Part 10

We have already seen that only certain metals are found native, while the hard metals under normal conditions remain in the form of oxide or mineral. They have a strong affinity for the oxygen of the air, and can only be separated and converted into metals by powerful chemical agents. There is _one_ substance which has a still more powerful attraction for oxygen than those metals. This is ignited carbon, which, in its fight with the metallic oxides, robs them of their oxygen.

Carbon has been separated from the carbonic acid of the air by the life-giving force of the sun, and vegetable life dependent upon it. But the isolated element waits impatiently for the impulse that will enable it to unite with the vital air under flame and heat. Men that know how to utilize this process of nature possess the means of resurrecting those metallic treasures which, without its powerful assistance, would remain forever hidden from their eyes. But accident, as we have said, pointed out the way.

In numerous places visited by primeval man, as hunter and fisherman, and afterward as nomad, conflagrations broke out. Not unfrequently whole forests were burned, either intentionally or not. It could not be otherwise than that the earth's surface would get red hot in such places, and if a strong wind favored it, this would suffice to open these treasures. The glowing charcoal would rob the ores of their oxygen and leave the pure metal as melted drops or cakes.[17] Copper, tin, and iron ores could have been reduced in this way; mankind not only knew the result but also the method of reducing metals.

This process took place not once merely, but thousands of times in various parts of the earth, and thus, in my opinion, metallurgy may have become known to different races of people and at different times.

A simple trench in the ground, in which a heap of glowing coals and some pieces of ore could be subjected to a strong draught of air, suffices, under favorable circumstances, for the preparation of the metal; the oldest metallurgists had scarcely any more complete means at hand for their work.

In such primitive furnaces the well known and soft metals would naturally be worked first, and afterward copper, tin, and iron would be obtained from their ores. A variety of substances that occur together in nature would be smelted together in mixtures, and different metals would naturally be mixed and a great variety of products obtained.

CHARACTERISTICS OF COPPER ALLOYS.

The oldest civilized races used bronze for a long space of time as their chief useful metal, although some neighboring races understood the metallurgy of iron. These facts, which are in glaring contradiction to the present condition of things, require some explanation.

First it must be mentioned that _iron_ frequently contains injurious contaminations, sulphur, phosphorus, etc., and that it must have been very difficult for these primitive metallurgists to remove these contaminations, and to introduce the proper quantity of carbon into the iron. We must also consider that even a good, pure steel would be a useless product unless it was worked by a skillful and experienced smith. Finally, iron is much more rapidly destroyed by oxidation than bronze. These negative considerations certainly favored the rule of bronze for a long time.

The following facts must be fixed in mind regarding the manufacture of bronze in olden times:

1. In many districts copper and tin ores are found near together (as in Cornwall), so that under these circumstances bronze could have been obtained by smelting both at once, and together.

2. In olden times only the upper horizon of copper deposits were worked in all districts. In these, as we know, the ores are mostly oxides (with native copper). Such ores are easily worked and yield largely.

3. In regard to the mixing of metals, the metallurgists everywhere must have soon learned by experience that the metal remained soft and red when too little tin was added, while too much tin made it light colored and lustrous, but, at the same time, very brittle. Hence, we find that among all peoples the alloys used for weapons contain from 6 to 16, or, more closely, 8 to 12 per cent. of tin. These mixtures have been found to do the best.

4. Bronzes, as we shall see below, by slight admixtures and certain treatment, can be made so tough and hard that they will compare with moderately hard steel.

So we see: The metal was useful, and there was an excess of rich and easily worked ores. Under such conditions, of course, the age of bronze would flourish a long time.

Zinc ores frequently occur on copper beds, and yet zinc is rarely found in quantity worth mentioning in the bronzes of the ancients. There are two reasons for this:

1. Near the surface of the earth zinc occurs as calamine (silicate of zinc), which is a gray, unattractive, earthy looking mineral, not heavy enough to be taken for a metallic ore, and would naturally be thrown away and not put in the furnace.

2. If some zinc ore did get into the furnace, part of it would be volatilized and part oxidized by subsequent smelting.

In later times, however, we find zinc ores used a good deal. We can distinguish three types of zinc alloys:

1. Copper with 10 to 20 per cent. zinc produces a red metal, red brass, which is similar to bronze that is poor in tin.

2. Copper with 20 or 30 (and even 40) per cent. of zinc, gives a yellow metal (yellow or ordinary brass), which has more of a golden color than bronze with much tin, but quite brittle.

