In L. Frank Baum’s book, The Wonderful Wizard of Oz, when Dorothy and her friends finally reach the Emerald City to meet the Wizard of Oz, they’re given special sets of glasses, to protect their eyes from the brilliance of the city. Later it’s found out that the glasses are simply tinted green, and that the city isn’t actually all green to begin with, but it’s all part of an illusion crafted by the Wizard of Oz. But you may have noticed that even regular glass has a green tint to it, as long as you view it on the thicker edge.
What you might have learned in school is that glass is comprised of SiO2, or silica. But the reality is slightly more complicated. Glass is a complex mixture, and glaziers (that’s what you call someone who makes glass) have been tweaking the recipes for different types of glass long before we ever really understood the chemistry.
The image here is of some soda-lime glass, so called because of the presence of added sodium and calcium oxides (soda and lime, respectively.) These give the glass certain properties. The soda makes the glass very workable and meltable, which is important for the glass-maker. As for the lime… well, that deserves its own post, but the short of it is that if you add soda, the glass would dissolve in water if you don’t add lime as well. The green color comes from a third component you may be familiar with, and that’s iron, specifically iron (II) oxide or FeO. If you are close to the desert or the beach, try this:
Gather some sand and a really strong magnet. spread a thin layer of sand on the table and see if anything in the sand moves when you wave the magnet over it. You’ll notice, depending on the strength of your magnet, that some dark particles may be attracted to your magnet. These are bits of a mineral called magnetite. The amount of magnetite you can come by will vary depending on your local geology, and it’s a mixture of iron oxides. In addition to magnetite, there is also hematite, but that doesn’t respond to magnets.
So it comes as no surprise that iron ever made its way into glass. Historically, the first crude attempts at glass likely came from working with whatever sand was laying around, minerals and all. The glass can also acquire a yellowish tint if the dominant iron impurity is Fe2O3 or iron (III) oxide, which you may be more familiar with as red or brown rust. A greener tint means that the iron impurities in the glass absorb their opposite color (redder light), and a yellow tint implies that the glass absorbs bluer light. This is used by glass manufacturers to make glass melt faster, since they can use methods of infrared heating (redder light beyond what’s visible to the human eye) to heat glass with iron (II) oxide impurities faster, since they know the glass will absorb that wavelength of light, and therefore the energy of that light.
So why isn’t everything green when you look through a window? Actually it is. Most of the glasses you look through are just too thin for you to pick up on the subtle filtering of red light. You’ve lived in the Emerald City your whole life, you’ve just never realized it.