What is glass?

This sounds like another dumb question, but an understanding of the properties of glass is fundamental to understanding an important form of matter.

Glass is defined as an amorphous (as opposed to crystalline) solid material that's usually transparent. The amorphous state of matter comes with some very interesting properties. One of the most interesting is that glass acts like a liquid even when it's not molten. We'll get back to this.

Glass usually refers to silicate glass, which means that its principle component is silicon dioxide. Quartz is considered a pure form of silica. Some glass formulas contain other materials to add strength and stability, including sodium carbonate, and other oxides of magnesium and aluminum.

Glass is actually a vitreous solid, which means that it's formed by rapid melt quenching, meaning that it's kept from solidifying for as long as possible. That's obvious if you've ever seen someone working glass to form bottles, glasses and other objects by blowing a hot plug of molten glass and slowly working it as it cools and keeping it hot as long as possible.

Basically, glass is considered a non-ordered material. Its molecules are not arranged in a geometric structure like crystals. This allows the molecules to flow over one another, slipping and sliding like water. However, glass will crystalize if not cooled slowly down past its transition temperature (the temperature in which a liquid will crystalize) and stay liquid. It's hard to understand how a liquid can be solid, but that's what glass is--a supercooled liquid. One of the ways that this strange property is exhibited is how old glass windows show sagging, being thicker at the bottom. The glass is always flowing, but it's doing it very slowly, literally over centuries. It's like the old adage of watching grass grow, only in this case it's glass flowing.

The main quality of glass that's important is transparency and the fact that it can refract light; although plastics have taken over the applications that were once the exclusive realm of glass. It turns out that plastics also act like amorphous glass.

Another important property is chemical resistance. That's one of the reasons that glass apparatus--beakers, flasks, tubing, and measuring cylinders--are used for chemical synthesis and processes. In most cases Pyrex glass is best for this because of its resistance to breakage because of thermal shock. It's possible to heat a Pyrex flask with a Bunsen burner and then plunge it into water without breakage. The only chemicals that affect glass are strong caustics (sodium hydroxide) and in some cases strong acids, especially phosphoric acid and hydrofluoric acid. Glass etching solutions contain bifluoride salts along with sulfuric acid.

As a chemist I used Pyrex glass beakers and flasks. Pyrex is a special borosilicate glass that's very heat resistance because it has a low thermal expansion coefficient. This property has to do with how much a material expands when heated. Pyrex is a Corning product and it's been around for quite some time and has also been used in cookware. The other place that Pyrex is used is in telescope mirrors for the same reason - low expansion coefficient.

There is a lot of technology involved in glass and glass products. I can't imagine a world without this amazing material.

Thanks for reading.