Sentence:  A light brown sand, of an exceeding fine grain, almost impalpable, from the river Sullane, near New-bridge, where it is found in plenty. It <WSD>scours</WSD> brass without scratching, cleans silver, and is excellent for casting the finer works, both of the brazier and silversmith. 
Meaning of the word marked with <WSD> tags:  To clean, polish, or wash something by rubbing and scrubbing it vigorously, frequently with an abrasive or cleaning agent.