Lead glass - WikiHQ

thumb|right|300x300px|[[Cut glass wine glass made of lead glass]]

Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass typically contains 18–40% (by mass) lead(II) oxide (PbO); modern lead crystal or leaded crystal, historically also known as flint glass for the original silica source, contains a minimum of 24% PbO. In marketing terms it is often called crystal glass.

The term lead crystal is, technically, not an accurate way to describe lead glass, because glass lacks a crystalline structure and is instead an amorphous solid. The use of the term remains popular for historical and commercial reasons but is sometimes changed to simply crystal because of lead's reputation as a toxic substance. It is retained from the Venetian word to describe the rock crystal (quartz) imitated by Murano glassmakers. This naming convention has been maintained to the present day to describe decorative holloware.

Properties

The addition of lead oxide to glass raises its refractive index and lowers its working temperature and viscosity. The attractive optical properties of lead glass result from the high content of the heavy 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 or below, while typical lead crystal has a density of around and high-lead glass can be over or even up to . This increased refractive index also correlates with increased dispersion, a measure of the degree to which a medium separates light into its component wavelengths, thus producing a spectrum of colors just as a prism does. Crystal cutting techniques exploit these properties to create a brilliant, sparkling effect as each cut facet in cut glass reflects and transmits light through the object. The high refractive index is useful for lens making, since a given focal 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 be achromatic.

The addition of lead oxide to potash glass also reduces its viscosity, rendering it more fluid than ordinary soda glass above its softening temperature (about ), with a working point of . 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 ). From the glassmaker's perspective, this results in two practical effects. First, lead glass may be worked at a lower temperature, facilitating its use in enamelling; 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 the clinking 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 of F and G sharp". Consumers still rely on this property to distinguish lead glass from cheaper glasses. Emil Deeg had published a major study on the ringing of the lead crystal in 1958.

Since the potassium ions are bound more tightly in a lead-silica matrix than in a soda–lime glass, the former absorbs more energy when struck. This causes the lead crystal to oscillate, thereby producing its characteristic sound. Lead-containing glass is frequently used in light fixtures.

{| class="wikitable"

! Use

! PbO by weight (%)

| Household "crystal" leaded glass || 18–38

| Ceramic glazes and vitreous enamels || 16–35

| High refractive index optical glasses || 4–65

| Radiation shielding || 2–28

| High electrical resistance || 20–22

| Glass solders and sealants || 56–77

History

Lead may be introduced into glass either as an ingredient of the primary melt or as an addition to preformed leadless glass or frit. 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 be smelted to produce metallic lead. The lead metal was 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, including 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. Glass 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). There, it was 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 first 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. 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. 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. 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 the Corning Glassworks, New York State, developed a new lead glass of high optical clarity. This became the focus of Steuben Glass Works, 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.

Lead glazes

The fluxing and refractive properties valued for lead glass also make it attractive as a pottery or ceramic 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%. From the Roman period, they remained popular through the Byzantine and Islamic periods in the Near 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 from galena (PbS), red lead (Pb3O4), white lead (2PbCO3·Pb(OH)2), or lead 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.

|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 by Corning Incorporated to the Schottenstein 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 the Arc International to Lifetime Brands in 2008

|Liuligongfang

|Taiwan

|1987

|Production continued

|Asfour crystal

|Egypt

|1961

|Production continued

Safety

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 2011 World Health Organization committee on food additives "concluded that it was not possible to establish a new PTWI (provisional tolerable weekly intake) that would be considered health protective." Citrus juices and other acidic drinks leach lead from crystal as effectively as alcoholic beverages. 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. 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.

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." Statistical evidence linking gout to lead poisoning has been published.

References

Sources

Properties

History

Lead glazes

Safety

See also

References

Sources