Tin Glass , commonly called crystal , is a variety of glass in which lead replaces the calcium content of ordinary glass. The glass lead usually contains 18-40% (by weight) of lead (II) oxide (PbO), whereas the modern lead crystal , historically also known as stone glass as the original silica source, contains a minimum of 24% PbO. Tin glass is desirable because of its decorative properties.
Originally discovered by British George Ravenscroft in 1674, the technique of adding lead oxide (in amounts between 10 and 30%) enhanced the appearance of glass and made it easier to melt using marine coal as fuel furnace. This technique also increases the "working period" that makes glass easier to manipulate.
The term "lead crystal" is, technically, not an accurate term for describing tin glass as glass, amorphous solid, has no crystal structure. The use of the term lead crystal remains popular for both historical and commercial reasons. It is preserved from the word Venetian cristallo to describe stone crystals imitated by Murano glassmakers. This naming convention has been maintained to date to describe decorative hollow fixtures.
Lead crystal glasses were previously used to store and serve drinks, but because of the potential health risks of lead, this has become rare. One of the alternative materials is crystal glass , in which barium oxide, zinc oxide, or potassium oxide is used instead of lead oxide. Tin-free crystals have the same refractive index to lead the crystal, but these crystals are lighter and have less power.
In the EU, "crystal" product labeling is governed by Council Directive 69/493/EEC, which defines four categories, depending on chemical composition and material properties. Only glass products containing at least 24% lead oxide can be called "lead crystal". Products with fewer lead oxide, or glass products with other metal oxides used as lead oxide, should be labeled "crystal" or "crystal glass".
Video Lead glass
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The addition of lead oxide to the glass increases its refractive index and lowers its working temperature and viscosity. The attractive optical properties of lead glass result from the high content of heavy metal tin. The high amount of tin atoms also increases the density of the material, since lead has a very high atomic weight of 207.2, compared to 40.08 for calcium. Soda glass density is 2.4 g/cm 3 (0.087 lb/cuÃ, in) or below, whereas typical lead crystal has a density of about 3.1 g/cm 3 and high lead glass can be more than 4.0 g/cm 3 or even up to 5.9 g/cm 3 .
The brilliance of lead crystal depends on the high refractive index caused by the lead content. Regular glass has a refractive index of n = 1.5, while the addition of lead produces a range of up to 1.7 or 1.8. This high refractive index also correlates with increased dispersion, which measures the degree to which the medium separates light into its component spectrum, as in a prism. The crystal-cutting technique exploits these properties to create a brilliant shimmering effect because every aspect of the piece reflects and emits light through the object. A high refractive index is useful for lens manufacture, since the given focal length can be achieved with thinner lenses. However, the dispersion must be corrected by other components of the lens system if it is to be acromatic.
The addition of tin oxide to the potash glass also reduces its viscosity, making it more fluid than regular soda glasses above the softening temperature (about 600 ° C or 1,112 ° F), with a working point of 800 ° C (1.470 ° C). Â ° F). The viscosity of the glass varies radically with temperature, but the tin glass is about 100 times less than regular soda glass across the working temperature range (up to 1,100 ° C or 2.010 ° F). From a glass-making perspective, this generates two practical developments. First, lead glass can work at lower temperatures, leading to its use in enamel, and secondly, a clear vessel can be made free of trapped air bubbles with much less difficulty than ordinary glasses, enabling the creation of very clear objects and flawless.. When tapped, lead crystal makes a ringing sound, unlike ordinary glasses. Consumers still rely on this property to distinguish it from cheaper sunglasses. Since potassium ions are bonded more tightly in a silica-lead matrix than in soda-lime glasses, the first when hit absorbs more energy. This causes the lead crystal to oscillate, resulting in a distinctive sound. Lead also increases the solubility of lead, copper, and antimony, which causes its use in enamel and colored glazes. The low viscosity of lead glass melt is the reason the lead oxide content is usually high in glass solder.
The presence of tin is used in glasses that absorb gamma-ray radiation and X-rays, which are used in radiation shields (eg in cathode ray tubes, where reducing exposure from the viewer to the gentle X-rays is of concern).
