Some substances will only change if they are heated very fiercely. Furnaces are used mainly to change a rocky ore so that it will release its metal for us to use. For example a blast furnace is used to release iron from its ore.

The blast furnace is a steel tower about 30 metres high. It is lined with a special kind of brick that will not crumble at very high temperatures. Iron ore, coke and limestone are dropped into the top of the furnace and hot air is blasted in at the bottom. The coke burns quickly and produces a temperature of up to 2000°C. This is high enough to melt the ore and limestone and to allow the iron metal to run free. It flows out at the bottom of the furnace.