🪟 Why do windows float on melted metal?

As we drive past houses and shops, look for a window. Its glass may have started out floating on melted metal! Why would a factory give glass a metal bath? To make it flat! In a factory, hot, liquid glass flows onto a bath of melted tin.

Tin is a metal. The glass floats on top, like a toy boat in a bathtub. The tin makes a smooth, level bed. The glass spreads across it, forming a sheet with smooth surfaces and an even thickness. There is no need to grind those surfaces flat.

No rubber duck gets invited to this bath. It is much too hot! As the glass cools, it becomes hard. Most modern window glass is made this way. Pick a window outside and imagine its glass taking that very unusual bath.

On this drive past rows of houses, every single home is wearing glass. Window glass seems simple, just a clear flat sheet. But how do you make something perfectly flat and see-through without a single bump? Long ago it was hard. Glassmakers blew big tubes of glass, sliced them open and flattened them, and the panes came out wavy. For really smooth glass, workers ground and polished both sides for hours, like sanding a floor that never ends.

Then, in the 1950s, a man named Alastair Pilkington at the Pilkington glass company in St Helens, England, tried something wild. People say he got the idea watching grease float on dishwater. His team built a long pool of melted tin, a soft metal that turns liquid when it's hot, and poured glowing liquid glass on top. And the glass floated! Here's why it worked. Glass is lighter than melted tin, so it rides on top instead of sinking, and it spreads out evenly, like pancake batter on a hot pan.

As it cools, it becomes a sheet that is flat and smooth on both sides. So your window spent its baby days floating on a metal swimming pool. Try counting windows on the next three houses. The floating trick spread around the whole world. Nearly every window we pass was probably born on a bath of melted tin.