Cut glass objects of lead crystal were developed in response to chemical properties and political factors.
Like regular glass, lead crystal is made mostly of silica. The difference is that in regular glass, the flux that glassmakers use to lower the temperature at which the silica melts is composed primarily of a lime mixture. With leaded glass, artisans use a type of flux that contains lead salts and yields a particularly sparkling type of glass.
Lead crystal was actually something of a misnomer since the substance was not strictly crystalline and fell somewhere between the properties of a disordered liquid and an ordered, crystalline solid. The substance received its name from the Italian cristallo, the rock crystal that Italian glassmakers imitated in their art. Flint glass was an earlier term used to describe this glass, and the term continued even after sand replaced flint as the source of silica in the process.
The addition of lead to glass resulted in two important changes. First, lead greatly increased the refractive index compared to regular glass, yielding beautifully, sparkling goblets, condiment trays, and decanters. Likewise, as lead has a much higher atomic mass than calcium, glassmakers began creating products that were substantially denser, which as it turned out, had major political and economic implications.
In the early 1700s, glass was sold by weight in Europe, so most objects were relatively plain and lacked much decoration. That changed in 1746 when the British government levied a tax by weight on the sale of glass. Glassmakers responded not by reducing the amount of lead in the glass to make lighter glass, but by creating more decorative forms of cut glass and vessels with hollow stems. The lead glass was softer to cut than calcium glass, which aided the development of this art form. In 1780, another government act granted Ireland free trade in glass, which shifted English glassmaking to the Irish cities of Waterford and Cork, both of which became known for their intricate cut glass products.
Development of artistic forms of lead glass continued in the 20th century through the Steuben glassworks in Corning, New York, and more recently through Swarovski glass crystals with special glass coatings to add an additional sparkle to their crystals.
Additional information about lead crystal may be found at http://www.brotheridgechandeliers.co.uk/36001.html and http://www.kinsalecrystal.ie/history.htm