You look through it all day: the window, the glass of water, the shower door. Ask what it actually is and most people say melted sand.
They're mostly right. The part they're missing is the clever bit.
Most of it really does start with sand.
By weight, typical float glass, the flat glass made for windows, is 71% silica (silicon dioxide), according to the glassmaker Pilkington. Silica is what the quartz grains in glassmaking sand are made of. And it isn't just any sand: the ingredients are fine-grained and closely controlled for quality.
The rest is where it gets interesting. A typical pane, by weight:
- 71% silica, supplied by sand
- 14% sodium oxide, from soda ash
- 9% calcium oxide, from limestone and dolomite
- 4% magnesium oxide, from dolomite
- The rest: alumina, iron and other traces
Those figures describe the finished glass, not the shopping list. Sodium goes into the furnace as soda ash, a carbonate, and ends up as sodium oxide in the glass. Calcium and magnesium arrive in limestone and dolomite.
Pure silica is stubborn stuff.
On its own, silica only melts at around 1,700°C, and even then it is so thick that Pilkington compares it to syrup on a very cold day.
Soda is the shortcut. It lowers the melting point and thins the melt, so the sand becomes glass at lower temperatures and with less energy.
But the shortcut has a catch.
Soda makes the glass easier to produce, but less chemically durable. For windows and drinking glasses, that's a problem. So limestone goes in as well, acting as a stabiliser.
Soda makes the sand easier to melt, but makes the glass less chemically durable. Limestone helps stabilise it.
When the melt cools, it doesn't go back to being grains. The silicon and oxygen link into a network with no regular crystal pattern, an amorphous solid, and the calcium and magnesium woven through it make it even harder for crystals to form.
One ingredient still gives itself away. Look at the edge of an ordinary window pane and you'll see a faint green: traces of iron that came in with the raw materials. Low-iron glass is made from specially selected ingredients to lose that tint. Ordinary clear glass keeps it.
Sources checked 28 September 2026·4 sources ↓