Lead glass
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.
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.
[5] The addition of lead oxide to glass raises its refractive index and lowers its working temperature and viscosity.
[6] This heightened refractive index also correlates with increased dispersion, which measures the degree to which a medium separates light into its component wavelengths, thus producing a spectrum, just as a prism does.
The sound was better when large quantity of the lead oxide was present in the glassmaking material, like in the British and Irish wine glasses of the 17th-19th centuries with their "rich bell-notes of F and G sharp".
[10] 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].
In particle physics, the combination of the low radiation length resulting from the high density and presence of heavy nuclei with the high refractive index which leads to both pronounced Cherenkov radiation and containment of the Cherenkov light by total internal reflection makes lead glass one of the prominent tools for photon detection by means of electromagnetic showers.
In the medieval period lead metal could be obtained through recycling from abandoned Roman sites and plumbing, even from church roofs.
[13] A red sealing-wax cake found in the Burnt Palace at Nimrud, from the early 6th century BC, contains 10% PbO.
These low values suggest that lead oxide may not have been consciously added, and was certainly not used as the primary fluxing agent in ancient glasses.
There, it was cast to imitate jade, both for ritual objects such as big and small figures, as well as jewellery and a limited range of vessels.
Since glass first occurs at such a late date in China, it is thought that the technology was brought along the Silk Road by glassworkers from the Middle East.
In the late 11th-early 12th century, Schedula Diversarum Artium (List of Sundry Crafts), the author known as "Theophilus Presbyter" describes its use as imitation gemstone, and the title of a lost chapter of the work mentions the use of lead in glass.
The 12–13th century pseudonymous "Heraclius" details the manufacture of lead enamel and its use for window painting in his De coloribus et artibus Romanorum (Of Hues and Crafts of the Romans).
[2] At first, his glasses tended to crizzle, developing a network of small cracks destroying its transparency, which was eventually overcome by replacing some of the potash flux with lead oxide to the melt, up to 30%.
In 1681, the year of his death, the patent expired and operations quickly developed among several firms, where by 1696 twenty-seven of the eighty-eight glasshouses in England, especially at London and Bristol, were producing flint glass containing 30–35% PbO.
English labour and capital then shifted to Dublin and Belfast, and new glassworks specialising in cut glass were installed in Cork and Waterford.
In Holland, local engraving masters such as David Wolff and Frans Greenwood stippled imported English glassware, a style that remained popular through the eighteenth century.
[4] Such was its popularity in Holland that the first Continental production of lead-crystal glass began there, probably as the result of imported English workers.
[13] Imitating lead-crystal à la façon d’Angleterre presented technical difficulties, as the best results were obtained with covered pots in a coal-fired furnace, a particularly English process requiring specialised cone-furnaces.
The fluxing and refractive properties valued for lead glass also make it attractive as a pottery or ceramic glaze.
Glazes with even-higher lead content occur in Spanish and Italian maiolica, with up to 55% PbO and as low as 3% alkali.
It must not craze, forming a network of cracks, caused when the thermal contraction of the glaze and the ceramic body do not match properly.
[16] Lead glass and glazes have a long and complex history, and continue to play new roles in industry and technology today.
[1] This higher refractive index also raises the correlated dispersion, the degree to which the glass separates light into its colors, as in a prism.
This finding is "consistent with ceramic chemistry theory, which predicts that leaching of lead from crystal is self-limiting exponentially as a function of increasing distance from the crystal-liquid interface.
"[22] It has been proposed that the historic association of gout with the upper classes in Europe and America was, in part, caused by the extensive use of lead crystal decanters to store fortified wines and whisky.
