I´ve seen it asserted at the general physics forum that melting point of amorphous substance is "always" lower than the "melting point" of corresponding crystal. Also, amorphous solids are often described as metastable against crystallization. Does it necessarily apply to all amorphous solids? For the viscosity of liquids is found in a broad range. Among simple common substances - water conveniently has viscosity of 1 cP at 20 C (less than 1.01). The viscosity rises on cooling - to less than 1,8 cP at 0 C. Water easily freezes. At boiling point of 100 Celsius, viscosity of water is 0,28 cP. Glycerine has freezing point of 18 Celsius, and freezing point viscosity of around 1700 cP. And glycerin readily supercools. On heating, viscosity of glycerin drops a lot. While heating glycerin is nasty (tends to decay to acrolein), the boiling point can be measured at 290 Celsius. Could not find viscosity measured or estimated at that region, but already at 170 Celsius, it is under 3 cP. Molten silica comes to true melting equilibrium with crystal - cristobalite - around 1710 Celsius. And at that region, the viscosity of the melt is about 3*109 cP. On heating, it falls. But hot molten silica is too hot to handle - it tends to attack solid vessel materials. Its boiling point is hard to measure, but estimated at 2700...2800 Celsius (cannot be measured better). Its viscosity under those conditions? Still around 50 000 cP. As you see, freezing point viscosity is found in a wide range. But beyond the range of these three? Any liquids with yet bigger freezing point viscosity? On cooling liquids, while viscosity changes continuously, in the range of 1015 cP some other features like thermal expansion and heat capacity undergo a rapid change in a narrow range (how narrow, in terms of viscosity?). It´s called glass transition. Are there any substances for which true thermodynamic equilibrium between ordered crystal and disordered phase does exist, but at conditions where the viscosity of the disordered phase is on the amorphous solid side of the glass transition? And therefore, a true thermodynamic equilibrium amorphous solid exists?