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Types of Crystal Glass
Cut Crystal Glass from the Glass Encyclopedia
A short explanation of Cut Crystal Glass
The process of cutting glass involves pressing the completed glass object against a large rotating wheel usually made of iron or stone. The wheel cuts grooves into the glass, which have straight sharp angles, and the effect is to greatly increase the sparkle of the glass, by adding more light-reflecting surfaces. The best effect is obtained by using glass with high lead oxide content, usually called "crystal glass" or "lead crystal glass". This kind of glass has a very high brilliance, and shows the cutting to greatest effect. It was "invented" by George Ravenscroft in England around 1676 and independently by the St-Louis Glass Factory in France around 1781, although the use of lead oxide to enhance the brilliance of glass had been known in Mesopotamia in ancient times. From the mid-eighteenth century for about a hundred years, cut crystal glass was the predominant form of decorated glass in England and Ireland, and was also produced in large quantities on the European continent, particularly in France. In the USA the 19th century "Brilliant Period" lasted through the second half of the 19th century. The popularity of cut crystal glass has never really died, and today there are major glassworks specializing in high-quality hand-cut crystal, such as Baccarat in France, Waterford in Ireland, and Webb Corbett in England. Glass Blowing
For nearly 2,000 years glass blowing by hand was the main method of forming glass articles. The last few years of the 19th century saw the beginnings of blowing glass by compressed air and the 20th century brought in the revolution of mechanization, although glass blowing is still carried out by craftsmen today.
For glass blowing, a hollow blowing-iron or pipe is dipped into a pot containing molten glass and the glass is gathered at the end of the pipe by rotating it, similar to gathering treacle onto a spoon. The collected glass, known as the 'gather', cools to about 1000oC and is marvered (rolled on an iron slab) to form a 'parison'. The parison is then manipulated by allowing it to elongate, re-heating it and blowing air into it to bring it into a shape that resembles the final article. It is then placed in an iron or wooden mould, which is kept wet by water and the glass, is blown to the final shape of the interior of the mould. There is no contact between the glass and the mould, due to the water which forms a cushion of steam. During the blowing the pipe is rotated continuously, preventing mould joints or other mould imperfections appearing in the glass.
There are many different types of glass with different chemical and physical properties. Each can be made by a suitable adjustment to chemical compositions, but the main types of glass are:
Commercial glass known as soda lime glass
Lead glass
Borosilicate glass
Glass fiber
Commercial Glass
Most of the glass we see around us in our everyday lives in the form of bottles and jars, flat glass for windows or for drinking glasses is known as commercial glass or soda-lime glass, as soda ash is used in its manufacture.
The main constituent of practically all commercial glass is sand. Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700oC. Adding other minerals and chemicals to sand can considerably reduce the melting temperature.
The addition of sodium carbonate (Na2CO3), known as soda ash, to produce a mixture of 75% silica (SiO2) and 25% of sodium oxide (Na2O), will reduce the temperature of fusion to about 800oC. However, a glass of this composition is water-soluble and is known as water glass. In order to give the glass stability, other chemicals like calcium oxide (CaO) and magnesium oxide (MgO) are needed. These are obtained by adding limestone which results in a pure inert glass.
Commercial glass is normally colorless, allowing it to freely transmit light, which is what makes glass ideal for windows and many other uses. Additional chemicals have to be added to produce different colors of glass such as green, blue or brown glass.
Most commercial glasses have roughly similar chemical compositions of:
Lead Glass
Commonly known as lead crystal, lead glass is used to make a wide variety of decorative glass objects.
It is made by using lead oxide instead of calcium oxide, and potassium oxide instead of all or most of the sodium oxide. The traditional English full lead crystal contains at least 30% lead oxide (PbO) but any glass containing at least 24% PbO can be described as lead crystal. Glass containing less than 24% PbO, is known simply as crystal glass. The lead is locked into the chemical structure of the glass so there is no risk to human health.
Lead glass has a high refractive index making it sparkle brightly and a relatively soft surface so that it is easy to decorate by grinding, cutting and engraving which highlights the crystal's brilliance making it popular for glasses, decanters and other decorative objects.
Glass with even higher lead oxide contents (typically 65%) may be used as radiation shielding because of the well-known ability of lead to absorb gamma rays and other forms of harmful radiation.
Borosilicate Glass
Most of us are more familiar with this type of glass in the form of ovenware and other heat-resisting ware, better known under the trade name Pyrex.
Borosilicate glass, the third major group, is made mainly of silica (70-80%) and boric oxide (7-13%) with smaller amounts of the alkalis (sodium and potassium oxides) and aluminum oxide. This type of glass has relatively low alkali content and consequently has good chemical durability and thermal shock resistance (it doesn't break when changing temperature quickly). As a result it is widely used in the chemical industry, for laboratory apparatus, for ampoules and other pharmaceutical containers, for various high intensity lighting applications and as glass fibers for textile and plastic reinforcement.
Glass Fiber
Glass fiber has many uses from roof insulation to medical equipment and its composition varies depending on its application.
For building insulation and glass wool the type of glass used is normally soda lime. For textiles, an alumino-borosilicate glass with very low sodium oxide content is preferred because of its good chemical durability and high softening point. This is also the type of glass fiber used in the reinforced plastics to make protective helmets, boats, piping, car chassis, ropes, car exhausts and many other items.
In recent years, great progress has been made in making optical fibers which can guide light and thus transmit images round corners. These fibers are used in endoscopes for examination of internal human organs, changeable traffic message signs now on motorways for speed restriction warnings and communications technology without which telephones and the internet would not be possible.
THE GLASS FORMING PROCESSES
Like treacle, glass is fluid at high temperature and its fluidity decreases as the temperature is reduced. Unlike water, glass has no specific melting or freezing point but is gradually changed from a solid to a liquid as the temperature is increased. It is this property of 'variable viscosity', which is used in forming a mass of glass into articles of beauty or utility.
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