Some scumd the drosse, that from the metall came; / Some stird the molten owre with ladles great[…].
To become covered with scum.
1769, Elizabeth Raffald, The Experienced English House-keeper, pp.321-322:
Take the smallest Cucumbers you can get, and as free from Spots as possible, put them into a strong Salt and Water for nine or ten Days, or 'till they are quite Yellow, and stir them twice a Day at least, or they will scum over, and grow soft
SOo by Merlyns aduys ther were sente fore rydars to skumme the Countreye / & they mette with the fore rydars of the north / and made hem to telle whiche wey the hooste cam / and thenne they told it to Arthur / and by kyng Ban and Bors counceill they lete brenne and destroye alle the contrey afore them there they shold ryde
Wandering up and down without certain seat, they lived by scumming those seas and shores as pirates.