Pilkington float glass process

Around the early part of the 20th century, the Pilkington company in England developed the Pilkington process for float glass. The float glass process still pulls out the sheet of glass, but instead of putting it on a table to cool and then go into the annealing furnace, they pull it out onto a bath of molten tin. Tin melts at around 400 °C, but the other good thing about tin is it doesn't vaporize until about 2,000 °C — it's got a very low vapor pressure. They would pull it out and the glass would anneal on this molten tin, and the top surface would just smooth out by gravity, and the bottom surface would be smooth because it's on top of this bath of molten tin. That's the float glass process. Everything that's in your house today is made by the float glass process.

Pilkington's 1920s development of floating molten glass on tin to eliminate the half-mile polishing line used by Pittsburgh Plate Glass. Tom's example of how a process innovation can collapse manufacturing cost.

So back in the old days, the bullseye glass, which was the center of this spun piece of glass, was the junk, and you would pay extra for the somewhat flat glass that went into windows like this. By the way, some of these at MIT from 1917 are original — I don't know how many are still original — you actually can see, by looking carefully, they're not completely flat. The way we made glass in 1900, companies like Pittsburgh Plate Glass — they would take the glass as a big sheet off a big mill, and they would have a mill that ran for half a mile to a mile just straight down, and they would polish the glass to make it flat. It was sort of wavy as the glass cooled, and you had to anneal it so it wouldn't shatter. Then someone — Pilkington in England, 1920s or so — came up with something called the float glass process, where you'd pull the glass off when it was hot, and you'd float it on molten tin, which is perfectly flat because of the earth's gravity. The top surface is also flat just because of surface tension, and you could eliminate all that half mile of polishing. Now you only have like a hundred yards of annealing on this float glass product. So newer glass is all float glass, and got rid of all the polishing.

Sir Alastair Pilkington, 1953–1957: continuous pour onto molten tin bath, natural-gas atmosphere, replaces the mile-long polishing line. The defining modern process. Also: Pilkington's earlier (~1900) wire-mesh laminated glass.

Referenced as background covered in casting lectures.

If you go to the research labs, they always have an escort. But you go into a manufacturing plant at Corning where they're making something like — the most recent one I've been to is where they're making the glass for your computer screens. I can't tell you too much because I had to sign confidentiality, but it's not all that different than a sheet form of this hot drawing. They do some other things so they get perfect structures on both surfaces. I don't want to get into glass technology right now — I may have covered some of this in the lectures on casting, the Pilkington process and stuff for making regular window glass.