|MadSci Network: Physics|
The term 'glass' is normally applied to silicate-based materials which are non-crystalline solids. However, 'glass' can be considered to be any material, including metals, which has solidified and become a rigid substance, with a non-regular crystal structure.
You are right in casting some doubt over the origin in the difference between the thicknesses at the top and bottom of a window. Glass has a very high viscosity.Viscosity is the resistance of a liquid to deformation by shear forces. Viscosity is measured in poise. The greater the number of poise, the less the 'flow' of the liquid under a shear stress. Water has a very low viscosity, about 0.01 poise, and thick treacle is about 100 poise.The viscosity of glass depends on how easily the molecular units (SiO4 tetrahedra) can move past one another.
The actual value of viscosity is a function of temperature. The greater the temperature, the lower the viscosity. Ordinary window glass has a viscosity of at least 10^28 poise at normal everyday temperatures. Calculations can show that this results in a viscous deformation, as a result of its own weight, of just a few nanometers (1 nm = 10^-9 m) every century (not very much..)! The explanation for the wedge shape effect in old windows can be attributed to the way which the glass was made. The glass was made by spinning a disc of glass. This method results in an increasing thickness from the centre to the rim. When it cooled, the disc was cut up into sections. The thicker edges were placed downwards, for purposes of stability.
Jim Intrater adds: Glass is a solid. Specifically it is a viscoelastic solid. It does not flow over time unless a critical (and usually sizeable) force is impinged on it. Two reasons why glass got the false label of being a liquid is because it has no crystalline structure and because of observations of old glass window panes. Actually, glasses have a crystalline diffraction pattern, it is merely very diffuse compared to a crystalline solid. Even if it's atoms were completely randomly oriented, it would still be considered a solid. [In that case it would be called an "amorphous" solid. Correct reference to a glass would be to call it "vitreous".] In old buildings, glass panes are found to be thicker at the bottom than at the top. Further, they tend to have vertical striations on their faces. However, this is a classic case of misinterpretation of observations. The reason why glass is thicker at the bottom in old buildings is not because it flowed over the years. The manner in which glass used to be made was imprecise. There would tend to be a thicker end to a piece. This would correspond to the region of the cut pane that was located closest to the center of a cast glass sheet. Builders installing glass windows knew this. They would gauge the pane and orient the thicker end in its most stable position (at the bottom). The reason why there are vertical striations on glass is that rain water running down the side of a building would become slightly alkaline through reaction with the brick. The vertical flow of the rain water would produce a gradual vertical etch on the glass. This would appear only on the outer face of the glass pane. In those cases where one sees glass structures sagging (e. g., telescope mirrors) they are nonetheless solid onjects exhibiting simple stress relaxation and compressive deformation from their own weigh. -Jim Intrater Ceramic Engineer
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