Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass typically contains 18–40% (by mass) lead(II) oxide (PbO); modern lead crystal or leaded crystal, historically also known as flint glass for the original silica source, contains a minimum of 24% PbO. Lead glass is desirable for a variety of uses due to its clarity. In marketing terms it is often called crystal glass.

The term lead crystal is, technically, not an accurate way to describe lead glass, because glass lacks a crystalline structure and is instead an amorphous solid. The use of the term remains popular for historical and commercial reasons but is sometimes changed to simply crystal because of lead's reputation as a toxic substance. It is retained from the Venetian word cristallo to describe the rock crystal (quartz) imitated by Murano glassmakers. This naming convention has been maintained to the present day to describe decorative holloware.

Lead crystal glassware was formerly used to store and serve drinks, but the health risks of lead have made this use rare. An alternative material is modern crystal glass, in which barium oxide, zinc oxide, or potassium oxide are employed instead of lead oxide.

In the European Union, labelling of "crystal" products is regulated by Council Directive 69/493/EEC, which defines four categories, depending on the chemical composition and properties of the material. Only glass products containing at least 24% lead oxide may be referred to as "lead crystal". Products with less lead oxide, and glass products with other metal oxides used in place of lead oxide, must be labelled "crystalline" or "crystal glass".

The addition of lead oxide to glass raises its refractive index and lowers its working temperature and viscosity. The attractive optical properties of lead glass result from the high content of the heavy metal lead. Lead, whose density is more than seven times that of calcium, also raises the density of the glass. The density of soda glass is 2.4 g/cm³ (1.4 oz/cu in) or below, while typical lead crystal has a density of around 3.1 g/cm³ (1.8 oz/cu in) and high-lead glass can be over 4.0 g/cm³ (2.3 oz/cu in) or even up to 5.9 g/cm³ (3.4 oz/cu in).

The brilliance of lead crystal arises from the high refractive index caused by the lead content. Ordinary glass has a refractive (n) of 1.5, while the addition of lead produces a range up to 1.7 or 1.8. This increased refractive index also correlates with increased dispersion, a measure of the degree to which a medium separates light into its component wavelengths, thus producing a spectrum of colors just as a prism does. Crystal cutting techniques exploit these properties to create a brilliant, sparkling effect as each cut facet in cut glass reflects and transmits light through the object. The high refractive index is useful for lens making, since a given focal length can be achieved with a thinner lens. However, the dispersion must be corrected by other components of the lens system if the lens is to be achromatic.

The addition of lead oxide to potash glass also reduces its viscosity, rendering it more fluid than ordinary soda glass above its softening temperature (about 600 °C or 1,112 °F), with a working point of 800 °C (1,470 °F). The viscosity of glass varies radically with temperature, but that of lead glass is roughly two orders of magnitude lower than that of ordinary soda glasses across working temperature ranges (up to 1,100 °C or 2,010 °F). From the glassmaker's perspective, this results in two practical effects. First, lead glass may be worked at a lower temperature, facilitating its use in enamelling; second, clear vessels may be made without trapped air bubbles with less difficulty than with ordinary glasses, allowing the manufacture of perfectly clear, flawless objects.

When tapped, lead crystal makes a ringing sound, unlike ordinary glasses. Wine glasses made of lead glass are valued for the "ring" made by the clinking of glasses. The sound was considered better when a large quantity of lead oxide was present in the glassmaking material, as in the British and Irish wine glasses of the 17th-19th centuries, with their "rich bell-notes of F and G sharp". Consumers still rely on this property to distinguish lead glass from cheaper glasses. Emil Deeg had published a major study on the ringing of the lead crystal in 1958. Since the potassium ions are bound more tightly in a lead-silica matrix than in a soda–lime glass, the former absorbs more energy when struck^{[dubious – discuss]}. This causes the lead crystal to oscillate, thereby producing its characteristic sound. Lead also increases the solubility of tin, copper, and antimony, leading to its use in colored enamels and glazes. The low viscosity of lead glass melt is the reason for typically high lead oxide content in the glass solders.