In spite of their fundamental importance in geophysics, the strong effects of water on the physical properties of silicate glasses and melts have long remained poorly understood because of the difficulties of making measurements at the kbar pressures under which water solubility is significant. Due to the slowness of water exsolution close to the glass transition, it is in fact possible to perform accurate measurements on water-bearing liquids supercooled at room pressure. Only hydration has, then, to be performed at high pressure. The purpose of this review is to summarise the wealth of information gathered during the past decade with this new procedure. Much effort has been devoted to viscosity, a property for which the depressing effects of water clearly increases with the degree of polymerization of the melt.
