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Lead glass

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See also:leaded glass (disambiguation)

Cut glasswine glass made of lead glass

Lead glass,commonly calledcrystal,is a variety ofglassin whichleadreplaces thecalciumcontent of a typicalpotashglass.[1]Lead glass contains typically 18–40% (by mass)lead(II) oxide(PbO), while modernlead crystal,historically also known asflint glassdue to the originalsilicasource, contains a minimum of 24% PbO.[2]Lead glass is often desirable for a variety of uses due to its clarity.[3]In marketing terms it is often calledcrystal glass.

The termlead crystalis, technically, not an accurate term to describe lead glass, because glass lacks acrystalline structureand is instead anamorphous solid.The use of the termlead crystalremains popular for historical and commercial reasons, but this term is sometimes changed to simplycrystalbecause of lead's reputation as a toxic substance. It is retained from theVenetianwordcristalloto describe the rock crystal (quartz) imitated byMuranoglassmakers. This naming convention has been maintained to the present day to describe decorativeholloware.[4]

Lead crystalglasswarewas formerly used to store and serve drinks, but due to thehealth risks of lead,this has become rare. One alternative material is modern crystal glass, in whichbarium oxide,zinc oxide,orpotassium oxideare employed instead of lead oxide.

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. Only glass products containing at least 24% of lead oxide may be referred to as "lead crystal". Products with less lead oxide, or glass products with other metal oxides used in place of lead oxide, must be labelled "crystalline" or "crystal glass".[5]

Properties[edit]

The addition of lead oxide to glass raises itsrefractive indexand lowers its working temperature andviscosity.The attractive optical properties of lead glass result from the high content of theheavy metallead. Lead also raises the density of the glass, being over 7 times as dense as calcium. The density of soda glass is 2.4g/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 relies on the highrefractive indexcaused 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.[6]This heightened refractive index also correlates with increaseddispersion,which measures the degree to which a medium separates light into its component wavelengths, thus producing a spectrum, just as aprismdoes. Crystal cutting techniques exploit these properties to create a brilliant, sparkling effect as each cut facet incut glassreflects and transmits light through the object. The high refractive index is useful forlensmaking, since a givenfocal lengthcan be achieved with a thinner lens. However, the dispersion must be corrected by other components of the lens system if it is to beachromatic.

The addition of lead oxide to potash glass also reduces itsviscosity,rendering it more fluid than ordinary soda glass above 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).[7]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 inenamelling,and second, clear vessels may be made without trapped air bubbles with less difficulty than ordinary glasses, allowing the manufacture of perfectly clear, flawless objects.

When tapped, lead crystal makes a ringing sound, unlike ordinary glasses. Consumers still rely on this property to distinguish it from cheaper glasses. 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.

Fluoroscopy room with control space separated bylead shieldingglass

The presence of lead is used in glasses absorbinggamma radiationandX-rays,used inradiation shieldingas 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 lengthresulting from the high density and presence of heavy nuclei with the high refractive index which leads to both pronouncedCherenkov radiationand containment of the Cherenkov light bytotal internal reflectionmakes lead glass one of the prominent tools forphoton detectionby means ofelectromagnetic showers.

The highionic radiusof 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 (108.5vs 106.5ohm·cm,DCat 250 °C or 482 °F).[8]Lead-containing glass is frequently used inlight fixtures.

Use PbO by weight (%)
Household "crystal" leaded glass 18–38
Ceramic glazesandvitreous enamels 16–35
Highrefractive indexoptical glasses 4–65
Radiation shielding 2–28
Highelectrical resistance 20–22
Glass soldersand sealants 56–77

History[edit]

Lead may be introduced into glass either as an ingredient of the primary melt or added to preformed leadless glass orfrit.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 besmeltedto produce metallic lead. The lead metal would becalcinedto form lead oxide by roasting it and scraping off thelitharge.In themedieval periodlead metal could be obtained through recycling from abandoned Roman sites and plumbing, even 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.[9]

Glasses with lead oxide content first appeared inMesopotamia,thebirthplace of the glass industry.[4]The earliest known example is a blue glass fragment fromNippurdated 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.[10]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 flu xing 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 Roadby glassworkers from the Middle East.[4]The fundamental compositional difference between Western silica-natronglass 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-glasspainting, 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.[10]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 glassof the body.

