Although modern-day silvered glass mirrors have pretty much destroyed the market for bronze mirrors, these highly polished pieces of metal once were the pinnacle of mirror technology. Due to the laborious process required these mirrors saw use essentially only by the affluent. That said, how hard would it be to make a bronze mirror today with all of the modern technologies that even a hobbyist can acquire for their shed? Cue [Lundgren Bronze Studios] giving it a shot, starting by casting something flat-ish to start polishing.
Just getting that initial shape to start polishing is a chore, with hammering out the shape possibly being also a viable method. When casting metal it’s tricky to avoid having air bubbles and other defects forming, though using a sand mold seems to help a lot.
After you have the rough shape, polishing using power tools seems like cheating, but as you can see in the video even going from 50 to 8000 grit with a rotating disc left countless scratches. Amusingly, hand sanding did a much better job of removing the worst scratches, following which a polishing compound helped to bring out that literal mirror finish.
A quick glance at the Wikipedia entry for bronze mirrors shows that a tin-bronze alloy like speculum metal was used for thousands of years as it was much easier to polish to a good mirror finish. The metallurgy of what may seem like just a vanity item clearly goes deeper than just polishing up a metal surface.
Kudos for the effort and the pretty good result.
He made his life way harder by starting from a cast. If he had bought a bronze disc that was cut from rod stock using modern tools it would have gone way easier. He probably could have got close to that result by polishing alone.
There are a few reasons why you shouldn’t do this, but it is admirable to try. There are machines that can do this type of machining/polishing but they cost more than you’d ever wish to spend. I would love to see a follow-up with speculum. Is it really that much easier?
I realized what I might have written could sound dismissive. He did a good job, despite his technique lacking early on, this is hard work. I think it’s important to say the surface roughness of a true mirror finish, at least for optical applications is literally atoms of variance.
Just a few more items and he will be ready for the Kraken
Umm…the gorgon?
Or Medusa.
Or invading flotilla / army. In sunny days only, of course.
It seems ‘speculum metal’ is a mixture of around two-thirds copper and one-third tin, and bronze is also a mix of copper and tin with 12%-12.5% tin, but often with things added like aluminium, manganese, nickel, or zinc.
So I guess you could go from copper to bronze to speculum metal, but did people actually do that like Maya’s statement suggests? The wikipedia article does not mention that at a quick glance, and I think it was likely just straight from copper and tin to speculum metal with the uncertainty of what else was in the bronze.
Now I am wondering why they did not use copper-tin-nickel for mirrors though.
Nickel wasn’t known (at least in pure form) for a long time. It’s not very abundant, and probably hard to smelt (having chemistry sort of hybrid between iron and copper). It often occurs as sulfides which associate with copper, so such alloys might’ve existed accidentally; iron alloys (a major use of the element today) wouldn’t have because sulfides ruin pig iron, and iron ores don’t associate nearly as much with nickel. (Not much way to separate or purify it either, until solution chemistry developed, I think? Maybe with tedious sorting of ores, if one was sufficiently committed to trying to smelt the stuff.)
An excessively tin-rich bronze forms a majority intermetallic compound, white rather than yellow, and the hardness (and complete absence of ductility) means it cuts, grinds and polishes like a stone — but a shiny one.