Lead glass, commonly calledcrystal, is a variety ofglass in whichlead replaces thecalcium content of a typicalpotash glass.[1] Lead glass typically contains 18–40% (by mass)lead(II) oxide (PbO); modernlead crystal orleaded crystal,[2] historically also known asflint glass for the originalsilica source, contains a minimum of 24% PbO.[3] Lead glass is desirable for a variety of uses due to its clarity.[4] In marketing terms it is often calledcrystal glass.
The termlead crystal is, technically, not an accurate way to describe lead glass, because glass lacks acrystalline structure and is instead anamorphous solid. The use of the term remains popular for historical and commercial reasons but is sometimes changed to simplycrystal because of lead's reputation as a toxic substance. It is retained from theVenetian wordcristallo to describe the rock crystal (quartz) imitated byMurano glassmakers. This naming convention has been maintained to the present day to describe decorativeholloware.[5]
Lead crystalglassware was formerly used to store and serve drinks, but thehealth risks of lead have made this use rare. An alternative material is modern crystal glass, in whichbarium oxide,zinc oxide, orpotassium oxide are employed instead of lead oxide.
In theEuropean Union, labelling of "crystal" products is regulated byCouncil Directive 69/493/EEC, which defines four categories, depending on the chemical composition and properties of the material. Only glass products containing at least 24% lead oxide may be referred to as "lead crystal". Products with less lead oxide, and glass products with other metal oxides used in place of lead oxide, must be labelled "crystalline" or "crystal glass".[6]
The addition of lead oxide to glass raises itsrefractive index and lowers its working temperature andviscosity. The attractive optical properties of lead glass result from the high content of theheavy metal lead. Lead, whose density is more than seven times that of calcium, also raises the density of the glass. The density of soda glass is 2.4 g/cm3 (1.4 oz/cu in) or below, while typical lead crystal has a density of around 3.1 g/cm3 (1.8 oz/cu in) and high-lead glass can be over 4.0 g/cm3 (2.3 oz/cu in) or even up to 5.9 g/cm3 (3.4 oz/cu in).[1]
The brilliance of lead crystal arises from the highrefractive index caused by the lead content. Ordinary glass has a refractive (n) of 1.5, while the addition of lead produces a range up to 1.7[1] or 1.8.[7] This increased refractive index also correlates with increaseddispersion, a measure of the degree to which a medium separates light into its component wavelengths, thus producing a spectrum of colors just as aprism does. Crystal cutting techniques exploit these properties to create a brilliant, sparkling effect as each cut facet incut glass reflects and transmits light through the object. The high refractive index is useful forlens making, since a givenfocal length can be achieved with a thinner lens. However, the dispersion must be corrected by other components of the lens system if the lens is to beachromatic.
The addition of lead oxide to potash glass also reduces itsviscosity, rendering it more fluid than ordinary soda glass above its softening temperature (about 600 °C or 1,112 °F), with a working point of 800 °C (1,470 °F). The viscosity of glass varies radically with temperature, but that of lead glass is roughly two orders of magnitude lower than that of ordinary soda glasses across working temperature ranges (up to 1,100 °C or 2,010 °F).[8] From the glassmaker's perspective, this results in two practical effects. First, lead glass may be worked at a lower temperature, facilitating its use inenamelling; second, clear vessels may be made without trapped air bubbles with less difficulty than with ordinary glasses, allowing the manufacture of perfectly clear, flawless objects.
When tapped, lead crystal makes a ringing sound, unlike ordinary glasses. Wine glasses made of lead glass are valued for the "ring" made by theclinking of glasses. The sound was considered better when a large quantity of lead oxide was present in the glassmaking material, as in the British and Irish wine glasses of the 17th-19th centuries, with their "rich bell-notes ofF andG sharp".[9] Consumers still rely on this property to distinguish lead glass from cheaper glasses. Emil Deeg had published a major study[10] on the ringing of the lead crystal in 1958.[11]Since the potassium ions are bound more tightly in a lead-silica matrix than in asoda–lime glass, the former absorbs more energy when struck[dubious –discuss]. This causes the lead crystal tooscillate, thereby producing its characteristic sound.[1] Lead also increases the solubility oftin,copper, andantimony, leading to its use in colored enamels andglazes. The low viscosity of lead glass melt is the reason for typically high lead oxide content in theglass solders.
