People sometimes claim that the stone age didn't end for lack of stone, and that the oil age will not end for lack of oil. This is often in the context of discussing renewable energy or other technological developments. This has me wondering about the end of other "ages" of human technology. My hazy, non-historian's understanding of the end of the bronze age and beginning of the iron age is something like the following. In the eastern Mediterranean / Mesopotamia region, a large international trade network was necessary to bring together tin and copper from widely separated areas in order to make bronze. In the late bronze-age collapse, these networks broke down, and making new bronze became prohibitively difficult. This somehow led to the development and adoption of iron technology, which requires only one type of ore and thus is not dependent on long-range trade networks. Thus, it might be said that the bronze age did end for lack of bronze.
My questions are, first, is this a somewhat accurate characterization of the historical facts as we understand them? And second, how was the transition from bronze age to iron age different or similar in, China, sub-Saharan Africa, or other regions? Did the development and widespread adoption of iron sometime take place in the context of decreasing supply of bronze, and sometimes not, or did it follow a more consistent pattern across regions and times?
Sort of, it's complicated.
There's no singular global "bronze age", there are just interconnected regional bronze ages.
Bronze has many advantages to explain why it became wildly popular and heavily adopted for tools and weapons after the neolithic. Like stone bronze is pretty corrosion resistant, more so than either iron or copper. Bronze can be used to create tools and weapons with sharp blades that can be sharpened after casting, and bronze can be recycled. Discovering how to extract the basic ingredients of bronze (copper and tin) is a pretty straightforward process. Both copper and tin ores have characteristic interesting appearances as strikingly colored rocks of particular hues in the case of many copper ores and as shiny rocks in the case of several tin ores. If you heat those rocks in reducing flames (which just means using a lot of charcoal) at high enough temperatures you'll convert the ore to metal and melt the metal to separate it from impurities, this is the sort of thing you can do in something equivalent to a camp fire. To cast bronze you mix the metals together, melt them, and pour them into a mold. All of this stuff is pretty easy to discover and is amenable to iterative improvement. The sophistication of bronze workers grew very high over time, but getting into bronze work was comparatively easy.
The biggest problem with bronze is that the ingredients are somewhat rare, especially tin, and are not geographically well distributed. Large scale bronze production necessarily relies on long-distance (continental scale) trade networks to have access to copper and tin. And that means bronze production in some areas is pretty heavily reliant on large scale geopolitical stability. In the Eastern Mediterranean during the Bronze Age most societies were organized into city states, and then perhaps into higher levels above that. Those city states were highly centralized in their organization, so when they fell they left behind a mess that devolved into anarchy rapidly. Large scale geopolitical instability and conflict at the end of the classical Bronze Age in the Eastern Mediterranean led to a huge collapse, including of trade, and this led to an absence of access to the materials to make bronze, forcing people to turn to alternatives including stone and iron.
Iron working is very different from bronze working. It requires significantly higher temperatures and different processes in order to get iron from iron ore using pre-industrial techniques. It requires very different methods to turn iron into useful tools and weapons. Bronze tools are cast using molten metal, iron tools are preferentially forged (formed from repeated hammering). It takes a great deal of skill to be able to make iron/steel tools and weapons that are as good as or better than the best bronze versions, and it wasn't until the middle ages before that level of skill was widespread. But the major advantage of iron is that iron ore is almost everywhere. Additionally, iron work is more easily repaired and mended using a small footprint of equipment and a comparably small amount of time, in contrast a lot of the equivalent work with bronze requires re-melting and re-casting which is time and resource intensive (and expertise intensive). That's why people turned to it in the Eastern Mediterranean after the Bronze Age collapse, it was what was available, it was suitable, and it was easier to work in a low-infrastructure context. As those societies crawled out of collapse they didn't have the same access to distant sources of copper and tin they once had, so they relied on iron much more heavily instead, and that forced them to develop iron working.
The story almost every else there was a bronze age (the rest of Europe and Asia as well as Northern Africa) is one of more gradual change and competing technologies. You have long periods of several centuries where both bronze and iron work were common and competed with each other. Iron mostly won out in these contests because of its marketplace advantages in terms of cost, ease of manufacture, availability, broad utility, etc. Although early iron/steel weapons do have some advantages, for the transitional period many of those advantages are pretty marginal and offset by other advantages of bronze, which is why outside of the context of the Eastern Mediterranean Bronze Age Collapse the transition to iron/steel weapons was generally pretty gradual and incremental.
There's also a whole landscape of culture and symbolism related to bronze and iron that I'm mostly going to gloss over. Think about the way that materials are perceived today, whether something is made of solid wood or chipboard, plastic or metal or carbon fiber, aluminum or titanium, etc. Those details have cultural resonance, and in some cases there are complex cultural, mythological, or religious traditions built around them. Major League Baseball uses wooden bats instead of aluminum, for example. You can imagine how similar feelings, traditions, expressions, and rituals would develop related to weapons or tools and their materials. That's one reason why the change of technology from bronze to iron was slow and gradual in much of the world.
