Featured Gold Jewelry and Tarnish

This is just a question thread, not to identify anything. Occasionally, I'll see pieces of antique jewelry that I think might be gold, but that have a more brassy tone to them than I'd expect. I've backed down on buying marked rings and unmarked pieces because of the tarnish. But now I'm wondering what sort of tarnish I can expect from gold jewelry.

For example, all of these pieces have some tarnish and are unmarked, but look like they might be gold. How can you infer that something might be gold? Any tips or tarnish level scales?

 

Since most gold jewellery is not 24K, it can discolor with time. This is actually at least 14K, but you would never know to look at it:



I know I can't reliably tell low karat gold from any of the other things that can look like it when I have a piece in my hand, much less when there are only photos to go by. Nor can I distinguish high quality gold from something that has been gilt/plated if the coating is unblemished & oxidation from the underlying metal has not 'bled through'. There's a particular look I associate - rightly or wrongly - with gilt silver, the grey/blue/purple of silver tarnish glinting through the gold. Think @Hollyblue could give us guidance on this, although seems to me she usually recommends 'get it tested'.

 

@Hollyblue schooled me on this subject as follows "24K will not tarnish,but the other Karat types alloyed with copper will tarnish." Sniped by Bronwen and her superior knowledge.

 

That's a stunning pendant. From its color, I would have been totally clueless about its metal type. (If I had seen it for sale, I'd have probably assumed it was some sort of gilded silver, because it's so dark). I feel like usually I can get a fair idea for a jewelry piece's make up based on the quality of the piece, but it's harder with like plain rings...

 

Nah, just superior attitude.

 

It's all chemistry, folks. I would think that in the chemical/metallurgical literature journals there might have been some studies done on the rates of oxidation, that would narrow down categories of age for a metal. Interesting project to take on... nope, not today.

Remember, boys and girls:
tarnish = oxidation = adding an oxygen molecule
like ordinary rust:
Iron + Water + Oxygen = Rust
i.e. 4 Fe + 6 H2O + 3 O2 → 4 Fe(OH)3
, and so iron rusts, dehydrates and you get Fe2O3.

24 K Gold = elemental gold = uniquely stable element that has no place for O to attach.
Alloys = molecular conglomerates [a solid solution, if you can imagine] of metals, like Cu, or Zn, or Sn added, which bind with Oxygen... so, Cu++ will bind with O-2... and it gets more complicated when you add other compounds...
"Verdigris is the common name for a ... natural patina formed when copper, brass or bronze is weathered and exposed to air or seawater over time. It is usually a basic copper carbonate (Cu2CO3(OH)2), but near the sea will be a basic copper chloride (Cu2(OH)3Cl).

[sorry, no superscripts or subscripts in the font so the formulas are hard to read]

So, for now, 'going by eye' maybe as effective, and "superior knowledge," as studying years of metallurgical chemistry on oxidation rates and colorimetry.

 

I have this pendant, earrings and chain sold long ago, all test as 14k and nothing is marked. I tested them multiple times because the gold is dark.

 

Good idea for a thread Barn Owl.

 

often "dark" = Sulfur exposure. so instead of reacting with O only, there's a "sulfide" group that attached to the alloy.

so, looking that up, I just learned that:

"Organic sulphur-containing compounds, present in the materials of storage boxes, are another known source that can cause tarnishing." Also in foods, like onions and pickles.

https://www.ganoksin.com/article/gold-jewellery-tarnishing/

Hm. whodda thunk that. might be a wood preservative, perhaps? Paints? Should they have a legal warning: "Caution: jewelry box may contain sulfur that damages your jewelry." Yeah, that'll sell.

"Recently, World Gold Council has had an opportunity to examine examples of blackened 22 carat gold returned to retailers in India. The analysis of the blackened layer has shown that it comprises silver (and copper) sulphide. This is a true tarnish layer. That such high carat golds should tarnish is unexpected and points to the jewellery being exposed to a particularly ‘corrosive’ environment at some stage. Perhaps, it is due to being worn during food preparation (some foods & spices are very high in sulphur compounds). "

 

A related issue comes up in the shell cameo world, so-called 'Byne's disease.' Some materials - oak is a particular culprit - provide just the right molecules to mix with atmospheric moisture & create acid that corrodes calcium carbonate.

For an explanation of what happens:

https://www.nps.gov/museum/publications/conserveogram/11-15.pdf

For cameo examples:

https://cameotimes.com/index.php/reference/byne-s-disease

 

In that same archeological revival style as my pendant, which is a locket. Would not be surprised if some patination process was applied to enhance the fresh-from-a-dig look.

 

Aha! scanned the cameo articles and found "cardboard"... now there's a 'call in the usual suspects' candidate. https://theassayoffice.com/tarnish_labexpert

lots of volatile sulfur in cardboard. Don't leave it in the crappy white box you bought it in! So Bynes and Gold tarnish have that in common.

 

Reiterating @Lucille.b: "Good idea for a thread @Barn Owl." thanks!

 

Since one of the Byne's-causing substances is formic acid, keep your jewellery away from ants, too.

 

@Bronwen -- Houston, we have a problem:

 

Don't leave it near eggs or bananas either.

 

Barn, very good question, and as you see, there is no simple answer, except get a testing kit.
The first one you showed is almost certainly gold, the others could be.

This one looked like tarnished brass, so could have been tombak or something resembling pinchbeck.
It was cheap, sold as costume, but it turned out to be 8k gold (Austro-Hungarian, they often had low finenesses). I suspected as much, given the delicately enameled details, which you wouldn't see on Bohemian costume.
There is still a lot of very persistent black goo which I am still in the process of cleaning out, especially behind the central rosette. The cleaned bits reveal minute stippling.

 

For 3.5 euros, I guess the first one was a steal then, heh.

That's absolutely gorgeous! I'll have to keep my eyes out for more AH jewelry... I really love it for some reason. I bid on a pretty AH belt buckle a few days ago, but I missed out on it at the last moment

 

This was the belt buckle I was bidding on...

 

Delicacy of the metal work is something I consider when gauging quality from photos. Pieces of genuine gold or silver tend not be clunky, with a lot of extra metal the way you see in things that are gold- or silver-tone. Does not help with brass/pinchbeck, which also allows for finer work.