‘The words of YHWH are pure words,
As silver tried in a furnace on the earth,
Purified seven times.
In contrast to the deceit and falsehood of men the words of YHWH are true and pure, and totally to be relied on. They are like silver which has been refined, yes refined ‘seven times' (totally and completely), as silver on earth needs to be. But the words of YHWH are so pure that they do not need such a refining process. They are already purer than any silver on earth, even though they are practical and effective on earth. Thus we can always rely on His word to see us through any situation. It has survived through the centuries, and will continue to do so. And it brings home truth to the heart.
There may also be the thought here that the words of YHWH themselves have such a purifying effect, making those who receive them pure.