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Thread: Black oxide on silver after melting

  1. #11
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    I don't think there is really an ideal , just the lowest temp you can get a complete casting from. With my kiln which may not be calibrated I can go down to 530 ish for heavy stuff and If its lots of filigree type work as much as 600 ish. Sorry I cant be more specific but its a bit of trial and error to find the perimeters. You should be able to pickle the black off the casting.

  2. #12
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    Quote Originally Posted by josef1 View Post
    I don't think there is really an ideal , just the lowest temp you can get a complete casting from. With my kiln which may not be calibrated I can go down to 530 ish for heavy stuff and If its lots of filigree type work as much as 600 ish. Sorry I cant be more specific but its a bit of trial and error to find the perimeters. You should be able to pickle the black off the casting.
    How long do you leave it at 530 degrees centigrade? By the term "filigree" do you mean if there was a very detailed wax pattern in the flask and consequently it requires a higher temperature to manifest these details after casting?

  3. #13
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    I usually soak the flask for 2 hours minimum. Yes very fine detail the hotter flask temperature keeps the metal liquid longer so it can fill all the smaller voids.

  4. #14
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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.
    What do you advise me regarding "warm safety pickle"? What kind of products? Sulfuric acid is certainly very effective but it seems to me possibly dangerous as a solution.

    In the casting scraps to be melted again, if a minimum of plaster investment remains, is there a way to remove it or should it be done in the crucible by removing the superficial slag with some tool?

  5. #15
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    Sulphuric acid definitely wouldn’t be classed as safety pickle . Many of us use either alum or a safety pickle from Cookson or other jewellery companies

  6. #16
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    Safety pickle is usually sodium bisulphate (NaHSO4). In water, it dissociates to form a highly acidic solution. It's safer to handle in its dry form than sulphuric, but the aqueous solution isn't that much "safer" than dilute H2SO4.
    In the context of slinging molten metal around, it doesn't seem to me to be the thing to worry most about.

  7. #17
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    Quote Originally Posted by ps_bond View Post
    Safety pickle is usually sodium bisulphate (NaHSO4). In water, it dissociates to form a highly acidic solution. It's safer to handle in its dry form than sulphuric, but the aqueous solution isn't that much "safer" than dilute H2SO4.
    In the context of slinging molten metal around, it doesn't seem to me to be the thing to worry most about.
    We weren’t asked how to dispose of it that’s another matter entirely! I just remember at ECA a large sulphuric acid bath steaming away all day in the workshop a few feet away from my bench. There was no health and safety in the 70s

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