In the meantime the evolution of artillery was taking a new direction. The large and relatively feeble ordnance of the Turks was, in the circumstances, not entirely unsuitable for the purpose for which it was intended: the smashing of masonry and the breaching of gates and walls. The maximum of effect was obtained from a missile of enormous mass projected with a low velocity. Nevertheless its disadvantages were obvious. Large cannon cast in bronze were necessarily of great expense and weight, their discharges were few and far between, they wore rapidly and were thus short-lived, and they possessed the dangerous property of becoming brittle when heated. An increase in power and a reduction in weight were required for the achievement of a portable artillery, and the progress of mechanical science pointed to wrought iron as the material of which such an artillery might be made. The extraction of iron in small quantities from ferruginous ore was a comparatively simple operation, even in primitive times. With the aid of bellows and a plentiful supply of wood charcoal the smith was able to make his furnace yield small masses of metallic iron of the purest quality. This iron, wrought on an anvil, could be drawn out into plate or bar as desired, the resulting metal being, by reason of the purity of the charcoal used in its extraction, of great toughness, homogeneity, and strength. In Spain and Italy were mines which had long been famed for their iron. In England the Roman had made good use of the metal found in the Sussex mines, and all through the middle ages the wealds of Kent and Sussex were the centres of the English iron trade. In the fourteenth century improved methods came into use; the adoption of water-power for driving the bellows, for crushing the charcoal, and for operating the tilt-hammers, had its effect on the development of the iron-smelting industry; higher temperatures obtained and larger masses of ore could now be treated; the iron, produced in larger quantities by improved methods, was perhaps purer and stronger than before. In wrought iron, then, a material was available which almost alone was suitable for the manufacture of the more portable sorts of gun. By its use guns could be made strong enough, without being of an excessive weight, to withstand the increasing stresses thrown on them, first, by the use of iron bullets instead of stone, and secondly, by the discovery of an improved gunpowder. Artillery underwent a dual development. On the one hand, for use with the weak cannon powder, was the large stone-throwing ordnance, made of cast bronze or of hooped bars of iron; on the other, for use with iron shot and a stronger propellant, were various denominations of small portable and semi-portable wrought-iron guns. These two distinct types developed side by side until the middle of the sixteenth century.