All wrought iron, after fusion, or after having been exposed to high temperatures sufficient to induce softening or pastiness, which is the case when iron is reheated to a white heat, consists of an aggregation of crystals of a cubical form.

In the act of rolling, these crystals are elongated into fibres, which form the mass of all good wrought iron.

Some authorities consider that when bar iron is subjected to continued vibration, constantly repeated loads, shocks, or blows, its structure becomes altered, and that it returns to a crystalline condition. On this point, however, there is considerable doubt (see p. 332).

The chemical constitution of the iron as well as its mechanical structure is altered during the process of rolling. When heated the surface is exposed to the oxidising influence of the atmosphere, the amount of carbon is considerably reduced, and a large proportion of other impurities may be got rid of.

Some experiments made at Woolwich on Bessemer wrought iron showed that this iron, when fused and run into a mould, had a tensile strength of 18.412 tons per square inch, but when the same iron was rolled its tensile strength became 32.4 tons per square inch, by which it appears that the operation of rolling has the effect of nearly doubling the strength of the iron.

The effect of rolling is illustrated also by the example given at page 333.

Iron, however, will not bear to be rolled too often, for it appears from Sir W. Fairbairn's experiments that it gains strength only up to the fifth reheating, and then its strength begins to fall off.

Professor Rankine says - "Good bar iron has in general attained its maximum strength, and the desired size and figure should be given to it with the least possible amount of reheating and working."