About Glass
Glass is a hard material, normally fragile and transparent as used commonly in our daily life. It is composed mainly of sand or silica and an alkali, soda ash or limestone (calcium carbonate or dolomite).
These materials at high temperature are in a molten viscous state and will fuse together. They are cooled rapidly to form a rigid structure without enough time to form a crystalline regular structure.
Adding soda will lower the melting point to 1000°C making it more manageable. The soda makes the glass water-soluble, soft and not very durable. Therefore lime is added increasing the hardness and chemical durability and providing insolubility of the materials. Other materials and oxides can be added to increase properties (tinting, durability, etc.), and to produce different effects and colors.
Types of glass
Soda-lime glass
The most common and least expensive type of glass is commercial glass or soda-lime glass, normally 60-75% silica, 12-18% soda, and 5-12% lime. A low percentage of other materials can be added for specific properties such as coloring.
There are many advantages of soda-lime glass and it is commonly used commercially for bottles, jars, everyday drinking glasses, and window glass. The disadvantages of soda-lime glass is that is not resistant to high temperatures and sudden thermal changes.
Lead glass
Lead glass is composed of 54-65% silica, 18-38% lead oxide, 13-15% soda or potash and various other oxides. When the content of lead is less than 18% it is known as crystal glass.
In moderate amounts lead increases durability; in high amounts it lowers the melting point and decreases the hardness giving a soft surface. Lead also creates a high refractive index giving high brilliance glass making it appropriate for decorating purposes.
Neither soda-lime glass nor lead glass will withstand high temperatures or sudden changes in temperature.
Borosilicate glass
Borosilicate glass is mainly composed of silica (70-80%), boric oxide (7-13%) and smaller amounts of alkalis such as sodium and potassium oxides.
Borosilicate glass substitutes boron oxide particles in place of the soda and lime particles found in soft glass. The boron oxide serves as a flux or glue to hold the silicate particles together with aluminum oxide and sodium oxide. Because the boron oxide particles are so small, the silicate is held together more closely resulting in a much stronger glass. The greater resistance to thermal changes and chemical corrosion make it suitable for industrial use and also in the home for cooking plates and other heat-resistant products (Pyrex and Kimax).
Metals and metal oxides are used to change glass color. For example, manganese and selenium is used to decolorized gas, cobalt for blue, copper for red, nickel produces blue, violet or black glass, titanium produces yellowish-brown, etc. Different precious metals such as silver and gold may be used to color the borosilicate glass in some very unique ways.
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