Informant38

....states that glass is a liquid. This is incorrect. Glass is a solid at normal temperature. It will not flow and experiments have been done to show this.Bubbles in obsidian, a volcanic glass, are still spherical after more than a million years in spite of the weight of overlying rocks, which exert a much larger force than gravity does on the glass alone. A shorter term experiment was performed by Fred Ernsberger of Corning Glass Works, who subjected glass rods to a constant bending stress of 150 000 pounds per square inch for 26 years. When the stress was released, there was no permanent deformation.

Glass is a liquid at temperatures above the glass transition (about 500�C for window glass) but below the transition it is a solid which is perfectly elastic up to its breaking point. Microbalances using glass springs take advantage of this property. They are extremely linear and never need to be rezeroed.Old glass window panes are usually thicker at the bottom because they were installed that way for convenience (they do not fall over easily and remain in position while they are pointed). Even so, old panes are sometimes found installed thick edge up, especially in stained glass windows where an inverted prism effect was desired.

If the panes are carefully measured they will also be found to taper from side to side. This double taper is obviously not due to gravity, which acts in only one direction, but is a product of the crown process of glass making, which created a disc of glass that tapered radially from the centre.

The crown process, which involved spinning a molten glass gob into a disc, also produced the flow lines and elongated bubbles that are seen in old glass. The window panes were cut from the disc after it had cooled. Crown glass window panes were produced in large volumes in England until the 1830s.

Ross Firestone, Winnetka Illinois