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.