The Semito-Hamitic races made excellent castings at a very early date.
The Phoenicians may be mentioned as particularly skillful. It is
reported that there were two immense bronze pillars that stood before
the temple of Gades in the 11th century before Christ. The Tyrian
founders also made a pillar for Solomon's Temple, and a metallic basin
10 ells in diameter and 5 deep. Similar large basins have been dug up
in Assyria.
The art of casting statues is no less ancient. Small statuettes were
cast solid; larger ones consisted of several pieces which were riveted
together. In the later Grecian and old Roman days the art reached
a high stage of perfection. Many cities had thousands of bronzes;
gigantic pieces were constructed. The Colossus of Rhodes was 30 meters
high and stood with outstretched legs astride the entrance to the
smaller harbor. Ships could pass through it with sails extended. A
statue of Jupiter in Tarent was 20 meters high, and one of Nero was
erected in Pliny's time, 30 meters high, costing a million dollars.[18]
These facts give us a good idea of the technical ability of the old
founders of bronze.
Analyses of antique bronzes give us some idea of their art of mixing
and coloring. We presume that they soon abandoned the use of copper
and pure bronze; the former yields porous casts and of a poor color;
the latter material was, in later times, too costly. Lead was probably
used at first for its fusibility only, but afterward it was certainly
introduced for economical reasons. This cheap material was often added
in very considerable quantity until they learned that leaden bronzes
did not have a fine color either while fresh and clean, or when old and
covered with patina.
We have also seen that zinc, as well as lead, was often added. As the
color of zinc alloys was red to light golden yellow (red metal, brass),
they tried to dispense with tin entirely, as its price was higher
than that of zinc (cadmia, as it was called). But they soon became
convinced that for fine statues, at least, a small quantity of _tin was
a necessity_. Generally a zinc-brass was used for statues.
To prevent the metallic constituents from separating during fusion, the
mass was kept thick and pasty by putting in old scrap bronze that had
been often melted and contained oxides. The smelters also knew that the
metals, particularly the tin, grew smaller every time it was melted,
in consequence of oxidation, slagging, and evaporation.[19] The Romans
therefore added, besides the scrap bronze, an eighth part of "silver
lead," _i. e._, a mixture of tin and lead.
Finally, in regard to the color of the castings, the ancients collected
valuable experiences. Cadmia (zinc ore) was used to impart a golden
color to the bronze.[20] Alloys rich in tin were used for mirrors, and
arsenic was employed to make them white.[21]
The moulds originally employed were very primitive. For simple objects
a corresponding hole was dug in the sand or clay soil. Complicated
figures had to be formed in clay, and the metal was cast in the clay
mould. If the mould was to serve for several castings, it had to be
made of baked clay, stone, brick, or other durable material. Organic
substances were mixed with the clay to prevent uneven shrinkage and
cracking.
Hollow casting is more difficult; first a core is formed corresponding
to the hollow in the figure; over this the figure is formed, and over
that the mantle, _i. e._, the negative, or mould. The latter is taken
off, the figure taken away from the core, the mantle replaced, and
the metal poured into the space between the core and the mantle. In
this case it is difficult to take off the mantle so clean and put it
back so accurately that the parts will not be disturbed. To avoid this
difficulty a wax model may be built on the core, and the mantle formed
over this, and then when the mould is dry it can be heated and the wax
melted out.
The Phoenicians and Egyptians must have used one or the other of
these devices for their hollow castings.
The Greeks appear at first as pupils and imitators of the
Phoenicians, but they soon surpassed their teachers in forms as well
as skill. They knew how to make their moulds so perfect, and were able
to place their cores so near the mantles, that the castings were as
thin as cardboard. The master founders of to-day have not reached that
perfection.