Prov. 25:4-5 expounds & applies the analogy of pure metals vs. dross. Another example in Proverbs utilizing similar picture language to Prov. 17:3. When the heart is tested by the refining fires of life, the process is designed to remove some elements and refine others.  When the heat is turned up, the difference between precious metals and dross or impurities becomes increasingly apparent.  The book of Proverbs serves as a literary version of this process.  Solomon turns up the heat of truth and when his words are taken to heart, the contrast between folly and righteousness is magnified.