The high ionic radius of the Pb 2 ion makes it very immobile in the matrix and prevents the movement of other ions; Therefore tin cups have high electrical resistance, about twice as high as glass soda-lime (10 8.5 vs. 10 6.5 OhmÃ, Â · cm, DC at 250 Ã, Â ° C or 482 Â ° F). Glass containing lead is therefore often used in light fixtures.
Maps Lead glass
History
Lead can be incorporated into a glass either as the main melting agent or added to unbranded glass or preformed frit. Tin oxides used in tin glasses can be obtained from various sources. In Europe, galena, lead sulphide, widely available, which can be melted to produce metal tin. The lead metal will be calcined to form lead oxide by baking it and eroding litharge. In medieval tin metals can be obtained through the recycling of abandoned Roman sites and aqueducts, even from the church roof. Metal tin is required in quantities for the silver cupellation, and the resulting litharge can be used directly by the glass maker. Lead is also used for ceramic lead glazes. The interdependence of this material shows the close working relationship between pottery, glassmaker, and metal craftsmen.
Glasses with the first tin oxide content appeared in Mesopotamia, the birthplace of the glass industry. The earliest known example is the blue glass fragment from Nippur dated 1400 BC containing 3.66% PbO. Glass is mentioned in clay tablets from the reign of Assurbanipal (668-631 BC), and the recipe for tin glaze appeared on the Babylonian tablet of 1700 BC. The red sealing candle cake found in Burnt Palace in Nimrud, from the early 6th century BC, contains 10% PbO. These low values ​​indicate that lead oxide may not be consciously added, and certainly not used as a primary rejuvenating agent in ancient glasses.
The glass lead also occurs in the Han-Chinese period (206 BC - 220 AD). There, he was cast to imitate jade, both for ritual objects such as big and small figures, as well as jewelry and various limited ships. Since glass first occurs on a late date in China, it is thought that the technology was brought along the Silk Road by the makers of goods from the Middle East. The fundamental composition difference between Western silica-natron glass and unique Chinese tin glass, however, can show autonomous development.
In medieval and early modern Europe, tin glass is used as a base in colored glasses, especially in mosaic tesserae, enamel, colored glass paintings, and jewelry, where it is used to imitate precious stones. Some textual sources that explain tin glass survive. At the end of the early 11th-12th century, the Schedula Diversarum Artium (List of Wheeled Handicrafts ), the author known as "Theophilus Presbyter" describes its use as an artificial gemstone, and the title of the missing chapter of the work mentioned the use of lead in a glass. 12th-13th century Pseudonymus "Heraclius" specifies the making of lead enamel and its use for window painting in his book De Coloribus et artibus Romanorum ( Of Hues and Crafts of the Roman). This refers to tin glass as "Jewish glass", possibly showing its transmission to Europe. A preserved manuscript in Biblioteca Marciana, Venetian, describes the use of lead oxide in enamels and includes a recipe for lead calcination to form oxides. The glass lead is ideal for enamel ships and windows because the working temperature is lower than the body forest glass.
Antonio Neri presents his four books of L'Arte Vetraria ("The Art of Glass-making", 1612) to lead the glass. In his first systematic treatise on glass, he again refers to the use of lead glass in enamel, glass, and for imitation of precious stones. Christopher Merrett translated this into English in 1662 ( The Art of Glass ), paving the way for the production of British lead crystal glass by George Ravenscroft.
George Ravenscroft (1618-1681) was the first to produce clear, clear crystal glass on an industrial scale. The son of a merchant with close ties to Venice, Ravenscroft has the cultural and financial resources necessary to revolutionize the glass trade, setting the foundation from which the British took over Venice and Bohemia as the center of the glass industry in the eighteenth and nineteenth centuries. With the help of Venetian glassmakers, especially da Costa, and under the auspices of the Semburus Company of London Glass Sellers, Ravenscroft sought an alternative to Venetian cristallo. The use of flint as a source of silica has led to the term stone glass to illustrate this crystal glass, although it later switched to sand. At first, his glasses tend to harden, developing a small crack network that destroys transparency, which is ultimately overcome by replacing some of the potash fluxes with lead oxide to melt, by up to 30%. The crizzling results from the destruction of glass tissue by the excess of alkali, and possibly caused by excess moisture and defects inherent in the glass composition. He was granted a patent patent in 1673, where production moved from a greenhouse in the Savoy area of ​​London, into the Henley-on-Thames exile. In 1676, after apparently overcoming the gravel problem, Ravenscroft was given the use of the crow's head seal as a quality assurance. In 1681, the year of his death, the patent expired and the rapid operation developed among several companies, where in 1696 twenty-seven of eighty greenhouses in England, mainly in London and Bristol, produced stone glass containing 30-35% PbO.
