In Jeremiah compared the judgment of God against His sinful people with a fire heated by a blacksmith. The fire was intensified with air from a bellows until it became intense enough to heat metal for shaping by the blacksmith’s hammer (read Isa 2;4).
The bellows of Bible times was a long bag made of animal skin that was placed on the ground in front of the fire. the blacksmith stepped on the bag to expel air into the fire, then pulled it open with a rope to fill it with air again.
JEREMIAH 6:29 – The bellows are burned [blow fiercely, NIV], the lead is consumed of the fire.