Glass is sometimes called a supercooled liquid because it does not form a crystalline structure, but instead forms an amorphous solid that allows molecules in the material to continue to move. However, Scientific American indicates that amorphous solids are neither supercooled liquids nor solids.
True solids form crystalline structures that lock molecules into place. They retain their shape unless the temperature increases to above their melting point. However, glass, amber and plastic do not form such solids when their liquid form cools. While glass is more rigid than liquid, it is not as ordered as crystalline solids.
High school chemistry teachers and tour guides often erroneously point to European cathedral windows as evidence that glass remains a supercooled liquid. The glass in these windows is thicker at the bottom than at the top. However, Scientific American refutes these claims due to the slow-moving nature of glass molecules and Egyptian glass that is older but has the same thickness at the top and the bottom. The reason for the melted appearance of cathedral windows is likely due to the European method of glass blowing, which included blowing cylinders and flattening them. For an unknown reason, the artisans building the windows preferred to install the heavier glass at the bottom of the window.