Glass consists of a network former, typically silica (SiO2), and network modifiers, including alkali fluxes such as potassium oxide or sodium oxide, and a stabilizer, typically calcium oxide. Lead oxide acts as both a flux and a stabilizer. Lead glass forms part of the silica-potassium-lead system, where lead replaces the calcium content of typical potash glasses. Lead glass contains typically 18–35 mol% PbO, whilst modern lead crystal, historically also known as flint glass due to the original silica source, contains a minimum of 24% lead oxide. Technically, the term crystal should never be applied to glass, as glass by definition lacks a crystalline structure, but the use of the term lead crystal remains popular due to historical and commercial reasons, originally stemming from the Venetian practice of calling rock crystal imitating the style of Murano glassmakers, cristallo. This is a naming convention which has been maintained to the present day to describe decorative hollowware.
The brilliance of lead crystal relies on the high refractive index (RI) caused by lead oxide. Ordinary glass has a refractive index of n=1.5, whereas the addition of lead produces a range up to 1.7. This heightened RI also raises the correlating index of dispersion, which measures the degree to which a medium separates light into its component spectra, as in a prism. This is why high-lead glass is favoured for achromatic lenses. Cut crystal decorating techniques exploit these properties to create its brilliant, sparkling qualities as each cut facet reflects and transmits light through the object.
The addition of lead oxide to potash glass also reduces its viscosity, rendering it more fluid than ordinary soda glass above softening temperature (about 600 °C), with a working point of 800 °C. The viscosity of glass varies radically with temperature, but that of lead glass is roughly 100 times less than that of ordinary soda glasses across working temperature ranges (up to 1100 °C). From the glassmaker’s perspective, this results in two practical developments. First, lead glass may be worked at a lower temperature, leading to its use in enamelling, and second, clear vessels may be made free from trapped air bubbles with considerably less difficulty than with ordinary glasses, allowing the manufacture of perfectly clear, flawless objects. When tapped, lead crystal rings, unlike ordinary glasses. Consumers still rely on this property to distinguish it from cheaper glasses. Since the potash ions are bound more tightly in a lead-silica matrix than in a soda-lime glass, the glass when struck absorbs less energy. This causes the glass 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.
Lead oxide additives first appear in Mesopotamia, the birthplace of the glass industry. The earliest known example is a blue glass fragment from Nippur dated to 1400 BC containing 3.66% PbO, and is mentioned in clay tablets from the reign of Assurbanipal (668–631 BC), and a recipe for lead glaze appears in a Babylonian tablet of 1700 BC. A red sealing-wax cake found in the Burnt Palace at Nimrud, from the early C6 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. Lead glass also occurs in Han-period China (206 BC – 220 AD). Here it was used in 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 occurs at such a late date in China, it is thought that the technology was brought along the Silk Road by glassworkers from the West. The fundamental compositional difference between Western silica-natron glass and the unique Chinese lead glass, however, may indicate a quite different development.
In medieval and early modern Europe lead glass was used as a base in coloured glasses, specifically in mosaic tesserae, enamels, stained-glass painting, and bijouterie, where it was used to imitate precious stones. Several textual sources describing lead glass survive. In his Schedula Diversarum Artium (List Sundry Crafts'), Theophilus describes its use as imitation gemstone, and the title of a lost chapter mentions the use of lead in glass. The 12–13th century Heraclius details the manufacture of lead enamel and its use for window painting in his De Coloribus et artibus Romanorum (Of for Huereds and Crafts Romans'). This refers to lead glass as “Jewish glass”, perhaps indicating its transmission to Europe. A manuscript preserved at San Marco, Venice, describes the use of lead oxide in enamels and includes recipes for calcining lead to form the oxide. Lead glass was ideally suited for enamelling vessels and windows due to its lower working temperature than the forest glass body.
Antonio Neri devoted his entire book four of his L’Arte Vetraria to lead glass, first published in 1612. In this first systematic treatise on glass, he again refers to the use of lead glass in enamels, glassware, and for the imitation of precious stones. Christopher Merrett translated this into English in 1662 (The Art of Glass), paving the way for the production of English lead crystal glass by George Ravenscroft.
