Charles H. Spurgeon on Psalms 17:3

"Thou hast visited me in the night," etc. In the night the soul is free from business with the world, and therefore freest for business with God; and then did God prove and visit David, that is, examine and sift him, by calling to his mind all his ways and works in former passages; and the issue of this trial was, he found nothing; not that his soul was empty of good things, or that there was nothing evil in him; but God, upon examination, found nothing of that evil in him which some men suspected him of; namely either any ill will or evil design against Saul, in reference to whom he called his cause a righteous cause, or "the right" (verse 1); "Hear the right, O Lord." —Joseph Caryl.

Verse 3 (third clause, New Translation.) "Thou hast smelted me, and found in me no dross." A metaphor taken from the smelting of metals to purify them from extraneous matter. —Geddes.