“Therefore thus says the Lord Yahweh, Because you are all become dross, behold therefore I will gather you into the midst of Jerusalem, as they gather silver and brass and iron and lead and tin into the midst of the furnace, to blow the fire on it to melt it. So will I gather you in my anger and in my fury, and I will lay you there and melt you.”
Ezekiel as a non-expert had no doubt witnessed the pouring of all these metals in their impure form into the furnace where the bellows blew them to white heat and they melted, and there they turned into purified metal and into dross. God showed him in it a picture of His wrath revealed against Jerusalem/Judah removing anything pure and leaving them as dross.
The gathering of them all ‘into the midst of Jerusalem' was itself a vivid picture. As the invading armies drew nearer the defenceless people would stream into Jerusalem to take advantage of her strong walls. But they would not realise, until Ezekiel revealed it, that they were being gathered into a furnace where they would be melted down and revealed as dross in the light of God's anger.