Black oxide on silver after melting

[mdk]

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.

[Dennis]

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.

[mdk]

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?

[Dennis]

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

[mdk]

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 :)

[CJ57]

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?

[china]

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.

[mdk]

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.

[josef1]

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.

[mdk]

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?

[josef1]

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.

[mdk]

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?

[josef1]

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.

[mdk]

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?

[CJ57]

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

[ps_bond]

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.

[CJ57]

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