Showing posts with label Glass Composition. Show all posts
Showing posts with label Glass Composition. Show all posts
Tuesday 3 February 2009
Prince Rupert's Drops
Though the underlying mechanism was not known at the time, the effects of "toughening/tempering" glass have been known for centuries. In the 1640's Prince Rupert of Bavaria, who was grandson of James I of England and VI of Scotland, and nephew of Charles I, brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of the King. These are remarkable tear-drop shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. The resulting tear drop shape is very strong, resisting hammer blows. But breaking the tail causes the whole to shatter.
Thursday 29 January 2009
Chemical Compositions of Glass
Nearly all commercial glasses fall into one of six basic categories or types. These categories are based on chemical composition. Within each type, except for fused silica, there are numerous distinct compositions.
Soda-lime glass is the most common (90% of glass made), and least expensive form of glass. It usually contains 60-75% silica, 12-18% soda, 5-12% lime. Resistance to high temperatures and sudden changes of temperature are not good and resistance to corrosive chemicals is only fair.
Lead glass has a high percentage of lead oxide (at least 20% of the batch by weight). It is relatively soft, and its refractive index gives a brilliance that may be exploited by cutting. It is more expensive than soda-lime glass and is favoured for electrical applications because of its excellent electrical insulating properties. Thermometer tubing and art glass are also made from lead-alkali glass, commonly called lead glass. This glass will not withstand high temperatures or sudden changes in temperature.
Borosilicate glass is any silicate glass having at least 5% of boric oxide in its composition. It has high resistance to temperature change and chemical corrosion. Not quite as convenient to fabricate as either lime or lead glass. Borosilicate's cost is moderate when measured against its usefulness. Pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware are examples of borosilicate products.
Aluminosilicate glass has aluminum oxide in its composition. It is similar to borosilicate glass but it has greater chemical durability and can withstand higher operating temperatures. Compared to borosilicate, aluminosilicates are more difficult to fabricate. When coated with an electrically conductive film, aluminosilicate glass is used as resistors for electronic circuitry.
Ninety-six percent silica glass is a borosilicate glass, melted and formed by conventional means, then processed to remove almost all the non-silicate elements from the piece. By reheating to 1200°C the resulting pores are consolidated. This glass is resistant to heat shock up to 900°C.
Fused silica glass is pure silicon dioxide in the non-crystalline state. It is very difficult to fabricate, so it is the most expensive of all glasses. It can sustain operating temperatures up to 1200°C for short periods.
The full article is available from the Corning Museum of Glass
Soda-lime glass is the most common (90% of glass made), and least expensive form of glass. It usually contains 60-75% silica, 12-18% soda, 5-12% lime. Resistance to high temperatures and sudden changes of temperature are not good and resistance to corrosive chemicals is only fair.
Lead glass has a high percentage of lead oxide (at least 20% of the batch by weight). It is relatively soft, and its refractive index gives a brilliance that may be exploited by cutting. It is more expensive than soda-lime glass and is favoured for electrical applications because of its excellent electrical insulating properties. Thermometer tubing and art glass are also made from lead-alkali glass, commonly called lead glass. This glass will not withstand high temperatures or sudden changes in temperature.
Borosilicate glass is any silicate glass having at least 5% of boric oxide in its composition. It has high resistance to temperature change and chemical corrosion. Not quite as convenient to fabricate as either lime or lead glass. Borosilicate's cost is moderate when measured against its usefulness. Pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware are examples of borosilicate products.
Aluminosilicate glass has aluminum oxide in its composition. It is similar to borosilicate glass but it has greater chemical durability and can withstand higher operating temperatures. Compared to borosilicate, aluminosilicates are more difficult to fabricate. When coated with an electrically conductive film, aluminosilicate glass is used as resistors for electronic circuitry.
Ninety-six percent silica glass is a borosilicate glass, melted and formed by conventional means, then processed to remove almost all the non-silicate elements from the piece. By reheating to 1200°C the resulting pores are consolidated. This glass is resistant to heat shock up to 900°C.
Fused silica glass is pure silicon dioxide in the non-crystalline state. It is very difficult to fabricate, so it is the most expensive of all glasses. It can sustain operating temperatures up to 1200°C for short periods.
The full article is available from the Corning Museum of Glass
Thursday 22 January 2009
Coloured Glass
Glass is coloured by (1) impurities in the batch ingredients, or(2) by one of three processes:
a. using a dissolved metallic oxide to impart a colour throughout
b. forming a dispersion of some substance in a colloidal state, and
c. suspending particles of pigments to form opaque colours.
