Zohar Ki Tisa
While Moses was around, the negative side of the Erev Rav was subdued. Now that he was away on the mountain for forty days, they felt that it was time to raise the power of the other-side. They wanted to be an integral part of the Israelites or have their own God leading them as God leads them.
Aaron thought that mixing the Erev Rav with the holy children of Israel if not acceptable. He chose to keep them away until Moses comes back.
Aaron intention was pure, but many from the Israelites were weak in heart and followed the Erev Rav. Aaron allowed them to create the Golden Calf to stay far from the Israelites. When Moses came back and saw them celebrating their new God, it angered Moses. He broke the tablets, and the Erev Rav run away from him (Ramban on Exodus 32). He took the Golden Calf, burned it and ground it to powder. Then he spread the dust over water and made the Israelites drink this water to find out those with a corrupted consciousness. He had to make this process of selection to remove the evil from spreading in the camps of the children of Israel.
The process of Moses use is similar to the method used by the priest to find out if a woman was unfaithful to her husband (check links below). Moses transformed the golden idol to thin powder that doesn’t have form or energy due to the process of fire and grinding. They were humiliated when they see the ‘Golden Calf, comes out with their body waste.
Another aspect of the process is to check if the corrupted consciousness gives energy back to the ‘consumed’ idol. Those with strong faith could overcome the idol’s force in their body. If the ‘immune’ system is active, the person is cleared from the sin, and if not, It would then be shown on the body.