Sheet glass, not only for windows

On the north gable of our building, we have a small exhibition of various other applications, including the fact that it doesn't have to be flat...
Note the windshield mounted over the door - it is said to have come from an Opel Rekord '58.

Glass with special surface properties

These two glasses have both been given very special properties in the very thinnest surface layers.

Above left is shown a self-cleaning glass. Such glass has a catalytic, light activated coating.
Organic material like dirt that sticks to the surface decomposes and fragments when ultra-violet sunshine hits the photocatalytic layer so that it no longer adheres to the glass surface. Thus the dirt is easily washed away by rain.

Below left you see a home-refrigerator glass shelf hardened for carrying capacity. Chemical processes, like adding special elements to the glass surface or, more common, by heat treatment, can be used to increase the mechanical strength of glass. The glass can even become (almost) bullet proof.
Both processes - chemical hardening as well as thermal - means that tensile stresses are introduced in the glass surfaces while compression stresses are introduced in the central parts of the glass. Hardened glass cannot be manufactured after hardening but must first be given its final shape and then hardened.

Glass with special surface structures

These two glasses have a macroscopic surface structure, a structure that you can see and feel.

Above right is shown a distorting glass, aimed for a bathroom window.
Such glass is made by rolling. The semi-liquid glass passes through two rollers where one - or both - are patterned. Several other types of decorative glass, "Culture glass" made to imitate windows produced by the cylinder glass-blowing technique, glass for front doors, glass for decorative candle-light lanterns and such, are produced in a similar way.

Below right you see a glass designed for use with solar heaters.
Since glass will always reflect light - and at some angles it will even reflect all incident light - will a solar heater covered with a simple glass at some instances work more like a mirror than as anything else: Much of the incident solar heat will bounce away rather than to enter the heater.
This glass has a rugged surface structure which means that whichever angle the incident light might have, a large portion of it will be caught and not reflected. This will increase the efficiency of the solar heater.

Glass with special inlays

These two glasses have specific properties because an extra material has been added inside the glass.

Above left is shown a metal-mesh glass.
The shards from such a window are, to a certain extent, kept in place by the mesh as the window breaks. This gives a certain protection from cutting wounds by scattered shards. If the window is placed in the door between two adjacent fire cells in a building the fact that the shards stay in place will become a hindrance for the flames from one side of the door to spread wildly to the other side.
This is not a fire resistant glass but it may give an extra minute for evacuation and may thus contribute to fire safety.

Below left you see a "real" fire protective glass, designed to protect from fire for 30 minutes.
This is a cassette with two glass sheets containing a special liquid between them. As the cassette is heated two things happen:
- the liquid absorbs heat
- it becomes opaque.
This prevents thermal radiation from passing through the window and the fire spread from one fire cell to the next is delayed.

By combining several cassettes, the time gained can be set at 30, 60 or 90 minutes.

Glass for cars

Glass for car cars need special properties no to worsen injuries in car accidents.

Above the door is a windscreen from a 1958 Opel Rekord.
At a car accident the front screen will scatter and it is quite simple to imagine the wounds that would result if it scattered in razor-blade sharp shards just in front of the drivers face, like a common window.
At the same time it is important that the windscreen is shaped so as to provide the best possible view for the driver. Therefore, it is a bit impractical to rely only on thermal hardening because, as you saw from the hardened glass description above, the hardening must be the last step.
So for windscreens laminated glass is commonly used. This means that a thin plastic layer is attached to a single glass or placed between double glasses prior to the final shaping. Just like with the metal-mesh glass above the plastic layer will retain the shards and thereby prevent cutting wounds.
The same process is used for the remaining windows in the car but the windscreen and the back window are a bit special because they often have quite a complicated shape. Today all the windows are also hardened which means that the glass shards become less sharp.

The oldest type of window glass

was made with the so called crown glass method (see below).

We have a small crown glass, made especially for us by Bosse Andersson here at Bergdala glassworks. It was given to us when we published our book on shet glass (only available in Swedish).
It is mounted in the window above the glassblower's chair.

Do you want to know more about sheet glass?

Sheet glass is much more common than most people think...
Most of the glass literature deals with art glass and other luxurious products.
But the economic backbone for the glass industry were the everyday products: mass produced eggcups, citrus presses, table salt containers. These products, if mentioned at all, are often treated in a patronizing way.
The same applies to sheet glass. But sheet glass is by no means used only in windows. Sheet glass was a major product at many Swedish glassworks for centuries - though not in Bergdala.

In 2012 our society published a book about sheet glass, its history, the methods etc. The book is called Planglas and can be bought here in the museum, or from a couple of places on the 'web. However, it only exists in Swedish...

Medieval sheet glass

The oldest technique to make sheet glass was called the crown glass method.
The idea is to make a bubble, open it and rotate it rapidly. This gives a plane, round disc, from which panes were cut. The disc will always have pontil mark at the centre.

We have a small example mounted in the window, above the glassblower's chair
Illustration to the right.

Cylinder-blowing was invented about 1400 and was used until the 1900's.

In the picture here you will see how a flat glass pane is made by first blowing a cylinder.
These cylinders could be about two meters in length but the wall thickness was only about two millimetres.
To extend the cylinder, the glassblower would swing it back and forth in a special cavity often provided in the shop floor.

Both ends of the cylinder would be removed and it was then opened along its length in a special furnace with a flat floor, a flattening furnace.
The glass sheet would then be flattened further and polished by a soaked block of wood. This is often visible - if one studies an old window made like that, there are often marks.

Below is a picture showing the method invented in 1688 by Bernardo Perrotto. This technique became the basis for Saint-Gobain.
Glass produced this way was better - but much more expensive. This explains why the cylinder production method survived for 500 years.