• (p. pr. & vb. n.) of Scum • (n.) That which is scummed off; skimmings; scum; -- used chiefly in the plural. • (n.) The act of taking off scum.
Printing problem that occurs when the non-image area of a plate accepts ink in random areas. 
Scum'ming noun (a) The act of taking off scum. (b) That which is scummed off; skimmings; scum; -- used chiefly in the plural. 
The formation of dull scum on the glaze surface, caused by gases present in the kiln atmosphere, or sulphates present in the body or in the kiln gases.