In this period, glass is sold heavily, and the typical forms are rather heavy and dense with minimal decoration. Such was his success in the international market, however, that in 1746, the British Government imposed a tax on profitable by weight. Rather than drastically reducing the lead content of their glasses, manufacturers respond by creating highly-decorated, smaller, finer shapes, often with hollow stems, known to today's collectors as the excise glass. In 1780, the Government granted Ireland free trade in glass without tax. Workforce and UK capital then shifted to Dublin and Belfast, and a new glass factory specializing in glass cuts installed in Cork and Waterford. In 1825, the tax was renewed, and gradually the industry declined until the mid-nineteenth century, when the tax was finally repealed.
From the 18th century, British tin glass became popular throughout Europe, and ideally suited to new tastes for enhanced cutting-edge glass decorations on the Continent due to its relatively soft nature. In the Netherlands, local carvings such as David Wolff and Frans Greenwood bandaged imported English glassware, a style that remained popular until the 18th century. Such was his popularity in the Netherlands that the production of first tin crystal glass began there, perhaps as a result of the importation of British workers. Imitating the lead crystal ÃÆ' la faÃÆ'§on d'Angleterre presents technical difficulties, because the best results are obtained with a sealed pot in a coal-burning stove, especially the English process that requires a special cone-furnace. Toward the end of the 18th century, tin crystal glass was produced in France, Hungary, Germany, and Norway. In 1800, Irish lead crystal had taken over lime-lime glasses on the Continent, and the traditional glass-making center in Bohemia began to focus on colored glasses rather than competing directly against it.
The development of tin glass continued into the 20th century, when in 1932 scientists at Corning Glassworks, New York State, developed a new tin glass with high optical clarity. This is the focus of Steuben Glass Works, a division of Corning, which produces decorative vases, bowls and sunglasses in Art Deco style. Lead crystal continues to be used in industrial and decorative applications.
Glazes of lead
The rejuvenating and bias properties that are valued for lead glass also make it attractive as pottery or ceramic glazes. Leading glazes first appeared in the first century BC to the first century AD Roman goods, and occurred almost simultaneously in China. They are very high in lead, 45-60% PbO, with very low alkali content, less than 2%. From the Roman period, they remained popular through the Byzantine and Islamic periods in the Near East, in pottery and tile ships throughout medieval Europe, and to this day. In China, a similar glaze was used from the 12th century to colored enamel on the stoneware, and porcelain from the fourteenth century. This can be applied in three different ways. Lead can be added directly to the ceramic body in the form of lead compound in suspension, either from galena (PbS), red lead (Pb 3 O 4 ), white lead (2PbCO 3 Ã, Â · Pb (OH) 2 ), or lead oxide (PbO). The second method involves mixing lead compounds with silica, which is then placed in suspension and applied directly. The third method involves frying lead compounds with silica, sprinkling mixtures, and suspending and applying them. The method used on a particular ship can be summarized by analyzing the interaction layer between the glaze and the ceramic body microscopically.
The tin-opactive glaze appeared in Iraq in the eighth century. Initially containing 1-2% PbO; by tall glaze of tin in the eleventh century has developed, usually containing 20-40% PbO and 5-12% alkali. It is used throughout Europe and the Near East, especially in Iznik ware, and continues to be used today. Glazes with higher lead content occur in Spanish and Italian maiolica, with up to 55% PbO and as low as 3% alkali. Adding lead to melt allows the formation of lead oxide more easily than in an alkaline glaze: tin oxide settles into crystals in the glaze as it cools, creating opacity.
