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How Is Glass Made?

Ken Wiedemann
Glass is made by melting silica sand in an extremely hot oven and then shaping it, sometimes with a puff of air.

How is glass made? Why does glass break? Why do bubbles pop? What's it like inside a bubble? We make everything clear in this episode! Our questions from kids in Arizona, Brazil, California and Cambodia.


Glass is made from from silica sand. This special sand is shipped to a glass maker or a factory where it's put into a great big furnace, or oven, at 2,400 degrees Fahrenheit. That's way hotter than your oven at home, which probably only reaches 500 degrees.

At Simon Pearce, a blown glass company in Windsor, Vt., the furnace runs all of the time and stays at temperature 24 hours a day, 365 days a year without ever cooling down.

Glassmakers use silica sand because it is very pure. Simon Pearce's Mike Cushing says it's very different than beach sand. "There are a lot of minerals and other items in sand which are filtered out for our process. A fun fact about glass is all glass is clear. If you see a piece of glass that's green or blue, they simply change the color by adding a metal to the process. When you melt the silica sand if you add copper to that mix a very small amount will turn your glass to blue."

Adding sulfur or lead will turn your glass yellow, nickel oxide for violet, iron for Brown. And there are other chemicals that can be added to take color out of glass if the silica sand is not quite pure.

The sand is melted in the hot oven, which results in liquid or molten glass, clear and very hot and it moves around like lava sort of.

Most glass is poured into molds to make the glass a different shape like bottles, the hot liquid molten glass is poured into a mold of the shape and then it cools down and voila, it's a jar. Things like windows that are very flat and straight are poured onto a flat surface and they melt in that shape. It's kind of like how if you pour water onto a plate and put it in the freezer it would come out as a frozen sheet of ice.

But there's another way that people make shapes in glass and it's called glass blowing.

At Simon Pearce the process is very similar now to the way they made glass a thousand years ago.
"The glass, when it's hot will adhere to a metal rod," Cushing says.  "We use hollow metal rods to gather the liquid glass from the furnace. It's very similar to honey when you're gathering out of the furnace. It's a similar texture and you take that glass from the furnace and we use various tools to shape that to a shape we'd like. And all of our pieces have a wooden mold. So when you get the glass to a temperature that's solid enough you can actually put that glass into the mold and you blow through the hollow pipe and it takes the shape of the piece you're trying to make."

Credit Jane Lindholm / VPR
At Simon Pearce in Windsor, Vt. artist shape molten glass by blowing through a hollow metal rod.

The glassmaker will blow their breath through the rod to shape the glass. Then they use a special tool to cut the glass off the rod.

Then using heat they finished the top of the object, like a drinking glass, so it's nice and smooth.

Working with glass can be dangerous, if glass isn't cooled at the right speed it can explode into millions of small pieces.

Blown glass is very artful, but much of the glass we use is for things like bottles and jars, which is more likely made in a factory by a machine, but the process is very similar, using steel molds and glass is poured into a vessel and a machine might provide the puff of air that shapes the piece.

Credit Jane Lindholm / VPR
What's it like inside a bubble? We learn all about bubbles with the executive director of the Montshire Museum of Science has some ideas.

Listen to our entire episode to learn why glass breaks. And we'll visit the Montshire Museum of Science to learn about why bubbles pop and consider what it's like to be in a bubble.

This site has great bubble recipes if you want to experiment with bubbles at home.


Credit courtesy from parents
Curious Kid Cameron, left, 6 from San Jose, Ca.; Madelyn, center, 7, from Scottsdale, Az.; and Oscar, right, 6, from Brattleboro, Vt.

Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.
Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
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