This is a fairly straight-forward subject, discovered probably by accident by a potter who noted that certain rocks melted when placed in the airless ovens which were in use around 6000 to 4500 BCE. Still, it was another thousand years before certain localized areas had learned how to forge bronze into hard materials, through introducing an impurity into the metal (arsenic was used long before tin).
Certainly localized cultures with copper and arsenic available, and later tin, benefited from the slightly lower melting point for metals forged at or near sea level: the Indus, Nile, Tigris-Euphrates and Yellow river valleys. Bronze was considerably stronger than copper, and once the process of forging was discovered, an item could be hammered until it was up to four times as strong as the metal when cast.
This led to bronze weapons and armor, which were the power weapons of their day ... enabling those river valley civilizations with the process to war successfully against their neighbours, thus unifying each culture for long periods. Although all were subject to infighting, the cultures became homogeneous in language, religion, political structure and social custom.
Thus, individual cultures were melded into Egyptians, Mesopotamians, Aryans and Chinese. As metalsmithing spread out into the hills, those tribes too unified – as the strongest dominated the weakest through the use of metal weapons.
It is a point that very few cultures have been successful at all without the discovery of metal, and none at all who were at some point in their history exposed to metal-wielders. Wood does not stand up well to metal – not even when the wood includes the bow and arrow, as most cultures did long prior to the discovery of metal weapons.
That is because early bows and strings are notoriously unreliable, and lack the power of the later longbow for overcoming heavy armor. Early bronze armor, incidentally, was little more than a large plate which hung on a strap around one’s neck, covering the front of the body – often of burdensome weight. But it was tremendously effective in battle, compared to those relying on non-metal armours. Thatch and leather had been effective against wooden weapons – but were not so much against a hard bronze sword with the benefit of an edge.
It is often thought that bronze is inferior to iron where it comes to weapons. I haven’t been able to find any data to support that conclusion. Copper was more easily identified than iron by the neoliths, and occurs more often in placer deposits (pure metal nuggets). Copper melts at 1,083 degrees Celsius, while iron melts at 1,535 degrees C. Thus it is easier to manufacture. When beaten and annealed, bronze is as hard and as dense as wrought iron – meaning that for two dark age knights hacking at each other with swords, a bronze sword is just as effective (and no more likely to dent or bend) than an iron sword.
However, it was discovered that iron – once identified and once means to found it was managed (quite early on, not long after the discovery of smelted copper) – is vastly more plentiful. Its appeal rose because it was as good as bronze and much cheaper. It was also available in a greater number of places.
By the time of the Middle Ages, copper in many ancient mines had played out, not to be found in many new places – and tin, the best addition to copper, was much rarer. Bronze fell out of favour.
From a D&D perspective, any truly ancient weapon ought to be constructed from bronze ... at least it would be a good indication that the weapon came from another age, and was made according to principles that were lost. That is the usual program, is it not? Was Excalibur necessarily made of iron?
But then we see weapons presented in movies as shiny, lightweight artifacts – not as metal clubs with points and edges. Without question that is how early weapons were used. Daggers and swords were heavy, the heavier the better, since that helped hold the edge and produced the best hit possible.
I feel both mirth and disgust at watching the fight scenes from the movie 300, which features weapons of iron with remarkable sharpness, slicing bodies apart as though they were not made of hard bone. Mystically, we are taught by the movies that weapons don’t dull and that if you swing hard enough, the human body offers no greater resistance than warm jello.
Granted, those scenes are for iron weapons and not bronze, but the principal is the same. As I say, a bronze weapon will match the power of an iron weapon, weight for weight. It was not until the 17th century that harder iron metals were managed.
This is not very helpful for your campaign, I know. You may consider that for the time period, the power of the priests, the development of cultures, food production and so on were brought together by the creation of bronze weapons to form what we think of as the Bronze Age. I will write more on this tomorrow, as I hope to introduce a unifying principle into these technologies.