You are right of course in all you say, this particular glass was called smalt.
Evan, Mani,
Smalt must be coarsely ground to function well as a blue pigment. The more finely it is ground, the more of its color it loses, and the less tinting strength there is. In mixtures, finely ground smalt would not have much influence on the color, thus it would function as a colorless pigment that aids drying. It seems Rembrandt used the ground glass (I don't recall that it was positively identified as smalt, but that would be a reasonable assumption) to add physical substance to glaze passages as well as to his thick impastos.
Virgil Elliott
