How is Glass Made

Discover how glass is made.

| June 2018

  • The first glass was made by nature, not man.
    Photo by AdobeStock/Leigh Prather
  • “Ask a Science Teacher” by Larry Scheckel answers over 200 common questions about science.
    Cover courtesy The Experiment

Ask a Science Teacher (The Experiment, 2011) by Larry Scheckel is sure to resolve the everyday mysteries you’ve always wondered about. You’ll learn how planes really fly, why the Earth is round, how microwaves heat food, and much more—before you know it, all your friends will be asking you! This section answers how glass is made.

Glass is a hard, brittle, and transparent solid. About 90 percent of the glass we use is soda-lime-silica glass. It is 75 percent silica, or just plain sand. The “soda” refers to sodium carbonate, and the “lime” comes from limestone, which is mostly calcium carbonate. The ingredients are put into a gas-fired furnace. Window glass is made by having the molten, liquid glass flow over a molten tin bath in a continuous sheet. Nitrogen gas pushes on the top surface to make it smooth.

Glass melts, or turns to a liquid, at a temperature between 2,500–3,000 degrees Fahrenheit. The melting temperature depends on the composition of the glass. Glass does not melt at an exact temperature.

Unlike ice, which melts at a precise temperature of 32 degrees Fahrenheit, glass goes through a phase-change transition. Glass gets softer and softer as its temperature increases, until it can start to flow. That’s a nice property of glass, because glass can then be molded into any shape we desire. Ingredients are added to the soda and lime to change the properties of the glass. Lead or flint causes glass to sparkle. Boron is added to glass to change its thermal properties; this makes borosilicate glass suitable for cooking. We also use a lot of borosilicate glass in the science lab. It has a very low thermal expansion rate and does not break when a very hot or very cold liquid is put in it. Pyrex is a brand name for the same kind of glass. Lanthanum oxide has light reflective properties and is used in eyeglasses. Iron is put in window glass to absorb infrared energy. Take a look at the edge of ordinary window glass, and you can see the green tint. That’s from the iron.

Glass used in optical equipment is generally made from low-dispersion crown glass or denser, high-dispersion flint glass. The glass is selected based on how light is bent, or refracted, through it. Dispersion is a term that describes how much light is bent or refracted as it passes through the glass.

Tempered glass is stronger than regular glass, but if it does break, it shatters into small pieces rather than the big sharp shards of ordinary glass that can cause major bodily harm. Regular annealed glass is placed on a table, which is rolled through a furnace; then the surfaces are quenched or quickly cooled with air, while the interior of the glass remains liquid, or molten. This process sets up stresses in the glass. Tempered glass is used in side and rear windows of cars and in glass doors that are not in a frame.

There is an old myth that glass flows over time. The distortions found in glass that is several hundred years old, such as the glass in Thomas Jefferson’s Monticello home, gave rise to this myth. Glass is an amorphous solid that does not flow. The variations in thickness that cause distortions were due to very imperfect manufacturing techniques.

The first glass was made by nature, not man. Ancient lightning strikes hitting common silica sand instantly heated the sand, melting it and fusing the grains into glass. The result, termed a “fulgurite,” takes the form of a natural glass tube. The longest fulgurite, found in Florida, is over sixteen feet long.

A black glass from volcanoes is called obsidian. It is produced when lava cools with minimal crystal growth. Chipped obsidian edges can be as thin as a few molecules, making obsidian excellent for ancient hunters’ arrowheads and modern medical scalpel blades used in heart surgery. An obsidian blade can be many times sharper than a metal blade.

Tomah has its own connection to glass. Cardinal IG (insulating glass) and Cardinal TG (tempered glass) are located in the industrial park on the eastern edge of Tomah. The IG plant fabricates low-emissivity doublepane glass for windows, used mainly in homes and office buildings. A low-emissivity, microscopic, thin metal coating, applied to one of the inside surfaces of a double-pane window, controls heat transfer. It keeps the heat out while letting light in.

More from Ask a Science Teacher:

Excerpted from Ask a Science Teacher: 250 Answers to Questions You’ve Always Had About How Everyday Stuff Really Works © Larry Scheckel, 2011, 2013. Reprinted by permission of the publisher, The Experiment. Available wherever books are sold.



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