3 Words to use with scum

If the fire is regular, it will not be necessary to add cold water in order to make the scum rise; but if the fire is too large at first, it will then be necessary to do so. VI.

The milk passes through slowly, and care must be taken not to let scum form on the milk.

Above them the white scum wall of the water-fall came tumbling down, and round about the river rushed along in a mighty torrent.

3 Words to use with  scum