Charles Ellicott Commentary Exodus 32:20

"And he took the calf which they had made, and burnt it with fire, and ground it to powder, and strewed it upon the water, and made the children of Israel drink of it." — Exodus 32:20 (ASV)

He took the calf. —To suppress the idolatry, the first step was to destroy the idol. Moses, who must have rallied to his side at once a certain number of the people, seized the calf and ordered its immediate destruction. He subjected it to fire, by which its form was destroyed, and the material, it seems, was calcined. This calcined material he reduced to a fine powder by rubbing or pounding, and then had the powder sprinkled on the surface of the stream which supplied the camp with water, so that the people might seem, at any rate, to swallow their own sin. Compare the action of Josiah (2 Kings 23:6; 2 Kings 23:12). No doubt, the process of destruction took some time. It is not meant that it was completed, but only that it was begun, before Moses turned to other matters.