  When a glassy polymer film is formed by evaporation, the region near the free
surface is polymer rich and becomes glassy first, as noticed long ago by
Scriven et al. We discuss the thickness of this "crust" and the time interval
where it is present -before freezing of the whole film. We argue that the crust
is under mechanical tension, nd should form some cracks. This may be the source
of the roughness observed on the final, dry films, when the solvent vapor
pressure is high (and leads to thin crusts).
