Storybook Homes

There is much debate about whether or not glass is actually a solid or a liquid. There have been many claims, especially by tour guides of very old churches, that glass is a liquid because there is an apparent “sagging” or “glass flow” in the glass at the bottom of each pane or piece of glass, and this has become a persistent myth. After much research, no real evidence exists that glass actually flows over time, and therefore we can favor the fact that glass is nearer by definition to be a solid, albeit a very special one at that, which has a molecular arrangement not exactly like a crystal (solid) but even less like that of a pure liquid (could glass be in a class by itself?).

The glass flow or sagging of old glass panes from the Medieval times has more to do with the process of making the glass itself. In those times, the molten glass was rolled,expanded, blown, and then spun into a disc before being cut into pieces. Remnants of the evidence of the discs can be sometimes seen in old glass panes which retained the center point with obvious rings or ripples concentrically extending outward from this center point. I have seen actual center pieces of glass from these old large discs most often in the panes of old doors, they look like unfinished magnifying lenses if you look close, and their costs to buy were less because of these imperfections.

In these older processes of making glass, the glass sheets were thicker towards the edges of the disc and were usually installed with the heavier side to the bottom, thus creating the appearance that the glass is flowing or sagging. Modern glass production called “float glass production” creates even thicknesses and have thus eliminated the the un-evenness like those of the older glass processes. Other interesting factors to consider in glass making are the rate at which the glass cools during the manufacturing process along with the additives such as lime, soda, calcium oxides, gold, lead, etc., which all play a part as to hardness and color of the glass. If you are an inquiring mind and need to know in detail more about the process and chemical structures and make up of glass, there are lots of good references on the internet, simply search “Making Glass”.

In conclusion, it is very difficult to verify with absolute certainty that glass does not flow, however, there seems to be more evidence that it does not flow than it does flow. There are some great examples of very old glass in the British Museum, from Roman times around the 1st century AD, that have shown no apparent signs of glass flow. Some of the oldest known stained glass windows remain in place from the 12th century, which are found in the Augsburg Cathedral in Germany and date between 1050 and 1150 and show no real signs of sagging. Other great examples exist in England and France dating back to the 13th century and of those which have been examined, have shown no verifiable evidence to glass flow or sagging. Old glass, and all glass by that matter, by our view then, should be considered a solid and not a liquid (at least for now).