Antonio Neridevoted 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 Merretttranslated 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 glassto describe these crystal glasses, despite his later switch to sand.[2]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.[11]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.[2]

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.[2]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 glasswere 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.[4]

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 Greenwoodstippled 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.[10]Imitating lead-crystalà la façon d’Angleterrepresented technical difficulties, as the best results were obtained with covered pots in a coal-fired furnace, a particularly English process requiring specialised cone-furnaces.[2]Towards the end of the eighteenth century, lead-crystal glass was being produced in France, Hungary, Germany, and Norway.[10][12]By 1800,Irishlead 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 Decostyle. Lead-crystal continues to be used in industrial and decorative applications.

Lead glazes[edit]

The flu xing 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%.[13]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 mi xing 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.[13]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 glazesappear 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.[13]Adding lead to the melt allows the formation oftin oxidemore 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 andfrittedprior 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 tensionof the glaze. It must not craze, forming a network of cracks, caused when thethermal contractionof 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.[13]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.[13] Lead glass and glazes have a long and complex history, and continue to play new roles in industry and technology today.

Lead crystal[edit]

Lead crystal beads

Lead oxideadded to the molten glass gives lead crystal a much higherindex of refractionthan normal glass, and consequently much greater "sparkle" by increasingspecular reflectionand 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 Groupsince 2005
Saint-Louis France 1781 Part ofHermèssince 1989
Lalique France 1920s Part of theArt & Fragrancesince 2011
Daum France 1878 Part ofFinanciere Saint-Germainsince 2009 after bankruptcy in 2003
Arc International France 1968 Production ofCrystal D'Arqueended 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 Crystalsince 2006
Waterford Crystal Ireland 1783 WWRD HoldingsofKPS Capital Partnersafter bankruptcy in 2009.
Galway Crystal Ireland
Tipperary Crystal Ireland 1987 Founded by formerWaterford Crystalcraftsmen.
Cavan Crystal Ireland
Tyrone Crystal Ireland 1971 Factory closed 2010
Dingle Crystal 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 ABsince 2005
Kosta Boda Sweden 1742 Part of the Swedish glassworks groupOrrefors Kosta Boda ABsince 2005
Holmegaard Glass Factory Denmark 1825 Production ceased in 2009
Val Saint Lambert Belgium 1826 Sold toOnclinwinemaker family for $5M in 2008
Mozart Crystal Brazil 2018 Production continued
Royal Leerdam Crystal Netherlands 1765 Merged with porcelain factoryDe Koninklijke Porceleyne Flesin 2008
Zwiesel Kristallglas Germany 1872 Management buy out atSchott AGin 2001. Only crystal manufacturer in Germany
Nachtmann Germany 1834 Trademark of theRiedel wine glass companysince 2004
Riedel wine glass company Austria 1756 World leading wine glass manufacturer
Swarovski Austria 1895 Production continued
Ajka crystal Hungary[14] 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 Incorporatedto 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 InternationaltoLifetime Brandsin 2008
Liuligongfang Taiwan 1987 Production continued
Asfour crystal Egypt[15] 1961 Production continued

Safety[edit]

There is no safe level of lead intake.[16]TheWHOofficially withdrew its previous safe levels in 2011 after reviewing the available research and concluding that the only safe level is zero. Studies on lead safety and lead migration from leaded glass into food from before 2011 will often cite lead tolerable intake levels (PTWI, or provisional tolerable daily intake) greater than zero;[17]those intake levels are obsolete and should not be treated as health advice.

Several studies have demonstrated that merely 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. The amount of lead released increases with the acidity of the substance being served. Vinegar has been shown to cause more rapid leaching compared to white wine, as vinegar is more acidic.[18]Citrus juices and other acidic drinks leach lead from crystal as effectively as alcoholic beverages.[17][19]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.[17]

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 winestored in lead crystaldecanters.[20]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.[21]

Lead leaching from the same decanterdecreaseswith 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."[19]

It has been proposed that the historic association ofgoutwith the upper classes in Europe and America was, in part, caused by their extensive use of lead crystal decanters to storefortified winesandwhisky.[22]Statistical evidence linking gout tolead poisoninghas been correlated.[23]

See also[edit]

Wikimedia Commons has media related toCrystal glass.

