Bronze, Tin, and a Broken Supply Line
A coastal workshop produced bronze tools for about 180 years. Slag and crucible fragments show two alloy recipes. Period A (layers 6-4): copper with 8-12 percent tin, consistent across 40 analysed objects. Period B (layers 3-1): copper with 2-5 percent tin in 30 objects, plus 6 objects of almost pure copper and 4 of arsenical copper. Lead isotope analysis of Period A copper points to one inland ore source 400 km away. Period B copper is isotopically mixed, including sources 900 km away. Tin in Period A is scarce in the local geology; Period B tin content falls but a few objects retain 10 percent. Imported pottery in Period B rises sharply. The excavator claims a tin-supply collapse forced a technological regression. A colleague suggests instead that recycling of old bronze explains the chemistry.
1. Evaluate the tin-supply-collapse hypothesis against the recycling hypothesis using the compositional data.
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Both hypotheses predict lower tin, so the discriminating evidence is in the distribution and the outliers, not the average.
Supply collapse predicts: a fairly uniform downward shift as new tin becomes unavailable, a rise in recycling of scrap, and increasing variability as smiths stretch dwindling stock.
Recycling predicts: mixing curves, where tin content clusters at intermediate values depending on how much scrap was melted, plus dilution with fresh pure copper. Critically, recycling should reduce isotopic diversity in copper, because old bronze carries the signature of earlier ore sources.
Observed: Period B copper is isotopically more mixed, including distant sources. That fits supply reorganisation (new, more distant copper sources entering the network) better than simple recycling, which would concentrate signatures. The 4 arsenical copper objects are a separate recipe, not diluted bronze, suggesting experimentation with alternative hardeners when tin was short. The few 10 percent tin objects show tin had not vanished, only become unevenly available.
Judgement: supply disruption is better supported, but the mechanism is reallocation and substitution, not simple collapse. Recycling probably occurred alongside it. The two hypotheses are not mutually exclusive, and the data cannot fully separate them.
2. Imported pottery rises sharply in Period B. Explain how this changes the interpretation of the workshop's decline, and what additional test would sharpen it.
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Rising imports cut against a story of isolation or regional collapse. If the settlement were cut off, imports should fall, not rise. Instead, the community remained connected to long-distance exchange while its bronze recipe changed. That reframes the problem: the network did not fail; its priorities shifted. Perhaps tin was diverted to other consumers, or copper from distant sources became more accessible than tin, or the workshop lost its privileged access to the inland tin route while gaining access to other goods.
The archaeologically visible pattern is therefore specialisation loss, not network collapse: a workshop that once secured consistent tin now competes in a broader, more volatile exchange system. The excavator's 'collapse' language overstates a reorganisation.
A sharpening test: compare tin content of bronze objects from contemporary sites in the same region. If tin falls everywhere at the same date, the cause is regional supply. If it falls only at this workshop while neighbours retain high tin, the cause is local access or choice. A second test is to source the imported pottery: if it comes from the same direction as the new distant copper, it supports a redirected trade route rather than a broken one.