"When we use lead oxide instead of calcium oxide, and potassium oxide instead of all or most of the sodium oxide, we get the types of glass commonly known as lead crystal. Such glass is composed of 54-65% SiO2, 18-38% lead oxide (PbO), 13-15% soda (Na2O) or potash (K2), and various other oxides. Lead glass has a high refractive index and relatively soft surface, making it especially suited for decorating by grinding, cutting, and engraving. The specific gravity of lead glass is higher than that of soda-lime glass, and it is mostly used as drinking glasses, vases, bowls, ashtrays, or as decorative items.
"Glass of the same type, but containing less than 18% PbO, is known simply as crystal glass. Glass with even higher lead oxide contents (typically 65%) may be used as radiation shielding glass because of the well-known ability of lead to absorb gamma rays and other forms of harmful radiation."