A fine gray scum floats on its surface.

It was cold, had a grey scum around the rim and was utterly tasteless.

This was a brown, evil-looking brew, topped by grey scum, with the taste and texture of fine silt.

Beneath it, a large woman was standing bare-armed by a cauldron in which uncertain things bubbled in grey scum.

As the gray scum starts to rise, skim it from the surface of the water.

The Daphne was afloat on a bed of gray and brown scum that made the sea look like a cesspool.

There beckoned a gray scum of marine organisms.

The springs are covered with gray scum.

Even now he could hear the thrumming of the furnaces of the great undercity, no doubt teeming with the gray scum.

What are we gonna do with this gray scum, anyway?