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Thread: Black oxide on silver after melting

  1. #1
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    Default Black oxide on silver after melting

    Hello everybody,
    when I pour molten silver (I usually use 800 silver) and pour it into plaster cylinders for lost wax casting, the silver oxidizes and becomes dark in color similar to black. Conversely, when I pour the silver into the sand casting mold (delft clay) the silver after it has cooled has a light yellow color without showing all that oxide that instead occurs in the other type of casting. I can't understand why this black oxide occurs and how to do to prevent this from happening. I state that I use a melting furnace with graphite crucibles and I use borax. Thanks in advance for your help.
    Attached Thumbnails Attached Thumbnails photo_2022-07-18_11-47-45.jpg   photo_2022-07-18_11-47-46.jpg  

  2. #2
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    Unlike sand, casting investment contains sulphur, so its no wonder that the surface of your casting is black with sulphides, both of silver, and the 20% base metals contained in your alloy.
    However this should present no problem, as it would easily dissolve in warm safety pickle, or diluted sulphuric acid.
    Dennis.

  3. #3
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    Quote Originally Posted by Dennis View Post
    Unlike sand, casting investment contains sulphur, so its no wonder that the surface of your casting is black with sulphides, both of silver, and the 20% base metals contained in your alloy.
    However this should present no problem, as it would easily dissolve in warm safety pickle, or diluted sulphuric acid.
    Dennis.
    Seeing some videos on youtube, why for many silversmiths the melting of the silver inside the flasks is not oxidized but the silver is clear immediately after the breaking of the plaster coating inside the water?

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    Don't know, but keep an eye on this space, because there will be more answers in a week or two. Dennis.

  5. #5
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    Quote Originally Posted by Dennis View Post
    Don't know, but keep an eye on this space, because there will be more answers in a week or two. Dennis.
    Thanks Dennis for your reply, I will be waiting for any other replies

  6. #6
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    I did answer last night but it seems to have disappeared!
    Not my area of expertise but I doubt anyone can explain the different outcomes you’ve found on YouTube videos by other silversmith.
    I did wonder as your silver is only 80% pure in terms of the 925 we are used to if the other alloys are causing the oxidisation?

  7. #7
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    My thoughts it is a low grade Silver my have caused the result should be easy enough the remove as said above, I think the most likely cause is that it was over heated.

  8. #8
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    Quote Originally Posted by china View Post
    My thoughts it is a low grade Silver my have caused the result should be easy enough the remove as said above, I think the most likely cause is that it was over heated.
    I use 800 silver because I have a lot of scrap to melt. I don't think the metal can be overheated since sand casting with the same type of metal does not show the oxide. Perhaps it could be the excessive temperature of the inner core of the flask. These are all hypotheses, however, I don't really know.

  9. #9
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    I've not cast with 800 but with sterling if its black I take it as the metal or the flask was to hot. You can put a charcoal block over the flask as soon as it poured this may help a bit. It should come off with pickle.

  10. #10
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    Quote Originally Posted by josef1 View Post
    I've not cast with 800 but with sterling if its black I take it as the metal or the flask was to hot. You can put a charcoal block over the flask as soon as it poured this may help a bit. It should come off with pickle.
    What should the ideal temperature be for the flasks to avoid blackening?
    Last edited by mdk; 21-07-2022 at 06:38 PM.

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