The use of lead glazes has several advantages over the alkali glaze in addition to the larger optical power. The lead compound in the suspension can be added directly to the ceramic body. Alkali glazing should first be mixed with silica and boiled before use, as they are water soluble, require additional work. Successful glazes should not crawl , or peel off the pottery surface after cooling, leaving the ceramic area without glaze. Lead reduces this risk by reducing the surface tension of the glaze. It should not be crazy, forming a crack network, caused when the thermal contraction of the glaze and ceramic body does not fit properly. Ideally, the glaze contraction should be 5-15% smaller than the contraction of the body, because the glaze is stronger under compression than under pressure. High lead glaze has a linear expansion coefficient between 5 and 7ÃÆ'â € "10 -6 /Ã, Â ° C, compared to 9 to 10ÃÆ'â €" 10 -6 /Ã, Â ° C for alkaline glazes. Those of pottery ceramics vary between 3 and 5ÃÆ' â € "10 -6 /Ã, Â ° C for non-calcareous bodies and 5 to 7ÃÆ'â €" 10 -6 /Ã, Â ° C for calcareous clay, or containing 15-25% CaO. Therefore, thermal contrast of lead glazes fits well with ceramics closer than alkaline glazes, making them less susceptible to grazing. A glaze should also have a low enough viscosity to prevent the formation of small holes as trapped gases escape during combustion, typically between 900-1100 ° C, but not so low as to escape. The relatively low lead glaze viscosity alleviates this problem. It may also be cheaper to produce than alkaline glazes. Glass and lead glazes have a long and complicated history, and continue to play a new role in today's industry and technology.
Crystalline lead
Lead oxide added to the liquid glass gives the crystal a much higher refraction index than the normal glass, and consequently a much larger "sparkle" by increasing the spectral reflection and the total angular range of total internal reflection. Regular glass has a refractive index n = 1.5; the addition of lead produces a refractive index of up to 1.7. This high refractive index also increases the correlation index of the dispersion, which measures the degree to which the medium separates light into its component spectrum, as in a prism. An increase in refractive index from 1.5 to 1.7 significantly increases the amount of reflected light (by a factor of 1.68 for reflecting light in the normal direction, see Fresnel equation).
In cut glass, which has been cut off hand or machine by side, the presence of lead also makes the glass softer and easier to cut. Crystals can consist of up to 35% of lead, at which point it has the most sparkle.
The makers of crystal lead objects include
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The California Department of Public Health leads advisory country, "children should not eat or drink from leaded crystal". Crystal glasses and leaded decanters are generally not considered to pose significant health risks, as long as these items are washed thoroughly before use, the drinks are not stored in this container for more than a few hours, and provided they are not used by children..
It has been proposed that the historic association of uric acid with upper classes in Europe and America, in part, is due to the use of an extensive lead crystal decanter for storing wine and fortified whiskey. Lin et al. has statistical evidence linking uric acid to cause poisoning.
The objects made of leaded glass can coat the tin into the food and beverages contained. In a study conducted at North Carolina State University, the amount of lead migration measured for wine ports was stored in lead crystal decanters. After two days, the lead level was 89 Âμg/L (micrograms per liter). After four months, lead levels between 2,000 and 5,000 Âμg/L. White wine doubled its main content in one storage hour and tripled within four hours. Some brandy stored in lead crystal for more than five years has a lead level of about 20,000 Ã,Âμg/L. To put this into perspective, the US Environmental Protection Agency's primary standard for drinking water is 15 Âμg/L = 0.015 parts per million. Orange juice and other acidic beverages containing lead from crystals are as effective as alcoholic beverages. Under the conditions of repeated use of the decanter, lead leaching declines sharply with increased use. These findings are "consistent with the chemical theory of ceramics, which predicts that the lead release of the crystals confines itself exponentially as a function of increasing the distance from the liquid-crystal interface." Lead leaching is still happening, but the amount dissolved into a glass of wine or other beverage left over for several hours is much smaller than the amount of lead that is consumed daily in a regular diet.
The regular diet contains about 70 Ã,Âμg of lead per day.
See also
References
Source of the article : Wikipedia
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