George Ravenscroft (1618–1681) was the first to produce clear lead crystal glassware on an industrial scale. The son of a merchant with close ties to Venice, Ravenscroft had the cultural and financial resources necessary to revolutionise the glass trade, allowing England to overtake Venice as the centre of the glass industry in the eighteenth and nineteenth centuries. With the aid of Venetian glassmakers, especially da Costa, and under the auspices of the Glass Sellers Guild, Ravenscroft sought to find an alternative to Venetian cristallo. His use of flint as the silica source has led to the term flint glass to describe these crystal glasses, despite his later switch to sand. 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%. Crizzling results from the destruction of the glass network by an excess of alkali, and may be caused by excess humidity as well as inherent defects in glass composition. He was granted a protective patent in 1673, where production and refinement moved from his glasshouse on the Savoy to the seclusion of Henley-on-Thames, and in 1676, having apparently overcome the crizzling problem, was granted the use of a raven’s head seal as a guaranty of quality. In 1681, the year of his death, the patent expired and operations quickly developed amongst several firms, where by 1696 twenty-seven of the eighty-eight glasshouses in England were producing flint glass containing 30–35% PbO, especially at London and Bristol.
At this period, glass was sold by weight, and the typical forms were rather heavy and solid with minimal decoration. Such was its success on the international market, however, that in 1746 the British Government imposed a lucrative tax by weight. Rather than drastically reduce the lead content of their glass, manufacturers responded by creating highly-decorated, smaller, more-delicate forms, often with hollow stems, known to collectors today as Excise glasses. In 1780, the Government granted Ireland free trade in glass without taxation. English labour and capital then shifted to Dublin and Belfast, and new glassworks specialising in cut glass were installed in Cork and Waterford. In 1825, the tax was renewed, and gradually the industry declined until the mid-nineteenth century, when they were finally repealed.
From this period English lead glass became popular throughout Europe, and was ideally suited to the new taste for wheel-cut glass decoration perfected on the Continent due to its relatively soft properties. 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. 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. 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. Towards the end of the eighteenth century, lead-crystal glass was being produced in France, Germany, and Norway. By 1800 Anglo-Irish lead crystal had overtaken lime-potash glasses on the Continent, and traditional glassmaking centres in Bohemia began to focus on colored glasses rather than compete directly against it.
The development of lead glass continues through the twentieth century, when in 1932 scientists at the Corning Glassworks, New York, developed a new lead glass of high optical clarity. This became the focus of Steuben glassworks, a division of Corning, which produced decorative vases, bowls, and glasses in Art Deco style. Lead-crystal continues to be used in industrial and decorative applications.
Tin-opacified glazes appear in Iraq in the eighth century AD. Originally containing 1–2% PbO, by the eleventh century high-lead glaze had developed, typically containing 20–40% PbO and 5–12% alkali. These were used throughout Europe and the Near East, especially in Iznik ware, and continue to be used today. Glazes with even-higher lead content occur in Spanish and Italian maiolica, with up to 55% PbO and as low as 3% alkali. Adding lead to the melt allows the formation of tin oxide more readily than in an alkali glaze, which precipitates into crystals in the glaze as it cools, creating its opacity.
The use of lead glaze has several advantages over alkali glazes in addition to their greater optical refractivity. Lead compounds in suspension may be added directly to the ceramic body. Alkali glazes must first be mixed with silica and fritted prior to use, since they are soluble in water, requiring additional work input. A successful glaze must not crawl, or peel away from the pottery surface upon cooling, leaving areas of unglazed ceramic. Lead reduces this risk by reducing the surface tension of the glaze. It must not craze, forming a network of cracks, nor peel. This is caused when the thermal contraction of the glaze and the ceramic body do not match properly. Ideally, the glaze contraction should be 5–15% less than the body contraction, as glazes are stronger under compression than under tension. A high-lead glaze has a linear expansion coefficient of between 5 and 7×10-6/°C, compared to 9 to 10×10-6/°C for alkali glazes. Those of earthenware ceramics vary between 3 and 5×10-6/°C for non-calcareous bodies and 5 to 7×10-6/°C for calcareous clays, or those containing 15–25% CaO. Therefore the thermal contraction of lead glaze matches that of the ceramic more closely than an alkali glaze, rendering it less prone to crazing. A glaze should also have a low enough viscosity to prevent the formation of pinholes as trapped gasses escape during firing, typically between 900–1100 °C, but not so high as to run off. The relatively-low viscosity of lead glaze mitigates this issue. It may also have been cheaper to produce than alkali glazes. Lead glass and glazes have a long and complex history, and continue to play new roles in industry and technology today.