Medieval Africans Had a Unique Process for Purifying Gold with Glass

I don't know if the Africans lacked lead, perhaps lead was simply valued more than scrap glass?

Mills have historically been on rivers for precisely this reason. They can move logs to the mill by floating them down the river; they can put processed wood on a boat and float it further down; and they can put a waterwheel in the river and directly use the mechanical power of the wheel (using a mechanical transmission much like the one in older cars) to power the belts feeding the mill. All of these choices are intended to decrease transport/labor costs in a world where those costs dominate other considerations.

And the last one, I think—mechanical power transmission from the river—is also a clear example of another idea: millwrights have a lot of institutional knowledge about “doing things with the mechanical power of a river”, but they might not know anything about electricity. So the “obvious” way to automate the mill, to a millwright, is by using the mechanical power of the river, rather than by sticking a generator on a waterwheel and then powering the mill electrically.

I would guess that in a similar sense, jewellers (who glass-making was originally associated with) know more about working with glass, and pretty much nothing about working with the sort of chemicals and materials you get from mined ore; so if you tell a jeweller to purify gold, they’ll try to use what they have laying around—other jewelry-making materials and processes—to invent a way to purify gold, rather than concluding that the correct method is to go mine some stuff far away and bring it back here.

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Or, for a more cynical take on what I just said:

If you take “1800s millwright”, “1800s electrician”, “early-BCE jeweller”, and “early-BCE chemical ore refiner” all as artisan guilds that were preciously hoarding their secrets of how they did the things they did, then the millwright wouldn’t have access to knowledge about electrical generators, just like the electrician wouldn’t have access to knowledge about mechanical transmissions; and the jeweller wouldn’t have access to lead-based gold extraction methods, any more than the ore refiner wouldn’t have access to glass-based gold extraction methods.

But if Guild A discovered that Guild B had somehow found knowledge of (or independently discovered) one of its own “trade-secret” processes, and was now doing that process as a step in their own work, the guild that “owns” that process would [threateningly] insist that they should be doing that part of the work themselves [increasing labor costs on the final product by a huge amount]—with the alternative option being simply grabbing all their guild-members and kicking over the other guys’ operation.

So each guild would be highly incentivized to invent solutions to its problems that only relied on its own guild’s knowledge, rather than bringing any other guilds into the process. The millwright wouldn’t want electricians up in his business; the jeweller wouldn’t want chemical ore refiners involved in his gold-making. So they’d each invent solutions that, while perhaps suboptimal, are entirely just “a millwright solution” or “a jeweller solution” to the problems.

(Mind you, I’m not saying that there were actual professional guilds in either case; but rather, that this sort of us-vs-them mentality among the trades is very common historically, whether formalized or not—especially when you have trades that follow racial/cultural lines (banking, fortune-telling); or were associated with a particular religious sect (beer-making, ink-making); or, of course, were themselves the sort of unique fighting skills such groups developed to protect their group, e.g. pretty much all martial arts developed throughout history. People would “rip off” any of these techniques at their own peril.)