What is Glass?

Mary McMahon
Mary McMahon

Glass is an amorphous solid that has been around in various forms for thousands of years and has been manufactured for human use since 12,000 BCE. Its status as a liquid, versus a solid, has been hotly debated. The short story is that glass is a supercooled liquid, meaning that it is rigid and static but does not change molecularly between melting and solidification into a desired shape. It is one the most versatile substances on Earth, used in many applications and in a wide variety of forms, from plain clear to tempered and tinted varieties, and so forth.

Empty wine glass.
Empty wine glass.

Naturally occurring glass is created when rocks high in silicates melt at high temperatures and cool before they can form a crystalline structure. Obsidian or volcanic glass is a well known example of naturally occurring types, although it can also be formed by a lightning strike on a beach, which contains silicate-rich sand. Early forms were probably rife with impurities and subject to cracking and other instability, but examples of beads, jars, and eating materials first appeared in ancient Egyptian culture.

Glass blower heating glass in a kiln.
Glass blower heating glass in a kiln.

When manufactured by humans, glass is a mixture of silica, soda, and lime. Other materials are sometimes added to the mixture to “frost” or cloud the glass or to add color. The elements are heated to 1800° Fahrenheit (982° Celsius). The resulting fused liquid can be poured into molds or blown into various shapes, and when cooled, glass is a strong, minimally conducting substance that will not interact with materials stored inside. As a result, it is frequently used in scientific laboratories to minimize inadvertent chemical reactions and to insulate power lines.

Pieces of shattered tempered glass.
Pieces of shattered tempered glass.

Silica is found in a wide variety of natural sources, including, most commonly, sand. Sodium carbonate, or soda, is used to lower the fusion point of silica, making glass light and workable. Soda is called a flux, because it brings the melting point of the mixture down. Lime is ground from limestone and makes the mixture more viscous, as well as making it less susceptible to the erosive qualities of water and acids.

A glass fuse.
A glass fuse.

Glass is a strange substance, defying easy scientific categorization. It is not a solid, not a gas, and not quite a liquid either. Generally, it is classified as a rigid liquid, maintaining liquid properties while acting like a solid. Heat can return it to a liquid and workable form, making it easy to reuse and recycle.

Glass bowls can be used in cooking or decorating.
Glass bowls can be used in cooking or decorating.

There are many reasons that glass is a favored material. It resists chemical interactions, it is easy to recycle, it does not leach chemicals like plastics do, and it can withstand extremes of heat and cold, although not at the same time. Tempered or safety glass is used in a wide variety of applications, and virtually all consumers use many forms daily.

Discarded glass can be melted down and repurposed.
Discarded glass can be melted down and repurposed.
Obsidian can sometimes be formed by a lightning strike on a beach.
Obsidian can sometimes be formed by a lightning strike on a beach.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


So is glass a mixture or a compound?


Blown glass is absolutely beautiful. Glass blowers used to set up a booth in the middle of the mall around Christmas and sell their art, and they also would create some items as people watched.

The whole process blows my mind. I don't know how they shape it so intricately.

I have a couple of blown glass dragonflies and one unicorn. All of these pieces feature clear glass in some spots and colored glass in others, and they all have a round mirror base. They are delicate, and the thinnest spots are the weakest.


I love it when facets are cut into glass items. I have a glass vase with so many facets that it's hard to see through.

If you fill one of these with water and the sunlight hits it just right, it will create rainbows on the floor or table. Something about the angles allows this to happen.


@Oceana – Glass baking pans and many other types of glass cookware are made with tempered glass, so they can resist shattering. I have a thick tempered glass measuring cup that I boil water in, and it doesn't crack at all. I put it in the microwave until the water is boiling.

If you were to put some sort of decorative glass that wasn't meant for cooking in the oven, I imagine it would shatter or at least crack. Decorative glass generally isn't tempered, because it is just made to look pretty.

As long as you use oven-safe, microwave-safe, and dishwasher-safe glass cookware, you should be fine. These items are tough and hard to break even if you drop them.


Glass dishes can break if you expose them to extreme temperatures. I once ran hot water over a glass that had contained iced tea, and it cracked.

I've heard horror stories of glass pans shattering in the oven as someone opened the door. I don't understand why anyone would use a glass baking dish if this is a possibility!


Are obscured glass and frost glass one and the same, or are they different? If so, what is the basic difference between them?


why is glass called a super cool liquid?


Is glass everlasting?


@anon127339: It is sometimes said that glass in very old buildings is thicker at the bottom than at the top because glass is a liquid, and so over several centuries it has flowed towards the bottom. This is not true.

In Medieval times panes of glass were often made by the Crown glass process. A lump of molten glass was rolled, blown, expanded, flattened and finally spun into a disc before being cut into panes. The sheets were thicker towards the edge of the disc and were usually installed with the heavier side at the bottom.


thanks a lot! this helped me a lot in my seminar and projects.


glass is considered a liquid in chemistry because it does flow but very slowly. if you look at a very old house's windows you will see that the glass is thicker at the bottom than at the top. because the glass has flowed down.


is glass a compound or a mixture, and does it move even when made up like a window?


is glass a mixture or a compound?


Thanks for the information. it really helped me with my assignment. there isn't much information you can get about glass on the internet.


thanks for the info, it's well written and easy to understand. helped me with my project. ~emily


Anon: Soda lime glass weighs 158 lbs/ft3 which works out to be 1.55lb at 3mil and 2.07lb at 4mil.


does glass affect hearing?


To answer #3, the square foot weight of 3/8 tempered glass is 4.9 lbs. 3/16 is 2.16 lbs.


Glass is melted sand, nothing more nothing less. I'm 14 and i know that.


I am concerned about the safety of glass drinking cups made in China. Is glass ever made with possible toxic substances. We are so afraid of Chinese made products. Our area has been deluged with toxic Chinese wallboard used in new houses.


why are large pieces of glass called stoces and where did this originate?


what is the effect of glass?


I think glass is like so totally the coolest weirdest substance.


What is the characterization of B2O3 based borate glass?


This helps me a lot espespecially in my assignment!

Thank you very much for your information.

May God Bless you all.



Is glass always breakable?


is glass man made or what?


is glass a compound or a mixture?


can glass table be set outside?


what is a Bioglass?


What is the per square foot weight of 3/8" & 3/16" TEMPERED GLASS


What types of sand is used in glass blowing?

And what is the sand mixed with to make the mixture to begin Glass Blowing?



What are some chemical changes in glass?

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