Take a look at the world around you. What would it look like without glass? No windows, no computer screens, no televisions…the world wouldn’t be the same, would it?
Have you ever noticed how all glass is not clear? The windows in your house are probably clear, but what about soda bottles? Have you ever seen stained glass windows in a church? What about colored glass ornaments on a Christmas tree?
Believe it or not, delicate glass gets its colors from some of the hardest substances known to man: metals. Did you even realize that there are metals in glass? It’s true!
Glass starts out as a finely-ground mixture of sand and scraps of broken or waste glass called cullet. Cullet is created when glass is recycled. These materials are sifted and put into a furnace with a variety of additives, depending upon what type of glass is being made.
The sand and cullet must be heated to their melting point to make glass. This means glass makers need a furnace that can produce temperatures over 2,900 ° F! When the sand and cullet melt, a red-hot glowing liquid is formed. This is glass in its purest form.
Long, long ago, glass was made by hand. The process was very slow and dangerous. Today, large factories produce huge amounts of glass.
However, some artists still choose to blow glass by hand in specialty shops. If you ever get the chance to visit a glassblower’s shop, don’t miss it. Seeing glass made by hand is a WONDERful experience!
The first people to experiment with glass had no control over its color. Over time, though, they learned — probably by accident — that adding certain substances to the molten glass mixture would produce different colors of glass when cooled.
The ancient Egyptians and Romans developed many different colors of glass. In the 8th century, a chemist named Gerber discovered that metal oxides were the secret ingredients for making colored glass.
Early religious groups helped to drive experimentation with colored glass. Stained glass windows were in great demand, as many churches and mosques were built hundreds of years ago. Stained glass artists required a wide variety of colors to create their beautiful works of art.
To make stained glass windows, artists use glass that has already been colored by the method described above. They cut pieces of many different colors of glass and fit them together to form beautiful windows that are truly fabulous works of art. Stained glass window makers often use a metal, such as lead, to make an outline of their picture, which they then fill in with small pieces of colored glass.
Today, recipes for colored glass call for the addition of powdered metal oxides, sulfides, or other compounds to the molten glass mixture. Here are some common additives and the colors they produce:
- Cobalt Oxide: blue-violet
- Cadmium Sulfide: yellow
- Gold Chloride: red
- Antimony Oxide: white
- Sulfur: yellow-amber
- Uranium Oxide: fluorescent yellow, green
- Chromic Oxide: emerald green
There are also certain compounds, such as Manganese Dioxide and Sodium Nitrate, which can be used with certain types of molten glass to remove other impurities to create clear glass.