The presence of lead is used in glasses absorbinggamma radiation andX-rays, used inradiation shielding as a form oflead shielding (e.g. incathode-ray tubes, thus lowering the exposure of the viewer to soft X-rays). Inparticle physics, the combination of the lowradiation length resulting from the high density and presence of heavy nuclei with the high refractive index which leads to both pronouncedCherenkov radiation and containment of the Cherenkov light bytotal internal reflection makes lead glass one of the prominent tools forphoton detection by means ofelectromagnetic showers.
The highionic radius of the Pb2+ ion renders it highly immobile in the matrix and hinders the movement of other ions; lead glasses therefore have high electrical resistance, about two orders of magnitude higher than soda–lime glass (108.5 vs 106.5 ohm·cm,DC at 250 °C or 482 °F).[12] Lead-containing glass is frequently used inlight fixtures.
| Use | PbO by weight (%) |
|---|---|
| Household "crystal" leaded glass | 18–38 |
| Ceramic glazes andvitreous enamels | 16–35 |
| Highrefractive index optical glasses | 4–65 |
| Radiation shielding | 2–28 |
| Highelectrical resistance | 20–22 |
| Glass solders and sealants | 56–77 |
Lead may be introduced into glass either as an ingredient of the primary melt or as an addition to preformed leadless glass orfrit. The lead oxide used in lead glass can be obtained from a variety of sources. In Europe,galena – lead sulfide – was widely available, and it could besmelted to produce metallic lead. The lead metal wascalcined to form lead oxide by roasting it and scraping off thelitharge. In themedieval period lead metal could be obtained through recycling from abandoned Roman sites and plumbing, including from church roofs. Metallic lead was demanded in quantity for silvercupellation, 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.[13]
Glasses with lead oxide content first appeared inMesopotamia, thebirthplace of the glass industry.[5] The earliest known example is a blue glass fragment fromNippur dated to 1400 BC containing 3.66% PbO. Glass is mentioned in clay tablets from the reign ofAssurbanipal (668–631 BC), and a recipe for lead glaze appears in a Babylonian tablet of 1700 BC.[14] A red sealing-wax cake found in the Burnt Palace atNimrud, 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 inHan-period China (206 BC – 220 AD). There, it was cast to imitatejade, 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 alongthe Silk Road by glassworkers from the Middle East.[5] The fundamental compositional difference between Western silica-natron glass and the unique Chinese lead glass, however, may indicate an autonomous development.
In medieval andearly modern Europe, lead glass was used as a base in coloured glasses, specifically in mosaictesserae, enamels,stained-glass painting, andbijouterie, where it was used to imitateprecious stones. Several textual sources describing lead glass survive. 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 hisDe coloribus et artibus Romanorum (Of Hues and Crafts of the Romans). This refers to lead glass as "Jewish glass", perhaps indicating its transmission to Europe.[14] A manuscript preserved in theBiblioteca Marciana, 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 theforest glass of the body.
Antonio Neri devoted book four of hisL'Arte Vetraria ("The Art of Glass-making", 1612) to lead glass. 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, setting the basis from which England overtook Venice and Bohemia 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 Worshipful Company of Glass Sellers of London, Ravenscroft sought to find an alternative to Venetiancristallo. His use of flint as the silica source has led to the termflint glass to describe these crystal glasses, despite his later switch to sand.[3] At first, his glasses tended tocrizzle, 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.[1] He was granted a protective patent in 1673, where production moved from his glasshouse in theprecinct of the Savoy, London, to the seclusion ofHenley-on-Thames.[15] In 1676, having apparently overcome the crizzling problem, Ravenscroft 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 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.[3]
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 asExcise glasses.[3] 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 incut glass were installed in Cork andWaterford. In 1825, the tax was renewed, and gradually the industry declined until the mid-nineteenth century, when the tax was finally repealed.[5]
From the 18th century, 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 andFrans Greenwood stippled imported English glassware, a style that remained popular through the eighteenth century.[5] 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.[14] Imitating lead-crystalà 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.[3] Towards the end of the eighteenth century, lead-crystal glass was being produced in France, Hungary, Germany, and Norway.[14][16] By 1800,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 theCorning Glassworks, New York State, developed a new lead glass of high optical clarity. This became the focus ofSteuben Glass Works, a division of Corning, which produced decorative vases, bowls, and glasses inArt Deco style. Lead crystal continues to be used in industrial and decorative applications.
