Silica alone can form an excellent glass, called fused silica, which has remarkable properties such as chemical inertness and thermal shock resistance. However, fused silica is very expensive to produce and has a very high viscosity, which makes it difficult to shape and mold. Therefore, other ingredients are usually added to silica to lower its melting point and modify its properties.
The other ingredients of glass: modifiers and stabilizers
The most common additives to silica are soda (sodium carbonate, or Na2CO3) and lime (calcium carbonate, or CaCO3). Soda lowers the melting point of silica to about 800 ÂC (1472 ÂF), making it easier to melt and work with. However, soda also makes the glass more soluble in water and more prone to corrosion. Lime acts as a stabilizer, reducing the solubility and increasing the durability of the glass.
The most widely used type of glass, accounting for about 90% of all manufactured glass, is called soda-lime-silica glass. It has a typical composition of 70% SiO2, 15% Na2O, and 10% CaO, with minor amounts of other oxides such as alumina (Al2O3) and magnesia (MgO). Soda-lime-silica glass is used for windows, bottles, jars, tableware, and many other applications.
Another important type of glass is called borosilicate glass, which has a higher content of boron oxide (B2O3) than soda-lime-silica glass. Boron oxide lowers the thermal expansion coefficient of the glass, making it more resistant to thermal shock and more suitable for high-temperature applications. Borosilicate glass also has a higher chemical resistance than soda-lime-silica glass. It is used for cooking and chemical glassware, laboratory equipment, optical fibers, and some types of light bulbs.
The color and appearance of glass: additives and treatments
Glass can be colored by adding small amounts of metal oxides or sulfides to the molten mixture. For example, iron oxide (Fe2O3) can produce green or brown glass, copper oxide (CuO) can produce blue or green glass, cobalt oxide (CoO) can produce blue or purple glass, manganese dioxide (MnO2) can produce purple or pink glass, chromium oxide (Cr2O3) can produce green or yellow glass, and gold chloride (AuCl3) can produce red or pink glass. The amount and distribution of the colorants affect the intensity and transparency of the color.
Glass can also be treated by various methods to alter its appearance and properties. For example, glass can be coated with thin layers of metal or oxide to reflect or filter light, creating mirrors or tinted windows. Glass can also be etched with acid or sandblasted to create patterns or textures on its surface. Glass can also be tempered by heating and cooling it rapidly to increase its strength and shatter resistance.
The conclusion: glass is not a simple substance
As we have seen, glass is not a simple substance with a fixed chemical formula. It is a complex mixture of various ingredients that can be adjusted to produce different types of glass with different properties and appearances. Glass is a versatile and fascinating material that has many uses and applications in our daily lives.
References:
[Glass - Wikipedia]
[Glass Definition, Composition, Material, Types, & Facts Britannica]
[Chemistry of Glass - Pilkington - First in Glass]
[What Is The Formula Of Glass? Mondoro]
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