What is a whitish substance which is cast up, as a scum, from the materials of glass in fusion, and, floating on the top, is skimmed off; -- called also glass gall. [Formerly written also sandever.] called?

sandiver — A whitish substance which is cast up, as a scum, from the materials of glass in fusion, and, floating on the top, is skimmed off; -- called also glass gall. [Formerly written also sandever.]. (Webster's 1913.)