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Topic: Silvered bottle turns black instead of becoming reflective  (Read 1739 times)

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Offline Mr. Figs

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Silvered bottle turns black instead of becoming reflective
« on: June 12, 2015, 12:50:47 AM »
Earlier today, in my AP Chemistry class, we did a "silvering bottles" lab, similar to this one: (I couldn't find the exact lab we did, but the procedure is close). The short version is: reduce Ag+(aq) ions, so that they precipitate onto the inside of a bottle. Everyone's bottles came out just fine, except for mine, even though I followed the instructions to the letter. Instead of a reflective inner coating, by bottle turned a dark-brown or black color everywhere but its neck and flat base, which both came out mostly normal (the base had spots of black). I can't figure out why.

I washed the bottle with tap water at first, just to remove dust and the like, and left it underwater for a few hours to remove anything that was still stuck to it (it hadn't seen use in several decades). Someone else did the same thing, in the same sink, and both of his bottles worked. I went on to rinse it with acetone and then distilled water, in that order, before starting the lab. I doubt there was a substantial amount of contaminants in the bottle. I tried running the procedure again after the bottle stopped showing change, but nothing visible changed. The black areas were already totally opaque.

Clearly, something precipitated from the solution after the reaction, and the only insoluble species involved is Ag(s). This leads me to believe that either the ions somehow precipitated incorrectly—maybe due to the texture of the bottle's inner surface?—or some other species became involved. The bottle in question was very old, probably from the 1920's or so, so it might be possible that it's not a normal soda-lime bottle, like most we see today, but I can't find any information on the composition of old glass bottles so support that.

One more confusing observation was that the reflective area did not fade slowly to black near the neck—latitudinal rings of reflective deposits travel around the "shoulder" region instead. I don't think that any additives could have been added in such a way, but I could be wrong; I don't know that much about bottles.

I would have taken pictures if I had a smartphone—I might ask a friend to take a photo of it for me tomorrow. But either way, if anybody here is highly knowledgeable about silver plating, I'd appreciate some guidance. I'm totally stumped.

Offline Borek

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Re: Silvered bottle turns black instead of becoming reflective
« Reply #1 on: June 12, 2015, 02:56:12 AM »
If I understand the part of the latitudinal parts correctly - these could point to the changing levels of the solution originally stored in the bottle (which further supports the idea that it is the property of the surface that was in contact with some chemical in the past that makes the coating black).
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Offline Arkcon

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Re: Silvered bottle turns black instead of becoming reflective
« Reply #2 on: June 12, 2015, 06:57:49 AM »
This process is pretty sensitive to traces of salts or tap water chlorine.  If you want to do it at home, for example, you need to rinse with distilled water.  Not an easy thing to get, back in the day.
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