A few historical examples:
The name of a colour was often used to describe a certain kind of glass, as in the Black bottle: a term for bottles of dark green or dark brown glass, the dark colour of which protected the contents from light. Often the "colour" black that appeared with reflected light was caused by the combination of iron, found in the sand used to make the batch, and the sulphur found in the smoke from the coal used to melt the batch. "Black" glass was first made in England in the mid-17th century.
Often, the name of the mineral added to give the glass its colour, as in Uranium glass (glass coloured with uranium oxide) was used. This brilliant yellow-green glass was first made in the 1830s.
Sometimes, a combination of both the additive and colour, as in Gold ruby glass was used. This is a deep red glass coloured by the addition of gold chloride to the batch. The method of making gold ruby glass was perfected shortly before 1679.
Optical terms can be used to describe the glass, as in dichroic glass. This is glass that is one colour when seen by reflected light and another colour when light shines through it (this is sometimes due to the presence of minute amounts of colloidal gold).
Iridescent glass is a deliberate effect (visually similar to the shimmering rainbow effect seen on the surface of soap bubbles, oil slicks, or fish scales). This is achieved by the introduction of metallic substances into the batch or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere. On ancient glass, iridescence is caused by interference effects of light reflected from several layers of weathering products.
Iron can produce greens, iron and sulphur can produce ambers and browns, copper can produce light blues, cobalt produces very dark blue, manganese can produce shades of amethyst colour, tin can produce white, lead antimony can produce yellow and various metals produce reddish glasses.
A decolorizer is a substance (such as manganese dioxide or cerium oxide) used to remove or offset the greenish or brownish colour in glass that results from iron impurities in the batch or iron or other impurities in the pot or elsewhere in the production process.
The full article can be seen at the Corning Museum of Glass
a. using a dissolved metallic oxide to impart a colour throughout
b. forming a dispersion of some substance in a colloidal state, and
c. suspending particles of pigments to form opaque colours.
A few historical examples:
The name of a colour was often used to describe a certain kind of glass, as in the Black bottle: a term for bottles of dark green or dark brown glass, the dark colour of which protected the contents from light. Often the "colour" black that appeared with reflected light was caused by the combination of iron, found in the sand used to make the batch, and the sulphur found in the smoke from the coal used to melt the batch. "Black" glass was first made in England in the mid-17th century.
Often, the name of the mineral added to give the glass its colour, as in Uranium glass (glass coloured with uranium oxide) was used. This brilliant yellow-green glass was first made in the 1830s.
Sometimes, a combination of both the additive and colour, as in Gold ruby glass was used. This is a deep red glass coloured by the addition of gold chloride to the batch. The method of making gold ruby glass was perfected shortly before 1679.
Optical terms can be used to describe the glass, as in dichroic glass. This is glass that is one colour when seen by reflected light and another colour when light shines through it (this is sometimes due to the presence of minute amounts of colloidal gold).
Iridescent glass is a deliberate effect (visually similar to the shimmering rainbow effect seen on the surface of soap bubbles, oil slicks, or fish scales). This is achieved by the introduction of metallic substances into the batch or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere. On ancient glass, iridescence is caused by interference effects of light reflected from several layers of weathering products.
Iron can produce greens, iron and sulphur can produce ambers and browns, copper can produce light blues, cobalt produces very dark blue, manganese can produce shades of amethyst colour, tin can produce white, lead antimony can produce yellow and various metals produce reddish glasses.
A decolorizer is a substance (such as manganese dioxide or cerium oxide) used to remove or offset the greenish or brownish colour in glass that results from iron impurities in the batch or iron or other impurities in the pot or elsewhere in the production process.
The full article can be seen at the Corning Museum of Glass
Monday 1 December 2008
Laminated Glass
Laminated glass is a type of safety glass that holds together when shatered. In the event of breaking, it is held in place by an interlayer, typically of PVB, between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic "spider web" cracking pattern when the impact is not enough to completely pierce the glass.
Friday 28 November 2008
The Glassy State
- Glass is a state of matter.
- Glasses combine some properties of crystals and some of liquids but are distinctly different from both.
- Glasses have the mechanical rigidity of crystals, but the random disordered arrangement of molecules that characterises liquids.
- Glasses are usually formed by melting crystalline materials at very high temperatures. When the melt cools, the atoms are locked into a random (disordered) state before they can form into a perfect crystal arrangement.
The complete description can be seen at the Corning Museum of Glass
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