The fluxing and refractive properties valued for lead glass also make it attractive as a pottery orceramic glaze. Lead glazes first appear in first century BC to first century AD Roman wares, and occur nearly simultaneously in China. They were very high in lead, 45–60% PbO, with a very low alkali content, less than 2%.[17] From the Roman period, they remained popular through the Byzantine and Islamic periods in theNear East, on pottery vessels and tiles throughout medieval Europe, and up to the present day. In China, similar glazes were used from the twelfth century for colored enamels on stoneware, and on porcelain from the fourteenth century.These could be applied in three different ways. Lead could be added directly to a ceramic body in the form of a lead compound in suspension, either fromgalena (PbS),red lead (Pb3O4),white lead (2PbCO3·Pb(OH)2), orlead oxide (PbO). The second method involves mixing the lead compound with silica, which is then placed in suspension and applied directly. The third method involves fritting the lead compound with silica, powdering the mixture, and suspending and applying it.[17] The method used on a particular vessel may be deduced by analysing the interaction layer between the glaze and the ceramic body microscopically.
Tin-opacified glazes appear in Iraq in the eighth century AD. Originally containing 1–2% PbO; by the eleventh century high-lead glazes had developed, typically containing 20–40% PbO and 5–12% alkali. These were used throughout Europe and the Near East, especially inIznik ware, and continue to be used today. Glazes with even-higher lead content occur in Spanish and Italianmaiolica, with up to 55% PbO and as low as 3% alkali.[17] Adding lead to the melt allows the formation oftin oxide more readily than in an alkali glaze: tin oxide 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 andfritted prior to use, since they are soluble in water, requiring additional labor. A successful glaze must notcrawl, or peel away from the pottery surface upon cooling, leaving areas of unglazed ceramic. Lead reduces this risk by reducing thesurface tension of the glaze. It must not craze, forming a network of cracks, caused when thethermal 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.[17] 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 and 1100 °C, but not so low 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.[17]Lead glass and glazes have a long and complex history, and continue to play new roles in industry and technology today.
Lead oxide added to the molten glass gives lead crystal a much higherindex of refraction than normal glass, and consequently much greater "sparkle" by increasingspecular reflection and the range of angles oftotal internal reflection. Ordinary glass has a refractive index ofn = 1.5; the addition of lead produces an index of refraction of up to 1.7.[1] This higher refractive index also raises the correlateddispersion, the degree to which the glass separates light into its colors, as in aprism. The increases in refractive index and dispersion significantly increase the amount of reflected light and thus the "fire" in the glass.
Incut glass, which has been hand- or machine-cut with facets, 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.[1]
Makers of lead crystal objects include:
| Name | Polity | Production began | Notes |
|---|---|---|---|
| NovaScotian Crystal | Canada | 1996 | Production discontinued March 2021 |
| Gus Crystal | Russia | 1756 | Production continued |
| Baccarat | France | 1816 | Part of theStarwood Capital Group since 2005 |
| Saint-Louis | France | 1781 | Part ofHermès since 1989 |
| Lalique | France | 1920s | Part of theArt & Fragrance since 2011 |
| Daum | France | 1878 | Part ofFinanciere Saint-Germain since 2009 after bankruptcy in 2003 |
| Arc International | France | 1968 | Production ofCrystal D'Arque ended in 2009; restarted in 2010 as lead-free Diamax. |
| Dartington Crystal | England | 1967 | Management buy out in 2006. |
| Cumbria Crystal | England | 1976 | Last remaining Luxury Cut Crystal producer in the UK. |
| Royal Brierley | England | 1776 | Trademark of theDartington Crystal since 2006 |
| Waterford Crystal | Republic of Ireland | 1783 | WWRD Holdings ofKPS Capital Partners after bankruptcy in 2009. |
