As folks update their homes with more energy efficient options, their older double hung single-panes get the axe. The surprising amount of windows Meg comes across is troubling, as they are almost always thrown in the dump. But this is glass? Why isn't it recycled? This sparked further research into glass manufacturing and building waste, here's a brief rundown: Window glass, known as "float glass" is aptly named from its production process. A mixture of raw materials is heated in a furnace, and then poured out onto a pool of molten metal (usually tin). It floats (get it!?) perfectly smooth across this pool cooling from about 1100 to 600 degrees C, enters a kiln to anneal, and then is cut into panels. Machine manufactured glass has only been in production since the 20th century. Before that, glass was hand blown, flattened, and then ground and polished, a pretty awesome thing to consider the next time you're in a very old building.
BECAUSE OF VARIATIONS IN GLASS "RECIPES", WINDOWS CAN'T BE RECYCLED LIKE OUR BOTTLES AS EACH RECIPE HAS A DIFFERENT MELTING POINT.
There is no standard recipe for window glass like there is for bottles (WHY?!?!), so there is no standard way to recycle them. Some specialty facilities exist, but it's mostly up to local businesses like ReStore (Habitat for humanity) and Urban Ore, and more recently creative re-use like glassphault or terrazzo flooring, building greenhouses, really cool handmade terrariums, etc, to keep the glass in circulation and out of the dump. Once in a landfill, it's speculated that it would take a million years to decompose, arguably never.