It's only really within the context of the Eastern Med. Bronze Age Collapse that the bronze age ended due to "running out of bronze", almost everywhere else it was more of a gradual process of iron out-competing bronze production. And, yes, certainly some aspect of that was because of the logistical advantages of iron vs. bronze, but it wasn't a step function it was more of a cross-fade.
You are right, China did not have a collapse. The production of iron was much cheaper than bronze, which combined with large scale political changes lead to mass production and use of iron on a scale far larger than was possible with bronze.
The Shang and Zhou dynasty had absolutely massive bronze production, of both weapons and ceremonial vessels, though not of agricultural tools. Thousands of bronze artifacts from the Shang, and at least 12,000 from the Zhou period have been found. During the Zhou, we have discovered a large royal foundry, smaller workshops in the capitals, as well as foundries in aristocratic households, though those were generally inferior. China has large copper and tin deposits, many within trading range the core Zhou regions (from the "Science and Civilisation in China" on mining). However, most of the largest tin deposits are in south west and southern China, outside the range of direct Zhou control. Isotope studies suggest some of the ores came from the south or Yunnan, but there is some textual evidence that there was tin mining in the north, as well as more recent finds of tin deposits, so it is still uncertain how much of their tin was mined and how much was traded for. Finally, there is increasing evidence of early bronze age civilizations outside of the Shang/Zhou core. Excavations in Northern Sichuan reveal evidence of both local production and large scale trade, and found one of my favorite bronze decorations, a four meter tall bronze tree. Some of them were produced in the middle Yangze, and there is further evidence of bronze in Yunnan and elsewhere in southern China, which isn't particularly surprising because they may have been one of the main sources of metals for the entire region.
Instead of a massive collapse disrupting access to bronze, bronze continued to see widespread use in ceremonial uses and weapons. There is evidence of wrought and bloomery iron production in the periphery to the north and west quite early, as well as Korea, perhaps being transmitted from the Scythians. Cast iron was discovered as early as the 5th century BC, probably in Southern China, perhaps in Wu in the Yangzi river delta, possibly for making agriculture implements. While early cast iron was not as strong as wrought iron, it was even cheaper. As this was happening, the control the Eastern Zhou state had over their vassals was evaporating, leading to centuries of war during the Spring and Autumn and Warring States periods. This led to a shift from chariot focused aristocratic forms of warfare and a decline of the patrimonial, almost feudal, state to large scale infantry warfare and increasing centralization of state control.
In order to arm these new infantry armies, states turned to iron. They set up massive, tightly supervised weapon producing manufacturers, and leased mining rights to individuals, producing massive amounts of wrought iron and steel. Qin undertook widespread forced migration of skilled workers to set up industry in newly conquered areas, some of whom became wealthy industrialists. Cast iron farm implements combined with large scale state led irrigation projects led to an increase of the productivity of farming, and peasants also started using iron for pots and cookware. While cast iron is generally weaker and more brittle, by the ~3rd century BC China knew how to cast a stronger malleable cast iron through subjecting cast iron to a multi-day heat treatment. Furthermore, it didn't really matter for some products such as cookware, while others, such as plows, would have a detachable cast iron edge that could be easily replaced if it broke.
Bronze continued to be used at the same time as well, for weapons and ceremonial or decorative purposes. The terracotta warriors were armed with bronze weapons, and graves in Chu also contain bronze weapons. Burials of common soldiers in the north contain a few bronze weapons along side mostly iron ones. Finally, crossbow triggers were almost all made of bronze, as they are small parts the cost of the metal mattered less than the ability to create small, intricate parts. These seem to have been created before the widespread use of iron weapons and did as much as iron weapons to revolutionize warfare.
Sources:
"The Economic History of China" by Glahn covers this overall process, and the broader economic impacts it had.
On early bronze production, "The Archaeology of China: From the Late Paleolithic to the Early Bronze Age" by Liu and Chen, as well as the section on bronze by Bagley in "The Routledge Handbook of Early Chinese History".
The volumes of "Science and Civilisation in China" on mining and ferrous metallurgy has a ton of detail on mining and iron making.
Finally, "Standardisation,Labour Organisation and the Bronze Weapons of the Qin Terracotta Warriors" by Xiuzhen Li is a very cool look into how Qin organized weapon production. She also has a paper "Crossbows and imperial craft organisation: the bronze triggers of China’s Terracotta Army" looking into the details of crossbow trigger production. The thesis has been turned into a book, but unfortunately being an academic book is hilariously unaffordable.
