Hello,
Here are two extracts from some related material.
“” The tin in molten solder rapidly dissolves gold. If sufficient gold is dissolved, brittle intermetallic compounds can be formed in the solder joint. “”This refers to soldering of wires to gold
plated contacts (Lloyd’s comment). The problem here is not the brittle joint on soldering, which normally is very quick. It is that the gold is dissolved, especially with extended heating since both solder and coin have to be heated. And this happens at soldering temperatures.
“” Gold and lead readily form a wide range of alloy compositions. Lead has a high affinity for gold and can be used as a collector for gold in pyrometallurgical processes. The lead can be subsequently separated by volatilization.””Alloys of gold/lead are known to occur down to 240 deg C, within the range of a decently powered soldering iron.
I
still say don’t put any heat to the coin. You will only aggravate the condition.
As to mechanical scraping against chemical/acid etching, consider first some facts. You have effectively four metals in various states. The coin material which is gold alloyed (for hardening) with probably a % of copper (plus maybe silver). There is the tin/lead solder layer. There is also a gold/lead/tin alloy/s interface. Lead is quite passive in battery acid, so its left on the coin. And there is risk of damaging the rest of the clean surface by copper removal.
On the other hand solder is soft (but so is the gold) so I think a
sharp blade, a steady hand and a
bit of luck, will be my
choice for some remedy.
I have tried the process on a silver sixpence of George IIII from a broken cufflink. It was silver solder which was removed with mixed results, for where the
alloy was the discolouration persisted. The alloying elements
had diffused deeply.