Converting iron to steel is a two-step process - first, pig iron must be manufactured, then the pig iron combined with raw iron to steel. Both the production of pig iron and that of steel take one carbon and one fuel (and a flux stone).
Thus, each bar of steel consumes one coal as reagent when working with magma, two bars as reagent and fuel (for every single bar of steel) when working without magma. One bituminous coal converts to nine coal with magma, to eight (effectively) when fuelled with coal; wood produces one charcoal per log.
I corrected the amounts of fuel, which were listed at barely half the actual requirements, but i'm not convinced writing out the info that's already clearly laid out in the paragraph above is useful.--Larix (talk) 17:12, 5 November 2015 (UTC)
"To go from raw materials to finished products (assuming 1 bar per crafted item) at a conventional forge, a ratio of 8 iron bearing ore: 32 flux stone: 9 bituminous coal (or 18 lignite or 72 logs) will have no leftover raw material and will yield 32 single-bar steel items."
9 bit. coal = 81 coke -9 (firing bit. coal -- fuel) -8 (smelting ore -- fuel) -32 (making 16 pig iron -- reagent *and* fuel) -32 (making 32 steel -- reagent *and* fuel) -32 (making 32 steel products -- fuel) ----- -32 (coke shortage)