If anyone has the book on Kindle, could they check if the word "pewter" shows up? It's a pretty common tin alloy used to make household items. If there's enough tin around to make pewter, there should be enough for bronze to be common as well.
The constant reference to copper-coating does make me wonder if there's some issue obtaining tin and zinc beyond those one would expect from obtaining copper (they all have to be mined, after all). There's actually
more zinc than copper available in the Earth's Crust, and coating steel in zinc to protect from rust (
galvanization) seems to date back to the 4th Century AD and remains fairly common to this day (zinc, however, does not develop as effective a patina as other metals, making it an arguably less efficient option).
Something that occurs to me is that maybe we're not accounting for the difficulties involved in transportation of bulk materials. In the real world, one can make a ship as big as one wishes, since they just need to float on water; in the world of the Cinder Spires, each ship requires at least 2 very complicated crystals just to float. Similarly, in the real world, mankind has been on the top of the food chain since time immemorial, making it simple (relatively speaking) to go prospecting for new mine locations; it occurs to me that in the world of the Cinder Spires, a
gold rush simply cannot happen, due to the environmental conditions (of everything trying to kill you).
It wouldn't surprise me if copper-coating is only a local custom, with other spires further away using other materials for the same purpose.