indeed, some authors have claimed that amorphous solids, such as glass and many polymers, are actually liquids with a very high viscosity (greater than 10 pa·s) and there are reported measurements to substantiate the claim. however, other authors dispute this hypothesis, claiming instead that there is some threshold for the stress, below which most solids will not flow at all (a claim disputed with experimental data), and that alleged instances of glass flow in window panes of old buildings are due to the crude manufacturing process of older eras rather than to the viscosity of glass.