GLASS FLATNESS and THICKNESS
Glass... what is it? A solid or liquid or both.
It was observed that in the medieval church windows many panes are thicker at the bottom than at the top. It was believed for a long time that over the centuries the glass was sagging by its own weight. That assumption is false; almost! According to calculations published in the Proceedings of the Royal Society in 1999 by Prof. Yvonne Stokes (Ref. 36) from the University of Adelaide, it would take at least 10 million years for a window pane to become just 5% thicker at its base.
From Roman times till now, the use of window glass has increased exponential; basically due to the decreasing manufacturing costs. One of the earliest types of flat glass is Crown glass. This glass was blown into a ball, cut open and spun into a disk; up to 1.5 meter (5ft.) in diameter. These circular pieces of glass had the thickness decreasing from center to the edge. As this Crown glass was cut into usable pieces, the resulting thickness would vary wildly.
During the 17ᵗʰ century, this technique evolved into blowing a cylinder, cut off both ends, open the cylinder and flatten it while reheating. It produced larger panes. The thickness of this cylinder glass was much more uniform than Crown glass. As the demand for window glass increased, the cylinder size grew as well. In the 19ᵗʰ century the method was industrialized with cylinder lengths up to 12 meter (40 ft) long at even more reduced cost.
In the mid 20ᵗʰ century, a real breakthrough was made by Alastair Pilkington (UK) with his invention of Float Glass. This method continuously produces very high quality flat glass at high production speeds and very low cost. A large furnace has a production capacity of 1000 ton/day. Nowadays the standard for producing flat glass is float glass.
So, it is as clear as glass, that the older the glass pane, the more variation in thickness. As can be seen in the image below, Katrain’s huge variances in thickness indicates that it’s pre 17ᵗʰ century Crown glass.
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