54 METALLURGY OF CAST IRON. the iron and take much of the sulphur itself. If the furnace is working cold so as not to properly fuse the limestone, then the iron will absorb and retain higher sulphur; and hence the greater sulphur found in the iron coming from a coldworking furnace, which often results in giving a hard or *' white iron.'' The way high silicon and low sulphur iron, or No. i pig iron, is generally obtained is by having a hot furnace, well but not excessively fluxed with lime. To make high silicon and high sulphur iron, as is often obtained, it is necessary to have a hot furnace poorly fluxed with lime. A cold furnace gives a thick, bad slag, the same as a cold cupola retards good fluxing or slagging out. A good working furnace sends the most silicon into the pig and sulphur into the slag; a poor working furnace reverses these conditions. it is not thoroughly extracted from the ore and non-metallic matter. This is generally due to an insufficient amount of fuel, or decrease in temperature from other causes. Moreover, too small an amount of silicon is reduced at the same time from the fuel and ore, and consequently the iron obtained is smaller in amount and silicon contents and richer in sulphur. "The furnace is working cold, or "off," and a greater per cent of fuel may make it work better.