3. Statuary metal, which is made of copper with quite a good deal of zinc and little tin (often lead) can be called brass containing tin.

All three types may be used for casting (ornaments, statues, and coin), but are not useful for tools or weapons, because they have not sufficient strength.

After discussing the natural association of ores, and the most important alloys of copper, we will turn to the analyses of antique alloys. I have found it necessary to divide them into two groups:

1. Alloys from which the weapons and tools were _forged_. These are pure and genuine bronzes. I shall designate them as malleable metals or weapon bronzes.

2. Alloys from which ornaments, vessels, statues, and coin were _cast_. Some of these contain lead, some zinc, and some are varieties of our brass. I shall designate these as cast metals or ornamental alloys. Those substances present in some quantity were evidently put in _intentionally_, and I have classed them as admixtures, while the unintentional ones in small quantities I have designated as impurities.

I.--WEAPON BRONZES. _________________________________________________________________ | | | Country. | Essential | Admixtures. |Impurities. | constituents. | | __________|___________________|______________________|___________ | | | Egypt |Copper+ 6 to 14 tin| .. |Iron. Assyria | " +10 to 14 " | .. | .. Greece | " +10 to 12 " | .. |Fe. Ni. Co. Italy | " +11 to 16 " |Lead and Tin. |Ni. Fe. Gaul | " + 2 to 15 " | .. | .. Britain | " + 7 to 14 " |1 to 3 per. ct. lead. |Iron Alps | " + 8 to 12 " |Trace to 1 p. c. lead.|Fe. Ni. Bohemia | " + 5 to 11 " | .. |Fe. S. N. Germany| " + 8 to 16 " | .. |Nickel. Denmark | " + 6 to 12 " |To 1 p. c. zinc. |Ni. Co. Russia | " + 9 to 16 " |Lead |Ni. __________|___________________|______________________|___________

II.--CAST METAL FOR ORNAMENTS. _________________________________________________________________ | | | Country. | Essential | Admixtures. |Impurities. | constituents. | | __________|___________________|______________________|___________ | | | Egypt |Copper+ 4 to 11 tin|7 to 17 lead. |Traces Assyria | " +10 to 14 " | .. |Pb. Fe. Ni. Greece | " + 6 to 12 " |Lead. |Fe. Ni. Italy | " + 1 to 7 " |Zinc, lead. |Fe. Ni. Gaul | " + 5 to 15 " |Lead | .. Britain | " + 5 to 15 " |2 p. c. lead. |Nickel. Alps | " + 4 to 12 " |Zinc. |Pb. Fe. Ni. Bohemia | " + 4 to 11 " |Lead. | .. N. Germany| " + 6 to 17 " |Pb. rarely zn. |Ni. Denmark | " + 5 to 12 " |1 p. c. zn. |Fe. Ni. Co. Russia | " + 7 to 16 " |Pb. zn. |Ni. __________|___________________|______________________|___________

The following general statements are based upon these tables:

We see that the peoples named forged their weapons and tools from very different alloys; pure copper at one extreme, bronze with 20 per cent. tin at the other. Experience had everywhere taught them that copper and bronzes poor in tin are too soft, while bronzes with an excess of tin could not be used for weapons and tools on account of being too brittle.

They had also learned that lead and zinc considerably lessened the strength and tenacity of weapon bronze, while small quantities of iron, nickel, and cobalt are, at least, not injurious. So all races, although we can prove that they tried very different mixtures, finally adopted very simple and tolerably constant alloys. The bronze weapons of all countries frequently contain from 6 to 16 per cent. of tin, but usually between 8 and 12, with slight contamination of iron and nickel. Few nations have allowed lead to be used, fewer yet some zinc.

For casting, the oldest races used the same kind of bronze as for weapons and tools. In many cases a few per cent. of lead were added to make the casting easier. The Romans used zinc in addition to lead in large quantity as a constituent of their alloys, and they made old bronze, bronze-brass, and brass. Afterward many nations of middle Europe used zinc alloys.

