The history of glass and the window: from crown glass to triple glazing

Photo: one of the Goudse Glazen, the famous stained-glass windows of the Sint-Janskerk in Gouda. Dosseman, Wikimedia Commons, licence CC BY-SA 4.0 (cropped)

Building facts #7 · Every Monday and Thursday one building topic, examined in depth

Look out of the window: you're looking through melted sand. We hardly notice glass any more, yet for most of history a clear, flat window was a luxury. The Romans glazed their first windows with thick, cloudy cast glass, medieval churches filled huge openings with small pieces of glass in lead, and in the Netherlands people once paid tax on every window. Today a sheet of HR++ or triple glazing keeps the warmth inside. In this seventh edition of Building facts we look at one topic: the history of glass and the window. (Missed edition #6? Read the history of the bath and the shower, from Roman thermae to walk-in shower.)

Where does the history of glass and the window begin?

Long before anyone made glass, people used it. Obsidian, a natural volcanic glass, was used for arrowheads and knives as far back as the Stone Age. Man-made glass objects date back to about 3500 BC, and archaeologists think the first true glass was made in coastal north Syria, Mesopotamia or ancient Egypt.

The Roman writer Pliny the Elder told a nice story: Phoenician traders were said to have stumbled upon glass by accident at the river Belus. Historians see it more as a reflection of the Roman glass trade, which prized the very pure sand from that area, than as the real discovery. The oldest written recipe is more down to earth. An Assyrian clay tablet from about 650 BC says: 'Take 60 parts sand, 180 parts ashes of sea plants and 5 parts lime, and you get glass.'

What is glass made of, and how is it made?

The Dutch heritage agency (RCE) puts it simply: glass is melted sand that cools into a solid without crystallising. Scientists call it an amorphous solid: its atoms have no regular crystal pattern, which is why light passes straight through.

Pure silica only melts at 1,723 °C, too hot for early furnaces. So glassmakers added a flux: first potash from the ashes of wood or plants, later soda. Soda lowers the melting point but makes glass weaker, so lime is added for strength and durability. Window glass is still essentially that trio, sand, soda and lime, usually with broken glass (cullet) mixed in, which also helps it melt. This soda-lime glass contains about 70% silica and makes up around 90% of all glass made today. Depending on the type, the batch is melted at 1,300 to 1,600 °C.

Look at the edge of a pane and you'll see a faint green tint: that comes from traces of iron oxide in the sand. Glassmakers once added manganese to make glass colourless, but manganese slowly turns purple in sunlight. That's why some houses on the Amsterdam canals have purple window panes.

When did windows first get glass?

Before glass, window openings were closed with shutters, or with thin, translucent materials such as flattened animal horn, paper or thin slices of marble. The English word window still remembers the open hole: it comes from Old Norse vindauga, 'wind eye'. Many Germanic languages took the Latin word fenestra for a glazed window instead, like German Fenster.

The Romans are the first people known to have used glass in windows. Glassblowing, invented on the Syro-Judean coast in the 1st century BC, made glass much cheaper: glass vessels became inexpensive compared with pottery. Around 100 AD glassblowers in Alexandria learned to make clear glass with manganese dioxide, and the Romans began using glass in buildings. Their cast window glass had poor optical quality, but it appeared in the most important buildings in Rome and in the luxurious villas of Herculaneum and Pompeii. It also let in the warmth of the sun while keeping warm air inside.

In our part of the world, window glass only really took off from the late Middle Ages, and at first mainly in churches. Clear window glass was so expensive that only the rich could afford it, in castles and large stone town houses. Even in England, glass only became common in the windows of ordinary homes in the early 17th century.

How was window glass made by hand?

For centuries, a flat pane of glass was a feat of skill. For crown glass (in Dutch kroonglas or maanglas) the glassblower blew a bubble, transferred it to an iron rod and spun it so fast that it opened up into a flat, round disc of about 1.20 metres across. The centre, with the scar of the rod, stayed thick: the 'bullseye', used for cheaper windows. From a round disc it was most economical to cut diamond-shaped pieces, and according to the RCE that is where the Dutch word for a windowpane, ruit (which also means diamond shape), comes from. Disc and crown glass were made from at least the 7th century until the early 20th century.

