Glass is a clear, hard, amorphous substance. It cannot be characterized by a single physical state or chemical composition, as the process of making the different types of glass can vary. Silica, from sand, is the chief component of most glasses. Although it occurs naturally, man-made glass also exists. Glass can be pulled and formed into numerous shapes under high heat, demonstrating its elasticity. Once formed, glass can be cut into patterns and shapes; its ability to be colored also adds to its appeal as a form of art and decoration. Glass is also utilized for an abundance of practical uses, ranging from architecture, cooking ware, light bulbs, automobiles, and bulletproof substances. The glass industry has existed for several centuries, dating back to the ancient Egyptians. Although the value of mass produced glass is clearly demonstrated today, individually created pieces are still greatly prized and admired.
Glass, in its basic form, is an amorphous substance characterized by hardness and transparency.   The physical state of glass is neither liquid nor solid, but parts of both. With the rigidity of crystals but the random molecular arrangement of liquids, glass is a unique state of matter. Glass can have various colors, depending on what chemicals are added to it during synthesis. Another physical property of glass is its hardness and mechanical strength; its surface is resistant to scratches and abrasions. Glass is actually only weakened by imperfections on its surface--the root of its reputation as a fragile substance. With special tempering, these imperfections can be reduced. Despite its strength, glass can also be elastic, rebounding to its original shape when under a certain amount of stress. Glass also possesses strong resistance to thermal shock, being able to withstand extreme amounts of heat, cold, and sudden changes in temperature. Instead of conducting heat, glass retains it, even better than metal. Electrical current resistance is also a property of glass, as well as efficient electrical storage. 
There is no single chemical composition that is characteristic of all glasses. The thousands of formulas used to make glass affect its chemicals properties, as well as its physical properties.  However, glass's resistance to water, moisture, acids, and alkalis is determined by its chemical properties.  Its chemical composition also determines glass's capabilities. When certain substances are added to glass, certain characteristics appear. For example, lead makes glass heavy, brilliant in appearance, and resonant. Boron adds to glass's thermal shock resistance, as evident by borosilicate glass. .  96% silicate glass is also capable of resisting high temperatures.  Resistance to chemical corrosion is also one of glass's properties, as it can withstand most industrial and food acids, and few chemicals can affect it.  An ion exchange reaction is the main factor in how chemically durable a piece of glass is. In atmospheric humidity (or water), hydrogen and hydronium ions exchange with alkali ions in glass. These alkali ions are then "leached" out of the glass, and react with the atmospheric water and carbon dioxide to form alkali carbonates and bicarbonates. This reaction results in the white deposits seen after dishwashing or in humid environments, and is called weathering. Glasses that do not contain much alkali are usually more resistant to weathering, such as borosilicate and aluminosilicate. Dissolution of the entire substance structure can occur when silicate glass encounters caustic alkalis, such as hydrofluoric, phosphoric, and perchloric acids. 
Glass is also known for its optical properties; it can reflect, bend, transmit, and absorb light very accurately.  These properties result from glass's chemical composition, melting process, and finishing methods. For obtaining specific optic properties, certain chemical compositions that create products with less chemical resistance are often chosen. 
Glass exists in many natural forms, all resulting from the heating of rocks and sand to high temperatures, followed by a rapid cooling. Some examples include molten volcanic rock, called obsidian glass, and glassy rocks formed when lightning, or even meteorites, strike the desert sand. Glass even exists underwater, in the form of silica skeletons on microscopic algae and sea sponges. 
Most manufactured glass is composed of a mixture of silica, retrieved from fine sand or pulverized sandstone, and mixture of oxides.   The glass making process has basically remained the same since ancient times. A mixture of dry materials, primarily composed of a substance such as silica, called former, is fused together at extremely high temperatures in fiber clay containers into a viscous state. Substances called fluxes are added to the mixture to lower the melting point of the formers; some common examples are soda (sodium carbonate) and potash (potassium carbonate)--both alkalis. Stabilizers are also combined with the former and flux to keep the glass from crumbling and to prevent the formation of crystals, as well as to make glass water resistant. Calcium carbonate is a common stabilizer. Cullet, or broken glass, can also be added to expedite the melting process.  
