The silver dross spoken of here is an oxide of lead that was separated in the old refining process, it was used to glaze pottery. At first glance it would make the pot appear to be silver, but on closer inspection it would be found to be only earth.
Fervency in speech is attractive like silver. Zeal and charisma in speaking will draw people, but if the heart of the speaker is wicked, it will end in disillusionment and sometimes even destruction.
Hitler had fervent lips and they swayed a nation. He held before them the silver vessel of a utopia, where every wrong (real or imagined) that the German people had ever suffered would be righted. Millions of lives later, the deception wore off and those who followed him found instead that they had been responsible for the systematic extermination of millions of people. They also found that they had brought the just retribution of the world upon themselves in bombed-out cities, and for many years, a divided land.
It would help us all to remember that it is not the vigour of speech that makes a cause right, but the underlying principles on which it stands. A silver vessel is still silver, even if it has been somewhat tarnished and is black instead of shiny, and the truth remains true whether held in silence or broadcast in eloquence.
On a more personal level to those of us who use our tongues to preach the Gospel, we should remind ourselves that our eloquence in the pulpit is no substitute for goodness in our heart. Let us be fervent like a shining silver vessel, but let our hearts also be silver in goodness, that we may be a vessel fit for every good use. Unto Him be glory in the Church!