Making a DIY mirror using copper, tin and polishing - Page 1
Forum section: electronics / general

author: daqq
author_rank: Super Contributor
timestamp: September 13, 2015, 10:39:39 am
content: 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

author: daqq
author_rank: Super Contributor
timestamp: September 13, 2015, 10:42:02 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2015, 11:06:07 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2015, 11:09:50 am
content: 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

author: Augustus
author_rank: Regular Contributor
timestamp: September 13, 2015, 11:12:55 am
content: 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

author: rs20
author_rank: Super Contributor
timestamp: September 13, 2015, 11:53:42 am
content: Quote from: Augustus on September 13, 2015, 11:12:55 amYeah, even a kid can do it!https://www.youtube.com/watch?v=ti_E2ZKZpC4&feature=youtu.be&t=12m7s

author: TerraHertz
author_rank: Super Contributor
timestamp: September 13, 2015, 02:38:50 pm
content: 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?

author: G7PSK
author_rank: Super Contributor
timestamp: September 13, 2015, 03:13:41 pm
content: 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.

author: daqq
author_rank: Super Contributor
timestamp: September 13, 2015, 08:19:27 pm
content: Hi guys,Thanks so far!QuoteAre 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.QuoteThanks, any tips on the lacquer? I was hoping for a raw metal top layer.QuoteWhy 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 adventurousQuote"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.QuoteScratch resistance is not a problem really, but the tarnishing might be...

author: tggzzz
author_rank: Super Contributor
timestamp: September 13, 2015, 09:03:56 pm
content: I suspect amateur astronomers are adept at making mirrors. Look on their forums.

author: IanB
author_rank: Super Contributor
timestamp: September 13, 2015, 09:33:55 pm
content: Quote from: G7PSK on September 13, 2015, 03:13:41 pmThere are techniques to casting to avoid such problems. One such aspect is to de-gas the molten metal prior to pouring it.

author: Mechanical Menace
author_rank: Super Contributor
timestamp: September 13, 2015, 09:52:41 pm
content: I'djustfinely polish a lump of aluminium and lacquer that. Cut and mill it into a nice shape first.

author: SeanB
author_rank: Super Contributor
timestamp: September 15, 2015, 05:00:48 am
content: 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.