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About Glass and How It Is Made
   Throughout history, people from cultures around the world have been making glass. People first found and used glass made by nature. For example, lightning can create tubes of glass when it strikes sand that has the right combination of minerals. Glass pieces produced by lightning are called fulgurites. So today we answer this question as we explore the art of making glass.
   Obsidian is a kind of black glass formed when the heat of a volcano melts the silica material in sand. Ancient cultures broke off pieces of obsidian to make knives and weapons such as arrows. The ancient Aztec civilization in current day Mexico used obsidian for making hunting tools and jewelry. The Aztecs made extremely sharp knives and weapons from obsidian. This is one reason experts say why they never developed the use of metal.
   Glass is considered a physical state of matter. It may look solid, but it is a liquid as well. This is because glass has the hardness of crystal materials while also having a disordered arrangement of molecules like a liquid.
   The chemical quality of glass is what makes up its color. Impurities in glass such as iron can give it a green or brown color. Adding chemicals to the glass can give it different color intensities and effects. For example, adding copper to glass can make it blue, while adding tin can make it white.
   One of the earliest methods developed for making glass containers is called core-forming. A glassmaker places a rounded piece of clay material on the end of a long metal stick. Once the clay dries, the glassmaker dips the form in a container of hot liquid glass until it is covered. The artist can then add a second color of glass to make designs over the first layer of glass. Once the glass form cools completely, it is taken off the metal stick. The clay inside is carefully cut out to form a glass container.
   Another ancient method of making glass that is still used today is called casting. Casting involves making a clay form in which the shape of the glass container is carved. Then, the artist puts small pieces of glass material inside the clay form. When it is cooked at a very high temperature, the glass pieces melt and take the shape of the clay form. Once the solid glass object cools, an artist uses special tools to carve an opening in the container.
   But it was another method of making glass — the blown glass method — that changed the glass industry of the ancient world. It was first developed in the Roman Empire about two thousand years ago. This new technology made glass production faster and less costly. A glass container made by casting or core-forming could take a few days to make. With glass blowing, an artist could make many containers in a day.
   In modern terms, the hot oven that the glassblower uses to quickly reheat the glass is called a "glory hole" . The artist can shape the hot glass using metal tools such as jacks, tweezers and shears. Or, he or she can place the hot glass on a metal table called a marver to shape the form by rolling it back and forth. Watching an expert glassblower is an exciting experience. The artist moves as quickly and as gracefully as a dancer.
   So now you have known the ancient and current methods of making glass. About the use of glass. We can see glass everywhere in our daily life...