Fe_{2}O_{3}.
Magnetite Fe_{3}O_{4}.
Siderite FeCO_{3}.
Limonite 2Fe_{2}O_{2}.3H_{2}O.

These ores always contain impurities, such as silica,
sulphides, and earthy materials. All ores, with the exception
of the oxides, are first roasted to expel any water and carbon
dioxide present and to convert any sulphide into oxide.

2. _Carbon._ Carbon in some form is necessary both as a fuel
and as a reducing agent. In former times wood charcoal was used
to supply the carbon, but now anthracite coal or coke is almost
universally used.

3. _Hot air._ To maintain the high temperature required for the
reduction of iron a very active combustion of fuel is
necessary. This is secured by forcing a strong blast of hot air
into the lower part of the furnace during the reduction
process.

4. _Flux._ (a) _Purpose of the flux._ All the materials which
enter the furnace must leave it again either in the form of
gases or as liquids. The iron is drawn off as the liquid metal
after its reduction. To secure the removal of the earthy matter
charged into the furnace along with the ore, materials are
added to the charge which will, at the high temperature of the
furnace, combine with the impurities in the ore, forming a
liquid. The material added for this purpose is called the
_flux_; the liquid produced from the flux and the ore is called
_slag_.

(b) _Function of the slag._ While the main purpose of adding
flux to the charge is to remove from the furnace in the form of
liquid slag the impurities originally present in the ore, the
slag thus produced serves several other functions. It keeps the
contents of the furnace in a state of fusion, thus preventing
clogging, and makes it possible for the small globules of iron
to run together with greater ease into one large liquid mass.

(c) _Character of the slag._ The slag is really a kind of
readily fusible glass, being essentially a calcium-aluminium
silicate. The ore usually contains silica and some aluminium
compounds, so that limestone (which also contains some silica
and aluminium) is added to furnish the calcium required for the
slag. If the ore and the limestone do not contain a sufficient
amount of silica and aluminium for the formation of the slag,
these ing