Miracle glass
Next time you look through a window, spare a thought for the miracle of glass. Never mind the view, just consider how incredible it is that you can see through what amounts to a wall, protected from the cold and rain, and yet take in every detail of the outside world. No wonder windows were once so valuable owners carried them around the country with them.
These days glass can be bullet-proof and strong enough to build skyscrapers. In these green and straightened times we’re even told double glazing will save not only the planet, but enough cash to solve the banking crisis. Hmm.
Like most teenagers I believed I knew everything at the age of 15, except for the dreary stuff examiners kept asking about. Something that does stick from those far off days is that glass isn’t actually a solid, but a liquid. Depending on who you ask you’ll get told it’s a supercooled liquid, an amorphous solid or a supersaturated solution. This gives rise to the myth that old windows are thicker at the bottom than at the top because the glass is gradually running down the frame. Nice idea, but not true because otherwise all those Roman vases would look like puddles. In fact it’s because the imperfections in old glass made it thicker in places and these thicker, and therefore heavier, parts of the pane were placed at the bottom of the frame.
This peculiar state gives glass some astonishing properties, but being green isn’t one of them. James I banned burning wood to make glass because we were getting through so much of the forests. The energy needed to make glass is enormous, and that’s before you make sure the materials it’s made from are pure enough for it to be clear. In fact, after ceramic tiles, making glass needs more energy than anything else in a house and by some estimates the manufacture of double glazing produces more CO2 than it will ever save.
The earliest windows were just that - wind holes, vents to let smoke from the open fire escape. Say it in a nicely rustic accent - ‘wind o’s’ - and you’ve got our modern word. Roman villas may have had glass windows but their skills went the same way as the Empire and using glass to keep the drafts out probably didn’t reappears until eight or nine hundred years ago in France. This was largely because they had perfect glass making sand, but it was usually stained because a bright colour (although nothing like as bright as Victorian glass) was preferable to the natural muddy brown of this early glazing.
Up until the twentieth century glass was always blown, blobs of the burning, viscous goo picked up on tubes that a man would blow through to create a bulb. This was rolled into a sort of jam-jar called a “muff”, which once cooled could be cut into square-ish panes. These were small by today’s standards and usually joined by metalwork into the sort of window we call leaded lights. It was the end of the 16th Century before this could even begin to be called commonplace, and glass could finally usurp shaved horn and skin as the window of choice.
During the eighteenth century someone had the bright idea of spinning and flattening the muff into a disc to allow much bigger panes, although these discs were rarely more than four feet across. Still, that meant you could have a pane perhaps 18” by 24” , a massive leap forward. It left a ‘bull’s eye’ at the centre of the disc where the glass was knocked off the blowing tube, perhaps used for an attic window, hidden away so no one could see what a cheapskate you were. Think of that next time you see a bull’s eye in someone’s front door…
The next improvement came with cylinder glass, another first for those pesky French. An evolution of earlier muffs, huge cylinders were blown out, cut open on worktops and flattened while still hot. This gave the larger and larger panes beloved of the Victorians, although still with plenty of character injecting flaws.
And that’s how things stayed until Pilkington earnt his knighthood in the 1950’s with float glass. Pouring molten glass onto liquid metal allowed perfect, homogenous panes as large as you liked, and their perfection meant they could be laminated to give strength. But the character was gone.
So if you’re ever tempted to replace your period home’s windows with modern glazing, remember you’ll be removing character and history as well as harming the planet. So please leave well alone, and your windows should last for many centuries yet.