| Galway Crystal | Republic of Ireland | ||
| Tipperary Crystal | Republic of Ireland | 1987 | Founded by formerWaterford Crystal craftsmen. |
| Cavan Crystal | Republic of Ireland | ||
| Tyrone Crystal | Northern Ireland | 1971 | Factory closed 2010 |
| Dingle Crystal | Republic of Ireland | 1998 | |
| Edinburgh Crystal | Scotland | 1867 | Trademark of the WWRD Holdings after bankruptcy in 2006 |
| Hadeland Glassverk | Norway | 1765 | Production continued |
| Kristal Samobor | Croatia | 1839 | Production continued |
| Magnor Glassverk | Norway | 1830 | Production continued |
| Orrefors glassworks | Sweden | 1913 | Part of the Swedish glassworks groupOrrefors Kosta Boda AB since 2005 |
| Kosta Boda | Sweden | 1742 | Part of the Swedish glassworks groupOrrefors Kosta Boda AB since 2005 |
| Holmegaard Glass Factory | Denmark | 1825 | Production ceased in 2009 |
| Val Saint Lambert | Belgium | 1826 | Sold toOnclin winemaker family for $5M in 2008 |
| Mozart Crystal | Brazil | 2018 | Production continued |
| Royal Leerdam Crystal | Netherlands | 1765 | Merged with porcelain factoryDe Koninklijke Porceleyne Fles in 2008 |
| Zwiesel Kristallglas | Germany | 1872 | Management buy out atSchott AG in 2001. Only crystal manufacturer in Germany |
| Nachtmann | Germany | 1834 | Trademark of theRiedel wine glass company since 2004 |
| Riedel wine glass company | Austria | 1756 | World leading wine glass manufacturer |
| Swarovski | Austria | 1895 | Production continued |
| Ajka crystal | Hungary[18] | 1878 | In 1991 opened porcelain studio |
| Moser | Czech Republic | 1857 | Production continued |
| Rückl | Czech Republic | 1846 | Production continued |
| Crystalex | Czech Republic | 1948 | Production continued |
| Preciosa | Czech Republic | 1948 | Production continued |
| Lasvit | Czech Republic | 2008 | Production continued |
| Steuben Glass | United States | 1903 | Sold byCorning Incorporated to theSchottenstein Stores Corp. in 2008. In 2008 Schottenstein closed factory |
| Rogaška | Slovenia | 1927 | Production continued |
| Hoya | Japan | 1945 | Closed in 2009 |
| Mikasa | Japan | 1970s | Sold by theArc International toLifetime Brands in 2008 |
| Liuligongfang | Taiwan | 1987 | Production continued |
| Asfour crystal | Egypt[19] | 1961 | Production continued |
Several studies have demonstrated that serving food or drink in glassware containing lead oxide can cause lead to leach into the contents, even when the glassware has not been used for storage. Due to an inability to "indicate a threshold for the key effects of lead," a 2011World Health Organization committee on food additives "concluded that it was not possible to establish a new PTWI (provisionaltolerable weekly intake) that would be considered health protective."[20][21]
The amount of lead released from lead glass increases with the acidity of the substance being served. Vinegar, for example, has been shown to cause more rapid leaching compared to white wine, as vinegar is more acidic.[22] Citrus juices and other acidic drinks leach lead from crystal as effectively as alcoholic beverages.[21][23] Daily usage of lead crystalware (without longer-term storage) was found to add up to 14.5 μg of lead from drinking a 350ml cola beverage.[21]
The amount of lead released into a food or drink increases with the amount of time it stays in the vessel. In a study performed atNorth Carolina State University, the amount of lead migration was measured forport wine stored in lead crystaldecanters.[24] After two days, lead levels were 89 μg/L (micrograms per liter). After four months, lead levels were between 2,000 and 5,000 μg/L. White wine doubled its lead content within an hour of storage and tripled it within four hours. Some brandy stored in lead crystal for over five years had lead levels around 20,000 μg/L.[25]
Lead leaching from the same decanter decreases with repeated uses. 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."[23]
It has been proposed that the historic association ofgout with the upper classes in Europe and America was, in part, caused by the extensive use of lead crystal decanters to storefortified wines andwhisky.[26] Statistical evidence linking gout tolead poisoning has been published.[27]
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