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Author: Subject: Gold mirrors
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[*] posted on 13-8-2013 at 21:01
Gold mirrors


I saw some references to gold mirrors in flasks. Exactly what reactions would be necessary to do this?
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[*] posted on 13-8-2013 at 21:40


I would guess starting from a soluble gold salt, precipitating the hydroxide, and reducing this to gold metal - possibly like a silver mirror...?



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[*] posted on 14-8-2013 at 04:09


Quote: Originally posted by CGH  
I saw some references...
Then show us.



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[*] posted on 14-8-2013 at 05:56


Quote: Originally posted by Formatik  
That's neat. Gold compounds can also form mirrors. I have made one by mistake once and it turned the bottom half of the inside of a glass flask into gold, similar to your Pt mirror. It could be wiped off with a tissue.

It's not much, but it indicates that it is possible.
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[*] posted on 14-8-2013 at 06:35


Quote: Originally posted by CGH  
I saw some references to gold mirrors in flasks.
There's a procedure in Wheeler's _Scientific Glassblowing_. The latter of two citations: Von Angerer, Ernst, Wien-Harms Handb. exptl. Physik, 1, 375 (1926).
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[*] posted on 14-8-2013 at 06:59


Quote: Originally posted by watson.fawkes  
Quote: Originally posted by CGH  
I saw some references to gold mirrors in flasks.
There's a procedure in Wheeler's _Scientific Glassblowing_. The latter of two citations: Von Angerer, Ernst, Wien-Harms Handb. exptl. Physik, 1, 375 (1926).

Great, thanks!
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[*] posted on 18-8-2013 at 14:56


Gold mirrors have been known for a long time. What I did above was reduce an aqua regia solution of gold with methylacetylenepropadiene. Filtered it twice, and let sunlight hit it during filtration during which the bottom part and walls of the flask (IIRC 25mL flask) turned into gold. The gold itself was a very fine black powder. The procedure was not reproducible, so it wasn't worth mentioning.

The Handbuch der Experimentalphysik reference mentioned above describes a gold mirror method made by Wernicke. I haven't seen this paper, but see comments below concerning his procedure.

Muspratt's theoretische, praktische und analytische Chemie (1891), Vol.3, 1602 describes several things relevant to gold mirrors: First is that Liebig had obtained a nice goldening of glass container by heating an alkaline solution of gold solution with alcohol, but according to Liebig himself this method occurs under some specific conditions which made technical application not practical.

Bleckrode noticed the southamerican extract of soap bark (Quillaja saponaria) formed gold mirror on containers using gold chloride solution. Chemisches Zentralblatt (1860), Vol.5, 768 describes an abstract of Bleckrode's finds: Water creates a solution of 20-25% of the bark. This solution is what reduces the gold chloride, and forms gold mirrors on container walls.

The third thing mentioned by Muspratt is a procedure for forming a gold mirror. The reference is way old, so there might be newer better methods for forming gold mirrors. The procedure is originally from Wernicke (said to be a sure method), but was simplified by Böttger. According to the latter, one solubilizes 1g of high karat gold (must be free of any traces of silver) in aqua regia. Then this is evaporated to dryness with a water bath. It is taken up with water and diluted to a volume of 120 ccm using water. Then 6g NaOH is solvated in 100ccm water. A reducing solution is made from: 2g "starch sugar" in 24 ccm water, which is mixed with 24 ccm alcohol and 24 ccm of the commonly available aldehyde of spec. gravity 0.870. The reducing solution needs to be freshly made since it loses efficacy upon storage. The solutions are mixed in the following proportions:

64 vol. gold solution
16 vol. soda lye solution
1 vol. reducing solution

This now well-mixed solution is immediately poured into a shallow container. And then the glass plate (which was previously cleaned with aqueous NaOH, not acids) is with a carrier placed onto the surface of the liquid. After 5 minutes the gold has partially deposited itself to a golden sheet, and the other part of the gold precipitated as a powder in the liquid. The powder can be collected through filtration. To golden hollow vessels, one adds the approximate amount of the above well-mixed solution into the container; then one uses uninterrupted agitation to ensure that the walls of the container are evenly wetted.

There might be methods for forming gold-colored mirrors which use cheaper metals as stating materials, though I haven't looked. Something like a brass mirror might also be made by electrical means (electroplating, electrodeposition).
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