What is soft solder repair?
Solder is the name given to any metal alloy with a lower melting temperature than the two metals being used to join together. This means that almost any metal can be used as a solder if it has a lower melting temperature than the two metals and can flow properly into the cavity between the two. A concrete example of this using precious metal soldering is using different karats of gold. 10K yellow gold and 14k yellow gold each has their own solder. But if a goldsmith was working on a 14k yellow gold piece and ran out of that solder they could use a piece of 10k yellow gold in place as it would melt at a lower temperature to the 14k gold and would flow into the cavity between the two pieces of 14k yellow gold. Gold works well as a solder because it flows well and becomes almost liquid when it melts. All precious metal solders work this way.
There is another class of solder called soft solder that is used when pieces cannot take a lot of heat and or precious metal soldering is not important. Soft solder does not become liquid when heated (poor flowing properties) in the same way as precious metal solder but it does melt and adhere to the surface of the metals. Because soft solder does not fill the small cavity between metals but is instead lumped on top, the bond is not as strong. There are times when a piece of jewellery cannot be fixed by precious metal soldering because the piece cannot take any heat. Soft solder may seem like the answer but the bond is not strong enough to make this a good solution. This type of solder can also have a corrosive effect and can eat away at the metal. Soft solder usually contains lead and tin as the main ingredients which also means they react to the skin, lending further reason not to use them in precious metal jewellery.
’20 Tips on maintaining our Jewellery’, Gillian E. Batcher, 2013