References[edit]

  1. ^abcdefgNewton, Roy G.; Sandra Davison (1989).Conservation of Glass.Butterworth – Heinemann Series in Conservation and Museology. London:Butterworths.ISBN0-408-10623-9.
  2. ^abcdeHurst-Vose, Ruth (1980).Glass.Collins Archaeology. London:Collins.ISBN0-00-211379-1.
  3. ^Benvenuto, Mark Anthony (24 February 2015).Industrial Chemistry: For Advanced Students.Walter de Gruyter GmbH & Co KG.ISBN9783110351705.
  4. ^abcdeTait, Hugh, ed. (2004).Five Thousand Years of Glass.University of Pennsylvania Press(orig.British Museum Press).ISBN978-0-8122-1888-6.
  5. ^"Council Directive 69/493/EEC of 15 December 1969 on the approximation of the laws of the Member States relating to crystal glass".
  6. ^Refraction of media tutorial.physics.info
  7. ^NIST viahttps://glassproperties /viscosity/
  8. ^James F. Shackelford, Robert H. Doremus (2008).Ceramic and Glass Materials: Structure, Properties and Processing.Springer. p. 158.ISBN978-0-387-73361-6.
  9. ^Fiori, Cesare; Mariangela Vandini (2004). "Chemical Composition of Glass and its Raw Materials". In Marco Beretta (ed.).When Glass Matters: Studies in the History of Science and Art from Graeco-Roman Antiquity to Early Modern Era.Florence: Olschki.ISBN88-222-5318-3.
  10. ^abcdCharleston, R. J. (1960). "Lead in Glass".Archaeometry.3:1–4.doi:10.1111/j.1475-4754.1960.tb00508.x.
  11. ^MacLeod, Christine (1987). "Accident or Design? George Ravenscroft's Patent and the Invention of Lead-Crystal Glass".Technology and Culture.28(4): 776–803.doi:10.2307/3105182.JSTOR3105182.
  12. ^"About us – Ajka Kristály".Ajka, Hungary: Ajka Kristály. Archived fromthe originalon 20 December 2012.Retrieved16 August2012.
  13. ^abcdeTite, M. S.; Freestone, I.; Mason, R.; Molera, J.; Vendrell-Saz, M.; Wood, N. (1998). "Lead Glazes in Antiquity—methods of Production and Reasons for Use".Archaeometry.40(2): 241–60.doi:10.1111/j.1475-4754.1998.tb00836.x.
  14. ^"FOTEX-group » Ajka Crystal LLC".Luxembourg, Belgium: Fotex Holding SE Plc. Archived fromthe originalon 2 January 2012.Retrieved16 August2012.
  15. ^"ASFOUR crystal » about us".Cairo, Egypt: ASFOUR Crystal international. Archived fromthe originalon 1 May 2013.Retrieved9 May2013.
  16. ^"Evaluations of the Joint FAO/WHO Expert Committee on Food Additives (JECFA)".apps.who.int.Retrieved22 July2022.
  17. ^abcGuadagnino, E; Gambaro, M; Gramiccioni, L; Denaro, M; Feliciani, R; Baldini, M; Stacchini, P; Giovannangeli, S; et al. (2000). "Estimation of lead intake from crystalware under conditions of consumer use".Food Additives and Contaminants.17(3): 205–18.doi:10.1080/026520300283469.PMID10827902.S2CID23911153.
  18. ^Hight, S. C. (1996)."Lead migration from lead crystal wine glasses".Food Additives and Contaminants.13(7): 747–765.doi:10.1080/02652039609374463.ISSN0265-203X.PMID8885316.
  19. ^abBarbee, SJ; Constantine, LA (1994). "Release of lead from crystal decanters under conditions of normal use".Food and Chemical Toxicology.32(3): 285–8.doi:10.1016/0278-6915(94)90202-X.PMID8157224.
  20. ^Appel, B R; Kahlon, J K; Ferguson, J; Quattrone, A J; Book, S A (1992)."Potential lead exposures from lead crystal decanters".American Journal of Public Health.82(12): 1671–1673.doi:10.2105/ajph.82.12.1671.ISSN0090-0036.PMC1694534.PMID1456345.
  21. ^Graziano, P (1991). "Lead exposure from lead crystal".The Lancet.337(8734): 141–2.doi:10.1016/0140-6736(91)90803-W.PMID1670790.S2CID11508890.
  22. ^Emsley, John (2005).Elements of murder.Oxford University Press.ISBN0-19-280599-1.
  23. ^Lin, Ja-Liang; Tan, Dan-Tzu; Ho, Huei-Hong; Yu, Chun-Chen (2002). "Environmental lead exposure and urate excretion in the general population".The American Journal of Medicine.113(7): 563–8.doi:10.1016/S0002-9343(02)01296-2.PMID12459402.


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