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.
| WEAPON BRONZES. | |||
|---|---|---|---|
| Country. | Essential constituents. | Admixtures. | Impurities. |
| 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 constituents. | Admixtures. | Impurities. |
| 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.