Then, the mixture is boiled down, skimmed, and cooled several hundred degrees. The molten glass is poured into molds, and then pressed, blown, or drawn depending on the shape needed. Once the glass is formed, it is often annealed, a slow cooling process that reduces internal stress. If the glass isn't annealed properly so that all areas of the glass are even, the resulting stress will likely cause the glass to crack. The cooling process, if done correctly, gives glass its unique physical state. When the glass cools fast enough, the atoms are locked in an arrangement similar to a liquid before they can settle into a perfect crystalline structure characteristic of solids. Finally, the glass can be decorated using a variety of methods: cutting, engraving, etching with hydrofluoric acid, enameling, gilding, and painting. Color is added to the glass itself by combining certain metallic oxides with the original batch. Iron turns the glass green; copper, light blue; cobalt, dark blue; and gold, a deep red. Manganese dioxide removes color, but when used in larger amounts, it will turn the glass purple--and in even higher amounts, the glass will become black.  
Glass has a very broad range of use. Depending on its chemical composition, glass can be used in architecture, lighting, electrical transmission, scientific instruments, household utensils, fabrics, packaging, appliances, decoration, and in renewable energy devices.    Based on the process used to form it, glass can be utilized in several ways.
Soda Lime or Soda-Glass Soda-glass is the most common and inexpensive type of glass to produce, comprising of 90% of all glass production. Common examples include windows, tableware, bottles, and light bulbs.  
Safety Glass Safety glass, also known as shatterproof glass, consists of a layer of plastic between two layers of glass. If the glass breaks, the shards stick to the plastic and do not scatter. This type is utilized in car windows, as well as bulletproof screens. 
Laminated Glass Laminated glass is a stronger form of safety glass, with several layers of safety glass stuck together with a polymer adhesive. These extra layers not only add to the glass's strength, but also can be used to reduce sound, resist fire, and filter ultraviolet rays. The automobile industry uses laminated glass in car windshields; it is also used in airplanes, and for the outsides of buildings.  
Optical Glass Optical glass, also called flint glass, is the softest glass type. Characterized by clarity and transparency, it is used for lenses, prisms, and other instruments to aid the eye. 
Pyrex Glass This glass is characterized by its resistance to heat and several chemicals, making it suitable for cooking ware used in the oven as well as laboratory instruments. Made with silica and boron oxide, it is also called borosilicate glass. 
Other Other types of glass include those with color and patterns, used in decoration and to add variety. Lead crystal glass, or cut glass, is known for its ability to refract light brilliantly, so it is used in making fine quality glassware. Glass can even be made to have a mirror effect, by applying a metal coating of silver, gold, aluminum, or chrome. Mirrored glass is often used in architecture.   Glass can even be used in fabric, as fiberglass, made from molten glass formed into filaments; this type can also be used for electrical insulation. When gas bubbles are trapped in glass, it creates a spongy texture known as foam glass, also used in insulation. 
Although glass has been used since ancient times, such as with small objects made of obsidian or rock crystal, the exact time period and location of the creation of manufactured glass is unknown. Egypt is home to the oldest known samples of manufactured glass, having an established industry around 1500 B.C. The Romans were also familiar with glass; some known types being cameo millefiore glass. Other common uses included window panes, mirrors prisms, and magnifying glasses. Not much is known about the glassmaking process from the time period between the fall of the Roman empire and the tenth century in Europe, when stained glass was first introduced. After the Crusades, Venice arose as the leader in the glass industry, keeping a severely strict hold on their secrets of the trade. However, this monopolization only lasted until 1688, when the invention of a new process for casting glass in France made the French the new head of industry. Their plate glasswork is evident in the Palace of Versailles. Then, at the end of the seventeenth century, England started to produce flint glass with lead oxide, creating a softer texture and brilliant appearance ideal for cut glass. 
On the other side of the world, America established its first glass factory in 1608, whose products were in the first cargo exported to England. The city of New Amsterdam was home to several of the glasshouses in the colonies, but the first large-scale, successful production came from New Jersey in 1739, set up by Caspar Wistar, a manufacturer from Germany. Another German manufacturer, H.W. Stiegel, owned glasshouses in Pennsylvania that produced some of the best pieces created in the colonial era. Then, the invention of the glass-pressing machine in 1827 made glass able to be produced inexpensively and on a much more massive scale. However, throughout the nineteenth and twentieth century, the craftsmanship of individually created glass was still highly valued and admired. Some remarkable glass artists include Louis C. Tiffany and Sidney Waugh of the United States, René Lalique and Maurice Marinot of France, and Edvard Hald and Simon Gate of Sweden. Since the start of the modern glass-making era in the 1800s, the glass industry has continued to grow, as evident by the numerous uses of glass today. 
Video demonstrating glass blowing and shaping
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