Small quantities of iron, nickel, and cobalt are found for well known reasons in nearly all bronzes as harmless impurities.

Traces of _sulphur_ are also found in them. This injures the quality of the alloy, and discloses the fact that such bronzes were not made from pure oxide ores, but from those containing sulphur pyrites. At the time when such bronzes were produced the mines had probably reached a considerable depth.

Some of the weapon bronzes made by the ancients contain traces of _phosphorus_, an element as important in hard bronze as carbon is in steel.

CASTING THE ALLOYS.

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.

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.

SUMMARY OF ALLOYS USED BY THE ANCIENTS.

_Egypt_.--The wrought metal of the Egyptians is a pure bronze with 6 to 14 per cent. of tin; 22 per cent. is an exceptional case; 1 per cent. of iron is not rare.

The Egyptian cast metal is a plumbiferous bronze, with 4 to 11 per cent. tin, and 7 to 12 of lead; in one case 16 per cent. tin; rarely 2 or 3 per cent. of zinc.

_Assyria_.--The Assyrian bronze is very pure. It consists of copper, 10 to 14 per cent. of tin, and traces of iron and nickel; in one case 18 per cent. of tin.

_Greece_.--Their wrought bronze for tools and weapons contains 10 to 12 per cent. of tin and traces of nickel and cobalt; in one case 18 per cent. of tin.

The cast bronze has in part the same composition as wrought bronze. (Statues were rarely cast from pure copper.) A small quantity of lead was sometimes added, especially in later times, for statues and coin. The later coins contained 5 to 7 per cent. lead, even 20 per cent. in exceptional cases. Macedonian coins were of quite pure bronze.

_Italy_.--Roman weapons (found at Hallstadt) contain 11 to 16 per cent. of tin, in some cases some zinc or lead, also nickel and iron as impurities. Roman hatchets found in Gaul contain 20 or 25 per cent. of tin. We have too few analyses to give us a correct view of the matter, but on the contrary we have numerous analyses of Roman castings.

Ornamental Roman bronze for flexible articles contains less tin and lead. For less flexible objects bronze-brass with 1 to 7 per cent. of tin, and 5 to 12 per cent. of zinc, was employed; and for brittle but brilliant objects, like buckles and mountings, an almost pure brass was used, with 15 to 24 per cent. of zinc and little or no tin. Lead is found in all these alloys in small quantities, rarely more than 1 per cent.

The statues contain from 6 to 10 per cent. of tin, 0 to 3 per cent. zinc (in one case 14), and frequently from 10 to 12 per cent. of lead (once even 20), so that Roman statue bronze may be called lead-bronze with zinc in it.

Coin metal varied its composition at different times. In the days of the Republic a lead-bronze rich in tin (5 to 12 per cent.) was used. Under the early emperors brass or impure copper came into use. After the time of Marcus Aurelius an improvement is noticeable; the metal then in use can be called stanniferous brass (1 to 4 of tin). Under the Byzantines, coins were again struck from impure copper.

These are the most important alloys of the Romans. In general we may say that the zinc alloys held an important place among the Romans.

_Gaul_.--For weapons they employed a very pure bronze with 2 to 15 per cent. of tin. Traces of nickel were rare. Cast bronze contained a few per cent. of lead.

_Britain_.--The weapon bronze contained from 7 to 14 per cent. of tin. Cutting weapons not infrequently contain 1 to 3 per cent. of lead, and traces of iron. Ornament bronze does not differ from weapon bronze. Traces of sulphur are not rare, which points to the use of pyritical ores.

_Alps_.--Swiss weapon bronze contains 8 to 13 per cent. of tin (in one case even 16 per cent.), not infrequently 1 per cent. of lead and traces of silver, very often ½ to 1 per cent. of nickel and traces of iron (once as much as 3 per cent. of iron). The Swiss ornamental bronze has the same composition.

_Bavaria_.--Wrought bronze contains 8 to 12 per cent. of tin (in tools 17 and even 25 per cent.), and often as much as 1 per cent. of lead, traces of nickel and cobalt. Ornamental bronze has the same composition. A few per cent. of zinc is also found.

_Bohemia_.--The wrought metal contains 5 to 11 per cent. of tin and traces of iron and sulphur, from which we conclude that their ores contained pyrites. Their cast metal also contains lead.