Crystal
Glass
Lead crystal glass, also
called crystal, is lead glass that has been hand or machine
cut with facets. Lead oxide added to the molten glass gives
lead crystal a much higher index of refraction than normal
glass, and consequently much greater "sparkle". The
presence of lead also makes the glass softer and easier to cut.
Crystal can consist of up to 35% lead, at which point it has
the most sparkle. The higher lead content also makes it much
more difficult to form crystal during manufacturing.
Englishman, George
Ravenscroft discovered crystal in 1676.
Makers of lead crystal
objects include Baccarat and J.G.Durand in France, Royal
Leerdam Crystal of the Netherlands, Steuben Glass in the
United States, Waterford Crystal in Ireland, Mikasa in Japan,
Swarovski in Austria and Preciosa in Czech Republic.
Also Ravenscroft, Gorham, Kosta Boda, Dansk, Edgar Berebi,
Spiegelau, Schott Zwiesel, Royal Doulton, Noritake, Faberge,
Lenox, Asta Glass, Orrefors, Miller Rogaska, Wallace, Galmer,
Rosenthal, Versace, Varewang, Riedel, Oneida Villeroy &
Bosh.
Crystal Glassware History
Lead may be introduced into
glass either as an ingredient of the primary melt or added to
pre-formed unleaded glass or frit. The lead oxide used in lead
glass could be obtained from a variety of sources. In Europe,
galena, PbS, was widely available, which could be smelted to
produce metallic lead. The lead metal would be calcined to
form PbO 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. Metalworkers and glassmakers required ceramic
crucibles, and lead was needed for ceramic lead glazes. This
material interdependence suggests a close working relationship
between potters, glassmakers, and metalworkers.
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 object such as bi 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 (On Divers Arts),
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 (On the Colours and Arts of the 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 guarantee 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
coloured 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.
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