The art of metallurgy had made great progress during the bronze epoch. There were at that time considerable workshops for the preparation of bronze, and small foundries for melting and casting this alloy. When once formed into weapons, instruments, and tools, bronze objects were fashioned by artisans of various professions. The moulder's art had already attained to a high degree of perfection, a fact which is proved by the gigantic bronze objects which we have already mentioned, as well as the castings, so many of which have been represented in the preceding pages. The phenomenon of _tempering_ was well known, that is the principal modifications which are experienced by bronze in its cooling, whether slow or sudden. It was well known how to vary the proportions of the tin and copper so as to obtain bronze of different degrees of hardness. All the means of soldering were also familiarly known. Damascening was introduced in order to diversify the appearance of wrought metallic objects. The cutting qualities of instruments were increased by forging them and consolidating them by hammering. They had even gone so far as to discover the utility of the addition of certain mineral salts in the founder's crucible in order to facilitate the fusion of the bronze. Thus at the end of the bronze epoch the knowledge of metals had attained to a comparatively considerable development. Hence we may conclude that the substitution of iron for bronze took place without any great difficulty. Owing to the natural progress and successive improvements made in metallurgic art, the blacksmith made his appearance on the scene and took the place of the bronze-moulder. What, however, was the process which enabled our earliest metallurgists to extract iron from its native ore? Native iron, that is metallic iron in a natural state, is eminently rare; except in aërolites it is scarcely ever found. According to Pallas, the Russian naturalist, certain Siberian tribes have succeeded, with a great amount of labour, in obtaining from the aërolites which have been met with in their country small quantities of iron, which they have made into knives. The same practice existed among the Laplanders. Lastly, we are told by Amerigo Vespucci that in the fifteenth century the Indians at the mouth of the La Plata river were in the habit of making arrow-heads and other instruments with iron extracted from aërolites.[39]