It contains between 18 to 40% of lead (II) oxide by weight. The nomenclature of lead glass is regulated such that any glass that contains a minimum of 24% PbO by weight is referred to as “lead crystal”, and any glass that contains less than 24% PbO by weight is referred to as crystal glass. Lead glass has a chemical composition of SiO2 and PbO along with other impurities or additives added to achieve certain properties.