CHRIS25 - 18-3-2014 at 03:43
I am looking for some understanding of the chemistry behind the following:
Two methods to patinate copper with silver: The following solution; 2g AgNO3 dissolved in 35mLs water when applied to cold copper turns it black,
when applied to hot copper turns it silver grey. Now I realise that copper displaces silver but my question is why does the heating of the copper
make this difference?
The second question: since the latter does not produce a nice silver look (too grey) I tried the following on Cold Copper: I added a further 75mLs
water with 1g NaCl and 4,5g Na2S2O3 to the above solution. I think I have added too much of the thiosulphate but at this point I see a more silvering
appearance slightly yellowy no doubt from the sulphur.
I am eager for a little chemistry insight here if someone may have a few moments to spare. thankyou.
edit: I should have added that I understand that the resulting solution in the second case is a silver thiosulphate, but I am also getting a much
darker solution with what I believe to be the insoluble silver chloride settling at the bottom.
[Edited on 18-3-2014 by CHRIS25]
MrHomeScientist - 18-3-2014 at 06:37
Well, temperature has a big part in reaction rate. Using the hot copper would probably lead to faster silver plating and therefore a different surface
appearance.
Adding chloride seems like a bad idea for silver solutions - as you saw, silver chloride will precipitate. Why did you decide to try salt and
thiosulfate? Do you have a reference for what this is supposed to do? I have no experience with patina formation, so maybe this is something someone
in the trade would know.
CHRIS25 - 18-3-2014 at 06:50
Hallo,
well I have another recipe that uses ammonium chloride, but no, these references never ever deal with the chemistry, everywhere I look I never see
anything about the chemistry which is why I decided to teach myself. Anyway, the thiosulphate is a sort of generalised colour interference catalyst,
I use it because it favours shifts in colours depending upon length of time in solution. I actually have a brilliant silver plated looking piece of
copper with the second solution. But I realise that that it is the silver thiosulphate that is at work here, Maybe the sodium is necessary otherwise
the you would stay with silver nitrate in solution, and you don't want this? So you need to get rid of the sodium by joining it with the nitrate?
These are just thoughts I have. The first solution simply did not work very well at all. I have no idea why the chloride is necessary, but in so
many recipes,no matter the colour, a chloride is usually provided, I see this all over the place.
BJ68 - 19-3-2014 at 03:23
There is a mixture what I found in the book "Lehrbuch der Anorganischen Chemie" Holleman/Wiberg, which was working very nicely.
Prepare
first a solution of silver nitrate in water (only a small amount (tip of a spatula) is used) and add fine powdered calcium carbonate (the exact German
term was "Schlämmkreide") until you get a stiff paste.
Now made a second solution of sodium thiosulfate in water and add this to the solution above and mix very fast and well. There will be a color change
to (so far I remember) light brownish. It´s important that the silver nitrate paste is stiff enough to tolerate the dilution with the sodium
thiosulfate solution.
After mixing apply this semi liquid mixture to polished piece of copper and rub it in; be fast, if you rub to long the silver layer will became to
thick and blistering away, so it´s a good idea to have a bucket with water, where you can wash away the liquid without wasting time.
I used my bare hands and got a very shiny layer of silver metal plated on the copper.
In the book the mixture is called "Anreibeversilberung".
Bj68
CHRIS25 - 20-3-2014 at 02:50
Hallo BJ, thanks, I recognise the brown mixture, unfortunately there are no measurements in this recipe so reluctant to try it since there are so many
variables one could use in water to chemical ratio. But thankou for the input. My main query was an attempt to understand the chemistry a bit more.
However if you tell me what amounts you were using that would give me a starting point. In solution 2 above I have, for all intense and purposes, an
authentic looking silver plated copper piece, quite magical really, just added 1 extra gram of silver nitrate to the solution and it all went nicely,
it goes on greenish and stays greenish until you rub with a cloth and then the silver shine appears. The plating is silver thiosulphate, I suppose
that is why it is green, then the rubbing I do not understand, and the point of the chloride I do not understand.
2AgNO3+Na2S2O3+NaCl=AgS2O3+NaNO3+NaCl. The NaCl seems redundant here, yet without it the solution does not work as well.