With cylinder glass the glassblower swung a blown bubble back and forth until it became a long, hollow cylinder. The ends were cut off, the cylinder was slit lengthwise and then rolled out flat in a flattening kiln. The method has Roman roots, and German craftsmen developed it into a way of making window glass in the 11th century. In the early 19th century the cylinders were about 1.8 to 2.4 metres long and 25 to 36 cm across, which limited the width of a pane. Cylinder glass is still made today for leaded-glass windows, in more than five thousand colours.

Why did old windows have small panes set in lead?

Because large panes simply couldn't be made. In the Gothic period window openings grew enormously, and they were filled with pieces of glass held in H-shaped lead strips, soldered together where they cross. To make the window waterproof, a soft, oily cement or mastic is worked in between the lead and the glass. The oldest stained-glass windows still in their original place in the Netherlands date from the 16th century, and the finest of them, the Goudse Glazen in the Sint-Janskerk in Gouda (the photo at the top), are world famous.

In houses, the Dutch kruiskozijn (a cross window, divided into four by a vertical mullion and a horizontal transom) was at first closed only with shutters. Later the two upper lights were given leaded glass, and after about 1650 the glass was set in wooden glazing bars (roeden) instead. The lower windows got shutters, and later glass as well. The sash window was developed in the second half of the 17th century. In England, counterweights replaced the pins that held the sash open after 1670, and the Netherlands got its first sash window in 1685.

Did people really pay tax on their windows?

Yes. England and Wales introduced a window tax in 1696; it was only abolished in 1851. The Netherlands got a tax on doors and windows on 1 January 1812, under French rule. The idea was simple: the inspector didn't have to go inside. He counted the outside doors and windows and multiplied them by an amount per door or window, which depended on things like the number of inhabitants of the town and the floor the window was on. In 1821 that could be 40 cents, but also more than a guilder. Homes with an annual rental value under 20 guilders were exempt, and so were roof windows and attics or cellars that weren't fit to live in.

People found ways around it. They bricked up windows, sometimes plastering the façade or even painting a fake window on the bricks. Anyone building a new house simply chose fewer but larger windows, because the size of a window didn't count. The reverse was true as well: a farmhouse with many windows in its front façade showed that a wealthy family lived there. The tax was only abolished in 1896, partly so it wouldn't stand in the way of badly needed house building in overcrowded cities.

Not every blind window is a tax story, by the way. In the Archipelbuurt in The Hague there are walls with window-shaped brickwork that were never real windows. (Curious about the bricks themselves? Read the history of the brick.)

Is glass a liquid? Why are old panes thicker at the bottom?

You may have heard that old church windows are thicker at the bottom because glass slowly flows downwards over the centuries. It's a myth. Once glass has solidified, it stops flowing. The ripples and thicker edges in old glass were already there on the day it was made: hand-made crown and cylinder glass was simply never perfectly even. A 2017 study of medieval glass from Westminster Abbey, dating from 1268, calculated that it could flow at most 1 nanometre per billion years. Impossible to notice in a human lifetime, or in the lifetime of the universe.

How did glass become large and perfectly flat?

Around 1900 machines took over. In 1901 the Belgian engineer Emile Gobbe managed to draw a band of molten glass straight upwards, and from 1905 he improved the process together with Emile Fourcault. This drawn glass has faint draw lines, which can distort the view when you look at it from an angle. For perfectly flat glass, for mirrors and shop windows, glass had been cast on an iron table and then ground and polished on both sides since 1685: plate glass, beautiful but expensive.

The real revolution was float glass. In the 1950s Sir Alastair Pilkington and Kenneth Bickerstaff in England developed a process in which molten glass is poured onto a bath of molten tin, on which it floats. The tin gives a perfectly smooth underside, the heat above the glass a perfectly smooth top. The raw materials are melted at about 1,500 °C, and the glass cools on the tin from about 1,100 to 600 °C before it is lifted onto rollers. The process was introduced in 1959 and gives a ribbon up to 3.21 metres wide. Almost all window glass today is float glass.

The Netherlands, with little suitable sand and wood, was never an ideal place to make glass. Float glass was used here from about 1962, and only really took off after Dutch production started in 1984. Larger sheets changed architecture: think of the Glaspaleis in Heerlen (1935, designed by Frits Peutz), with its glass façades. A tip for spotting old glass: buildings from before 1915 originally had cylinder or disc glass; after 1915 drawn glass quickly took over.

