A furnace - The word in the original is Egyptian, and occurs only in the Pentateuch.

And Mount Sinai was altogether on a smoke,.... Not from nature, as volcanos, but for a reason after given; it seemed to be one large body of smoke, nothing else to be seen but smoke; an emblem of the darkness of the legal dispensation, which was full of obscure types and figures, of dark shadows and smoky sacrifices, to which the clear day, of the Gospel dispensation is opposed, see 2 Corinthians 3:12.

because the Lord descended upon it in fire; in flaming fire, as the Targums, which set the mountain on fire, and caused this prodigious smoke; for if he, who is a consuming fire, but toucheth the hills and mountains, they smoke, Psalm 104:32.

and the smoke thereof ascended as the smoke of a furnace such an one as that which Abraham in vision saw, Genesis 15:17.

and the whole mount quaked greatly; to which circumstance Deborah refers in her song, when she speaks of mountains melting and flowing from before the presence of the God of Israel, and particularly of Sinai, Judges 5:4, and the psalmist, who makes mention of the earth shaking, and the heavens dropping, and of Sinai being moved at his presence, Psalm 68:8, it is probable there was an earthquake at this time, which sometimes attends thunders and lightnings, see Revelation 16:18.

And mount Sinai was altogether on a smoke, because the LORD descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount {g} quaked greatly.

(g) God used these fearful signs, that his law would be held in greater reverence, and his majesty even more feared.

19:16-25 Never was there such a sermon preached, before or since, as this which was preached to the church in the wilderness. It might be supposed that the terrors would have checked presumption and curiosity in the people; but the hard heart of an unawakened sinner can trifle with the most terrible threatenings and judgments. In drawing near to God, we must never forget his holiness and greatness, nor our own meanness and pollution. We cannot stand in judgment before him according to his righteous law. The convinced transgressor asks, What must I do to be saved? and he hears the voice, Believe in the Lord Jesus Christ, and thou shalt be saved. The Holy Ghost, who made the law to convince of sin, now takes of the things of Christ, and shows them to us. In the gospel we read, Christ hath redeemed us from the curse of the law, being made a curse for us. We have redemption through his blood, even the forgiveness of sins. Through him we are justified from all things, from which we could not be justified by the law of Moses. But the Divine law is binding as a rule of life. The Son of God came down from heaven, and suffered poverty, shame, agony, and death, not only to redeem us from its curse, but to bind us more closely to keep its commands.