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The time now is Nov 21, 2018 10:48

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A general guide for using the Forum with some rules and tips
Bismuth lost its colours.
  
  Index -> Conserving, Preparing and Cleaning Minerals
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cornz




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PostPosted: Nov 05, 2018 14:27    Post subject: Bismuth lost its colours.  

Hi all.
I have a lovely piece of bismuth but all its colours have disappeared.
I thought it might have oxidised so tried some polish, no different so i decided to up the ante and clean it in a mild solution of citric acid.
Its come up lovely, a beautiful silver colour but thats it. The coloured banding has vanished completely.
Short of remelting it, i assume there is no way to recover the colours?
Thanks.
Cornz.
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alfredo
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PostPosted: Nov 05, 2018 14:59    Post subject: Re: Bismuth lost its colours.  

The short answer is no, you can't, or at least not easily. Those colors are an optical thin film effect, produced by an extremely thin oxide layer on the surface, just a few molecules thick, and you've now cleaned that layer off. So what you're seeing now is the TRUE color of bismuth, silver with a very slight pinkish tint, which I find preferable to the fake rainbow colours.
if you really want to reestablish the colourful oxide layer, you could try giving it a bath in some oxidizing liquid, but lots of experimentation will be needed to get the right thickness, because a tiny change in the thickness of that surface film is going to make a big difference to the colours you see, or whether you even get any colours at all.
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Recovering - Academic




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PostPosted: Nov 05, 2018 15:04    Post subject: Re: Bismuth lost its colours.  

cornz wrote:
Hi all.
I have a lovely piece of bismuth but all its colours have disappeared.
I thought it might have oxidised so tried some polish, no different so i decided to up the ante and clean it in a mild solution of citric acid.
Its come up lovely, a beautiful silver colour but thats it. The coloured banding has vanished completely.
Short of remelting it, i assume there is no way to recover the colours?
Thanks.
Cornz.


Hello:

Bismuth melts at 271 degrees C. Try heating it to about 150 degrees C (electric cooking oven) and keep the Bismuth at this temperature to see if the colors return. If not try increasing the temperature but avoid going too close to the melting point (<210 degrees C).

When I have cast Bismuth I have noted that the colours occur just after the melt solidifies.

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“The further a society drifts from the truth, the more it will hate those that speak it.”

― George Orwell
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Pete Modreski
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PostPosted: Nov 05, 2018 18:10    Post subject: Re: Bismuth lost its colours.  

I never heard of those colors on bismuth fading before, so, wonder why that happened to yours?
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mmauthner




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PostPosted: Nov 06, 2018 01:32    Post subject: Re: Bismuth lost its colours.  

Pete,
The same thing happens to the iridescent colours of the Yukon (Husky Mine) stephanites and polybasites. Actually, all of the minerals from there that have the iridescent coating...
Anyone that has such a specimen is advised to keep it in the dark if one wants to preserve those colours.
Mark
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