HARD BRONZES OF THE ANCIENTS.
We have already seen that only very pure bronze is suitable for weapons and tools. It must be well "cooked," and all sulphur, lead, and tin must be completely removed by oxidation. The best results are obtained with from 8 to 12 per cent. of tin. A bronze having this composition is tenacious and has a hardness of at least 4.
But the ancients were able to make much harder wrought bronzes, as proved by our collections of weapons and tools.
Unfortunately we have no record of the devices employed; but as we are able to make just such products and with simple means, we may assume that the ancients employed essentially the same methods. In our experience the following conditions are essential for the manufacture of hard bronze:
1. A particular treatment.
2. A small amount of phosphorus.
It is well known that normal weapon bronze, unlike iron, is softened by rapid cooling, but is hardened by hammering and rendered more compact.[22]
By repeating this process, the bronze gains in hardness and strength, and sheet bronze becomes lamellar by hammering or rolling, and hence acquires a certain elasticity.[23] Besides, a slight admixture of iron or nickel seems advantageous, but a slight amount of phosphorus is of the highest importance. The latter point may be somewhat enlarged on.
Ordinary bronze always contains oxides of copper and tin, the quantity increasing with the number of times it is recast. This oxide makes it pasty, so that the different constituents do not separate, and the casting is homogeneous.[24] This admixture of oxide does no harm for castings in which strength is not demanded; but is of importance for weapon bronze; the strength of which is considerably diminished by the presence of the oxide.
In this respect a slight amount of phosphorus is an advantage by preventing the formation of oxides, and consequently the mixture remains a thin fluid until it begins to solidify. On the other hand the metals are liable to separate. This evil can be avoided if the alloy is allowed to cool nearly to solidification before casting, and then cooled rapidly. Under these circumstances a homogeneous alloy is obtained that is nearly fifty per cent. stronger and about 200 per cent. more tenacious than bronze that contains oxides. The hardness and strength can be still further increased by chilling and hammering.
Besides the indirect influence of phosphorus, it also has the direct effect of hardening the bronze, because the compounds of phosphorus with copper and tin have a very considerable hardness. These facts, as well as the circumstance that we possess antique bronzes of extraordinary hardness, induced me, with the consent of Baron Sacken, to test the hardness of the bronze weapons in the Vienna Cabinet of Antiquities. Some hard pieces,[25] were sent to Prof. Ludwig, who followed the question with interest and agreed on the method of making the analyses. The results were satisfactory. The bronzes contained traces and up to one-fourth per cent. of phosphorus. Its presence had prevented the formation of oxides in these bronzes, and consequently the weapons were of extraordinary hardness. It now remains to ascertain how the ancients made these phosphorus-bronzes. It is evident that the phosphorus was not put directly into the metal, as is generally done at present. There is another method so simple that we can assume that the ancients employed it unintentionally. I refer to smelting the copper or bronze with charcoal and any salt of phosphorus. In this case the carbon would liberate phosphorus from the phosphoric acid, and it would be taken up by the melted metal.
The ancient metallurgists may have made use of the eruptive rocks that contain apatite, and with which copper ores are so often associated, for slag or flux, or the phosphates that occur in the gangue may have been smelted along with the ores; in both cases some phosphorus would get into the metal. Finally it is not impossible that the ancients did not put in phosphorus salts in some form. First of all I would mention certain vegetable and animal substances that are rich in phosphorus, especially blood,[26] which was a favorite with the old metallurgists and alchemists as having a powerful enchantment. In each of the cases referred to some phosphorus got into the metal, which thus acquired a considerable hardness that could be increased in the well known manner by chilling and hammering. Under certain circumstances weapons and tools were made almost as hard as steel.
We can easily comprehend how bronze with these excellent qualities could compete with steel at a time when rich ores were still abundant, and thus it checked and restrained the development of the iron industry.