EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: daqq on September 13, 2015, 10:39:39 am
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Hi guys,
I want to make something for my wife for Christmas. I was thinking a mirror - I would make a small, shallow "bowl" by bending copper sheet metal - say 10x10 cm square, with edges 3mm or so high. Into this I would put some tin alloy. I would heat this with a heat gun, add flux. This should melt the tin, bind it to the bowl. There should be enough tin to fill it up to the edges.
To flatten the surface after the whole thing has cooled down I would mill 0.5mm to 1mm off of the surface to get a wholly flat surface.
After this I would polish the surface with polishing creams, each finer than the previous. Ideally I would get a mirror like silver flat finish.
I am not a casting expert and I've never done anything similar. I would like to ask whether this is a good way to go about it.
Thank you,
David
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Added: Also, some recommendations for the proper alloy for this kind of enterprise would be helpful. Preferably something off the shelf, available at farnell or similar.
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Whatever you use you need to give it a protective lacquer coat after the final polish. Tinned copper makes a good mirror, not the best ( a final coat of silver will be better till it tarnishes) but still good. This has been the traditional mirror until they were able to sputter aluminium onto glass ( or silver from silver nitrate decomposing) in the modern mirror.
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That'll work, but you'll have a lot of work milling it to optical standards I think. Not that it needs to be telescope flat (within a wavelength, interferometrically), but it better be a good program and a tight mill. Or use a lathe with a tightly adjusted curve cutter?
The other downside is tin will tarnish, and it's very soft so it will scratch extremely easily as well. It might be reasonable to silver plate, in which case the tarnish may develop more slowly, and be easier to deal with (it should be a relatively heavy plating, to account for wear due to polishing, and for diffusion into the tin, which will slowly dissolve it from the underside).
As for alloys, 110 copper is cheap enough and easy to work, I guess. A good lead-free tin candidate might be the stuff used for soldering plumbing (typically 95%, Sn 5% Sb).
If you're feeling more adventurous about casting, you could try making up some white brass, which will still tarnish, but will be much harder (scratch resistant). Or the original: speculum metal (I think around 20% tin bronze, which is white, and very brittle -- you may need to grind rather than machine it).
Anything brass or bronze of course requires a furnace capable of 1200C, and all the handling equipment. Any alloys containing zinc give off fumes when molten, use strong ventilation and avoid the fumes. Guessing you don't have quite this much equipment... but if you do, it's certainly possible at home!
Tim
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Why don't you deposit metallic silver on a flat glass surface by chemical means, like they did with christmas balls back in the days? Youtube should have some vids on that :-+
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Why don't you deposit metallic silver on a flat glass surface by chemical means, like they did with christmas balls back in the days? Youtube should have some vids on that :-+
Yeah, even a kid can do it! https://www.youtube.com/watch?v=ti_E2ZKZpC4&feature=youtu.be&t=12m7s (https://www.youtube.com/watch?v=ti_E2ZKZpC4&feature=youtu.be&t=12m7s)
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If you have a vacuum chamber big enough to hold a piece of glass for your mirror you could sputter gold onto it. Isn't that the most romantic mirror possible?
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You need a vacuum chamber for casting the tin as well. If you just cast tin it will have at best micro porosities and worst porosities that you can see, either way it is unlikely that you will achieve a good mirror finish. It would be better to polish the copper sheet and then electro plate it with tin or silver, but just polished copper will make a fair mirror.
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Hi guys,
Thanks so far!
You need a vacuum chamber for casting the tin as well. If you just cast tin it will have at best micro porosities and worst porosities that you can see, either way it is unlikely that you will achieve a good mirror finish. It would be better to polish the copper sheet and then electro plate it with tin or silver, but just polished copper will make a fair mirror.
Are you sure about this? I've seen some casting, and never seen it done in a vacuum chamber. I'm counting on that the top layer (with all the nasty) will be milled away. The internal structure should be OK. While true, a vacuum chamber might help, shouldn't a similar effect be achieved by means of flux on the top?
As to the other means (chemical/vacuum deposition of silver onto glass), I really would prefer a real metal mirror - mostly for aesthetic reasons, partly I want to try it out and I want to hammer in a few letters. Also, silver reacts rather poorly with oxygen.
Whatever you use you need to give it a protective lacquer coat after the final polish. Tinned copper makes a good mirror, not the best ( a final coat of silver will be better till it tarnishes) but still good. This has been the traditional mirror until they were able to sputter aluminium onto glass ( or silver from silver nitrate decomposing) in the modern mirror.
Thanks, any tips on the lacquer? I was hoping for a raw metal top layer.
That'll work, but you'll have a lot of work milling it to optical standards I think. Not that it needs to be telescope flat (within a wavelength, interferometrically), but it better be a good program and a tight mill. Or use a lathe with a tightly adjusted curve cutter?
Why should there be a lot of milling? Milling should only be the first step to level out the rough "cast". Then its onto polishing in water by means of polishing pastes - I've seen alluminum get a mirror like (even if bumpy) finish.
As for the brass and advanced casting, I'm not that adventurous :D
If you have a vacuum chamber big enough to hold a piece of glass for your mirror you could sputter gold onto it. Isn't that the most romantic mirror possible?
"Honey, I evaporated our wedding ring onto this piece of glass! It's a mirror!"
"That's very nice!"
...nope. I'm sure it can be done, without melting down heirlooms, but I do not have a vacuum chamber.
The other downside is tin will tarnish, and it's very soft so it will scratch extremely easily as well.
Scratch resistance is not a problem really, but the tarnishing might be...
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I suspect amateur astronomers are adept at making mirrors. Look on their forums.
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You need a vacuum chamber for casting the tin as well. If you just cast tin it will have at best micro porosities and worst porosities that you can see, either way it is unlikely that you will achieve a good mirror finish. It would be better to polish the copper sheet and then electro plate it with tin or silver, but just polished copper will make a fair mirror.
There are techniques to casting to avoid such problems. One such aspect is to de-gas the molten metal prior to pouring it.
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I'd just finely polish a lump of aluminium and lacquer that. Cut and mill it into a nice shape first.
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Best lacquer is to get an optically clear UV cure epoxy, put a few drops on the finished mirror and spin it to spread it evenly, then cure it. Otherwise a clear spray on lacquer in a can, applied in a few very thin coats to cover without runs.