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Gold surface structure

Hi. I was recently looking at a gold plate, recent date and about 22-karat gold. The gold sheet was about 1 mm thick. The surface structure puzzled me. Any thoughts? My guess is acid etched, but I haven’t seen anything quite like this before. An alternative was that it was electroformed, but as the rather 2-dimensional decorative elements were soldered flat onto it on, not integral to it, that seems unlikely. I don’t have an exact scale for the photo, but I reckon it is about 10-12 mm across, so quite a coarse structure - which I imagine also counts against electrofroming.

And talking of acid - has anyone used chromic acid on gold to clean or give a matt surface?

Would love to hear any thoughts. Thanks

Jack!

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I don’t know if it’s just me or not, but I can’t get the photo to load from your link. I’d love to see what this puzzling surface is like!

1 Like


Here it is - don’t know why it didn’t load before… Sorry

What that looks like to me is the dendritic rental structure of gold, in this case with lots of porosity. Knd instance I’ve seen is with casting into a too cool mold, where metal hitting the mold surface freezes, but then subsequent thermal shrinkage of the still molten metal within the body of the casting causes metal starvation at the surface, exposing that crystal structure and pulling metal away again, resulting kn that porosity. But I have only seen this in casting, or pouring some ingots. Your description has me assuming some fabrication, such as rolled sheet metal, which I assume would preclude this texture. Perhaps over annealing, too hot, for too long? If the metal was electrostripped, or bombed, incorrectly, perhaps one of those processes might do this too…
Peter Rowe

Thanks, I agree Peter, looks like a cast structure, but it is a dish in sheet gold just 1mm thick and appears to have spinning striations on it… I assume it was spun, annealed and then strongly etched. XRF shows up traces of chromium everywhere - so my guess is that chromic acid or chromic acid + sulphuric acid was used (chromic acid does dissolve gold). Such mixtures are often used in industrial processes and, I believe, for stripping gold from electronic components. I can’t post a photo of the whole object as it is the center of a court case (was sold as ancient).