When did double glazing arrive in the Netherlands?

Glass is a poor insulator, so a house loses a lot of heat through its windows. In the 1870s people in Scotland, Germany and Switzerland started fitting a second pane. A sealed unit of two panes was patented in the United States as early as 1865, but it only became a commercial product in the 1930s and 1940s. In the Netherlands double glazing appeared in the late 1940s (the RCE mentions both 1946 and 1948). The big breakthrough only came after the oil crisis of 1973, when the government offered subsidies and people realised that double glazing could save around 40% energy compared with single glass.

How does it work? Two panes are bonded to a spacer so that a sealed cavity of dry air or gas forms between them. The spacer contains a drying agent that absorbs any moisture in the cavity. If that seal fails, the pane mists up on the inside, between the two sheets, and it has to be replaced.

What do HR, HR+, HR++ and triple glazing mean?

HR stands for hoog rendement, high efficiency. The secret is an invisible coating: an extremely thin metal layer, applied to the glass under vacuum, that reflects heat back into the room while letting light through. Milieu Centraal explains the difference: HR and HR+ glass have ordinary dry air between the panes, while HR++ and triple glazing contain a noble gas, usually argon, which insulates better. How well glass insulates is expressed as the U-value: the lower, the better.

HR++ has a U-value of 1.2 or lower and often fits in existing frames; triple glazing usually needs new, insulating frames. The newest step is vacuum glass: two panes with a gap of only 0.2 mm from which the air has been removed. It was first produced in Japan in 1996 and introduced in the Netherlands in 2004. At only about 8 mm thick, it insulates as well as triple glazing, according to Milieu Centraal.

Glass is only part of a warm house: draughts along window frames and worn sealant joints count too. We don't repeat those checks here; read getting your home ready for winter.

What are some fun facts about glass and windows?

• The Dutch word ruit (windowpane) comes from the diamond shapes that were cut from round discs of crown glass.
• Purple panes on the Amsterdam canals are old glass with manganese that changed colour in the sunlight. Later, glass was made purple on purpose, to look wealthy.
• The English word window literally means 'wind eye'.
• Laminated safety glass started with an accident: in 1903 the French chemist Edouard Benedictus dropped a flask that had held a solution of nitrocellulose. The glass cracked but didn't break apart. He patented his safety glass in 1909.
• After a V2 rocket hit the Archipelbuurt in The Hague in January 1945, glass was so scarce that broken windows were repaired by joining small pieces of glass with lead strips. Such windows were visible there until long after the war.
• In a listed building in the Netherlands, a broken pane may be replaced without a permit.

What do you need for working with glass and windows today?

Making glass is a job for the factory, but cutting a small pane, drilling a hole or sealing a joint around a window is something many DIYers do themselves. Glass edges are sharp, so wear cut-resistant gloves and eye protection; how cut levels work is explained in cut-resistant gloves, cut levels A to F. Below are a few products where you can recognise this history.

What is the delivery time at Saniso?

Order on a workday before 15:00? Then your order is delivered the next workday, as long as it fits in a parcel. Pallet shipments have a different delivery time. Saniso has thousands of products in stock in the Netherlands.

How do you order at Saniso if you're not a customer yet?

Your first order is easy to place:

1. Create an account via register.
2. Already have an account? Log in here with your email address or phone number.
3. Choose your product and click "Add to cart" or go straight to "Buy now".
4. Check out: all prices include VAT. Rather not pay shipping? You can also pick up your order for free in our showroom.

How do you stay up to date on offers?

Want to know when there are new offers? Sign up for the newsletter: scroll to the bottom of any page on saniso.nl, enter your email address under "Newsletter" and click "Subscribe". You'll get updates on offers and discount vouchers, and we don't spam. With an account you also collect loyalty points for a discount on your next purchase.

Questions about ordering, your account, payment, shipping or returns? Find all the answers in the Saniso FAQ.

Frequently asked questions about the history of glass and the window

Building facts takes one building topic under the magnifying glass every Monday and Thursday. This is edition #7. Read the previous edition too: the history of the bath and the shower, from Roman thermae to walk-in shower, and edition #1: the history of the brick. Fun link: next time you walk past an old brick façade, look for bricked-up windows from the days of the window tax.