Lead Glass - History

History

Lead may be introduced into glass either as an ingredient of the primary melt or added to preformed leadless glass or frit. The lead oxide used in lead glass could be obtained from a variety of sources. In Europe, galena, lead sulfide, was widely available, which could be smelted to produce metallic lead. The lead metal would be calcined to form lead oxide by roasting it and scraping off the litharge. In the medieval period lead metal could be obtained through recycling from abandoned Roman sites and plumbing, even from church roofs. Metallic lead was demanded in quantity for silver cupellation, and the resulting litharge could be used directly by glassmakers. Lead was also used for ceramic lead glazes. This material interdependence suggests a close working relationship between potters, glassmakers, and metalworkers.

Glasses with lead oxide content first appeared 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 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. 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 Middle East. 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 owing 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 owing 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, Hungary, 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 continued 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.