About Glass and How Its Made

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I’m Barbara Klein. And I’m Steve Ember with EXPLORATIONS in VOA Special English.

At the Smithsonian American Art Museum, you can see a large heart-shaped sculpture made of blown glass. The deep red colored heart is topped with a burning flame also made of glass. It is called the “Sacred Heart of Healing” and was made by the artist Tim Tate. How did he make this interesting glass form? Today we answer this question as we explore the art of making glass.

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.

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 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.

It is hard to say exactly when humans first started making glass. The Roman historian Pliny said that Phoenician sailors accidentally discovered how to make glass over three thousand years ago. The sailors landed on a beach and started a cooking fire near some containers of the mineral natron. The next day, they realized that the sand and natron under the fire had melted then cooled into glass. Other experts say glass making first started four to five thousand years ago in ancient Mesopotamia, present day Iraq and Syria.

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 of 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.

Glassblowing involves gathering hot liquid glass on the end of a metal pipe called a blowpipe. The glass reaches a temperature of about one thousand degrees Celsius. At this temperature, the glass is a bright orange color.  The glassblower must turn the pipe constantly so that the thick liquid glass does not fall off the end. He or she then blows through the pipe so that the glass expands into a rounded bubble form. The blown piece of glass can be worked and formed to create many different kinds of shapes. To reshape the glass, it must be continually reheated to stay soft.

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.