Montshire Minute: Glassblowing

Originally aired during the week of February 21, 2000

Monday

It can be transparent or opaque, or somewhere in between. It can be colored or colorless. It can be made thinner than eggshell or molded into blocks several feet thick. It is strong and durable, and impervious to almost any type of acid. Some new kind of space-age material? Well, actually humans have been making things out of glass since ancient times. Most glass consists of silica (a mineral in the form of sand); sodium oxide (an alkali which lowers the temperature at which the silica melts); and lime. Toss it all into a furnace and you end up with a gooey liquid which can be blown or molded to give it a shape. When cooled at a controlled rate, the material hardens. Glass is hard and brittle, but it's not really a solid - it's a super-cooled liquid. At room temperature, the changes in the composition of glass are so small we just don't notice it.

Tuesday

When molten glass cools, there is no specific temperature at which it suddenly goes from liquid to solid. Glass, even the kitchen glass you use to pour water into, is considered a supercooled liquid. That's right, supercool! Think of a stick of butter taken from the fridge, and compare it to a stick that's been sitting on the counter for a while. It's a lot softer, at room temperature, right? Butter is also a supercooled liquid. Glassmakers use several techniques to shape molten glass. It can be can be pressed into a mold (a technique commonly used for making optical lenses). Skilled craftspeople can also blow glass (First lesson: Don't breathe in!) Of course, glass has been used for centuries as a material for containers and window glass. Today, glass is also used in textiles, optical instruments, electronics, and complex industrial machines.

Wednesday

In ancient times, glass objects were considered luxury items. That's because making one bottle could take days to cast and cut. Then the Romans discovered how to form glass vessels by balling up molten glass at the end of hollow blowing pipe and inflating it like a bubble. The shape could be altered with a few simple tools, and this time saving method made it possible for anyone to own glass objects. Glassmakers eventually figured out a way to make flat glass used in windowpanes. In one method, the glassmaker blew a large bubble and spun it rapidly while the glass was still soft. The centrifugal force created a round, flat disk of glass attached to the blowpipe. After the pipe was removed, the disc was cut into square or rectangular panes. A little dimple called the "bull's eye" was left behind at the point where the blowpipe was attached. This impurity is now considered a cool design effect sought after by antique glass lovers.

Thursday

Glass lets the sun in, but still provides some protection from the weather. Glass light bulbs illuminate a room while keeping out oxygen that would cause the hot filaments to burn up. Glass mirrors reflect light, showing off our flawless complexions. Glass jars and bottles can reveal their contents without being opened. Glass is all around us! In the 17th century the Englishman George Ravenscroft discovered that by adding lead oxide to glass he created a new type of brilliant, sparkling glass. Heavier and softer than other kinds, this "lead glass" could bend light rays at different angles, and it became the preferred material for lenses in high-quality microscopes and telescopes. Lead glass also opened up new possibilities for craftspeople who could cut and grind it to create more intricate designs.

Friday

Glass has some crystal clear advantages over other packaging materials. It's not made of harmful chemicals. And glass can be used again and again - it is naturally 100% recyclable, forever. So today's jam jar can be converted into tomorrow's pickle jar. It takes more resources to produce new glass from scratch than it does to make it out of recycled material. About ten to twelve percent of the glass we use in this country is recycled. Still, according to the Earthworks group, Americans throw away enough glass bottles to fill the twin towers of New York's World Trade Center every two weeks! So we can do better! Support your local recycling program by leaving used glass containers out for curbside pickup or take them to your recycling center.