, 1968. Glass is made from sand, limestone, soda ash (sodium carbonate) and recycled waste glass. Pyrex is a heat resistant glass where boric oxide has been added. The ingredients are heated to a high temperature, in a furnace, until they melt together. Bottles may be moulded from the molten glass. Pyrex has a low coefficient of thermal expansion which makes less susceptible to thermal shock than conventional soda-lime-silica glass.