What Is The Name Of The Acid Used To Test Silver?

Fairly certain that'd be Nitric you're thinking of.

But, from what I've heard/read, Nitric Acid can destroy a ton of a coin's value...so, removing solder (although you're not going to be purposely putting acid on the coins), be careful, whatever you do.

The only acid that will attack gold is aqua regia, which is a concentrated mixture of hydrochloric and nitric acids.

Concentrated nitric acid is red fuming, poisonous, highly corrosive, explosive, and most probably carcinogenic as well.
Besides all of that, it darn dangerous to handle!
For these reasons, it is not obtainable unless you have an industrial license.

Problem with the gold coins mentioned here is the fact they are 10% alloyed with copper, which IS subject to attack by other acids than aqua regia.
I certain that the surface of the gold coins will be affected by any acid that may be used for the testing of silver.

Probably the best way to remove the coins would be by mechanical means. Unfortunately, evidence of the work would also be reasonably obvious. Many are the times I have seen an auction lot description: "Removed from jewelry mount".

Well, highly toxic acid: deadly, explosive, and heavily regulated by the government...

or going at it with a hammer and chisel...

I'd like to add my crazy idea... if it's solder why not take a soldering iron to it? Soft solder is typically thought of when solder or soldering is mentioned, with a typical melting range of 90 to 450 °C (190 to 840 °F). Melting points for Copper 1084C and Gold, 24K Pure 1063C.

Do a little more research, maybe find a welder in your area or ask a jeweler if they can help it with out charging you tons, like I said a crazy idea, but not much worse than the others

is Hydrochloric on the copper dissolving list?

It depends on what the composition of the solder is.

The main metals that that are found in solder, to my knowledge, are Lead, Tin and sometimes Silver.

I know that the Vaporization point is much higher than the boiling point but...

Lead Boils at roughly 1740 °C
Tin Boils at roughly 2270 °C
Silver Boils at roughly 2210 °C

and gold at melts at roughly 1064 °C

so... if someone tries to melt and then vaporize solder even of it is made of just lead (which has the lowest boiling point of the three main metals listed above) they would end up melting the coin and producing poisonous gas which is produced by the lead.

I would recomend just using a soldering iron and melting the solder so the coin can be removed from the necklace. you may never be able to fully remove the solder from the gold coin but at least you can remove the coin from the necklace along with some of the solder.

Three crazy ideas, don't be to hasty to try any of them, pics here would be nice, and try to ask local professionals. Please keep up posted, with before and after pictures.

GOLD Luck to you

Edit: WAIT... Fourth crazy idea, try all three, a drop of acid, then touch it with a hot soldering iron then take the pliers to it, always think outside of the box, unless it's a PROOF SET!

Let me just add, that nobody should attempt to make their own acid here. You'll get spills, it's less expensive to buy it, and it has a shelf life of 3 months ideally. After 3 months your acid seeps out of the containers which is why they sell them in wooden boxes and fill the baggies with sawdust. If you have the acid anywhere near steel tools they will get eaten up just by living near the acid.

Soldering iron is for low melting points. And your tip had better be super clean or anything on it will give off fumes.

If your metal is boiling, that means it's already melted. I could give you some pointers, but really, just take it to a jeweler, who will be less likely (than you) to melt the entire piece. I'd say your gold coins are done for. If you took them to a refinery they would account for the solder on there (lower karat). Boiling metal is bad for it; there's a threshold.

Ideally you want to do no damage to a coin, which is why there are bezels. But garage hobbyists will insist on drilling holes and using tin solder on gold when there are a zillion better ways to mount a coin. My guess is that the coins were viewed as scrap, or just canvas. Is the solder all over the place, or is it visible only at the joint? Whoever soldered the coins already damaged them, so balance out how badly you want them out of there.

Desoldering - extremely difficult. The reason being that the heat conducts very quickly through all the metals in the piece, with a standard iron I doubt it would be even possible. Ever try to remove solder from a copper pipe?