Burden materials charged to the top of the blast furnace descend slowly. The materials stay in the furnace 4-6 hr. Gases evolving in coke burning near the tuyeres flow against the descending charge at a great velocity throughout the cross section of the furnace. The heating of fusible materials depends on their distance from the hearth and permeability to gas. Decomposition of hydrates begins at 200o C and can be completed at  >= 550o C.