GLASS FACTS by Glass & Art

GLASS FACTS by Glass & Art

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Help! The windows are changing shape!

Quite often you hear someone say: glass is a liquid and hence the medieval windows the glass is thicker at the bottom. Slowly, they say, the glass changes shape!

Though it is true that in old churches the glass is thicker at the bottom, it has always been like that. Glass is liquid only when it is molten, and that is definitely not the case at room temperature! Glass is amorphous solid, or - to put it less technical - glass is a bit of a mess at the molecular level. Crystals are molecularly nicely arranged in beautiful geometric structures. Glass is not, it is disturbed. Hence, it has not a melting point (like water melts from ice at 0 degrees Celsius), but a trajectory: when lowering the temperature, gradually the glass becomes more and more solid. Almost all types of glass are fully solidified at 500 - 550 degrees, and will not change shape below that temperature.

Why then, are windows of old churches thicker at the bottom? That is because hand-blown sheet glass was made by spinning the glass into a huge disk. Such a disk is thicker in the middle than at the outside. The disks were cut in pieces, and the middle parts (the beautiful bullseyes) were sold, relatively cheaply,  to houses and shops. The pieces for the churches were cut out of the best parts of the disks, but even so, the thickness was not equal. If you apply this glass in a stained window, you punt the thickest part at the bottom for more strength.