Oil slick silver casting

In my almost 3 years of casting weekly I have never seen this happen- a type of oil slick surface on my casting.

Does anyone have any idea how this could have happened and how to rE-create it?

This is what I did-

Set up to cast with perforated flask
Using an electric melter and graphite crucible with vacuum casting machine
Used all new anti firescale casting grain (Rio)
Casting came out relatively normal directly after casting

I then put the pieces in the ultrasonic to remove the rest of the investment and then pickled.

Sawed off the sprues etc, used a brass bristle brush in my Fordham to see if I could clean up the surface, grinded down the sprues, then used bobbing compound, threw them in the tumbler, noticed a yellow/red/brownish tarnish happening so I picked it again, brought them to the polishing wheel, and then was completely stumped when I saw this oil slick pattern come up.

Tried doing it to the other pieces but to no avail.

I think my pickle pot had bronze in it prior to using it today, could that be what happened?

So curious, any input is greatly appreciated

Something has definitely created a thin oxide film of sorts.

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What could cause a thin oxide to be on the pieces? Im baffled. Some cross contamination perhaps? But of what?

Not sure, maybe some overheating during grinding.
It looks just like Stainless Steel after welding.

I’m not sure at all what caused it but if you tumbled in SS shot perhaps your shot needs cleaning? I’ve never done this myself but have read about people needing to clean it. As far as recreating it it looks very much like the finish I got on some reticulated silver, almost a black stainless look. I happened upon that accidentally after dipping the metal in liver of sulphur to achieve a very black finish then put it in the tumbler for 30-40 minutes with Shinebrite Burnishing Compound (I ordered this off Amazon because I was too impatient to wait for shipping from Rio) I attached a photo of it. The surface texture isn’t great but you can see how the finish looks. Mine is less blue and more a very shiny black but it looks kind of cool and I’ll probably try it again with some castings just for instance.
Good luck!

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I reached out to some people who work with patinas etc, one of them told me this must be due to an electric current in the ultrasonic. This makes sense to me. Re-creating it, however, is another story! I did watch a few videos on anodizing silver with a battery- here is the link for anyone who is curious

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Thank you for sharing that video! That looks really cool!

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I’m not suggesting this is the cause, but I’d like to mention that the iridescence I see in your pics is exactly the colouration I used to get on fine silver when I exposed it to hydrogen sulphide fumes.

Also, our older books of fine silver leaf (and sometimes white gold) show the same iridescence at the outer edges where they are exposed to the patina solutions we use (all based on hydrogen sulphide).