This paper describes methods of calculation, which give the conditions to be fulfilled in order to tap the same quality of iron and have a smooth running furnace when a new matericd is injected into the blast. These calculations also show the effects of the injection upon the regularity of the iron, the production of the furnace, the coke rate, and the quality of gas produced. Included in the paper are the results of two industrial campaigns: one with natural gas and the other with light fuel oil.