21mm sterling silver padlock clasp, i want to solder a jump ring to it but im worried that even if the spring on the inside of the clasp doesn’t melt, will the spring anneal and cause the mechanism of the clasp to fail early on in use?
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The temperature of hard soldering will likely draw the temper of the spring making it lose its springiness. If the clasp were made of karat gold or platinum you might have a chance, but since silver is such an excellent conductor of heat it will be nearly impossible to keep the spring from over-heating.
However, laser or spot welding isolates the heat to the immediate area, so the spring would stay springy. If there’s someone in your area with a laser or puk welder take the clasp to them.
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Thank you Elliot , sounds like a nice intro project for the laser
I’m not sure what you mean by a “padlock clasp”, if you mean a lobster clasp, I would approach this task as follows since the 21cm lobster clasp is relatively large, and if you are using a medium to large jump ring, you can wrap a wet tissue around the clasp. Using tweezers to hold between the clasp and jump ring. Where the joint for the jump ring is rotated away from the clasp, place a small clip of easy silver solder that is held by tension of the jump ring. (The clip of solder, is fresh, un-heated, so it will melt at the lowest temperature.) I’m right handed, so I would hold the tweezers and clasp in my left hand with the jump ring on top and the clasp wrapped in wet tissue below. With your right hand, place a small drop of flux on the joint. (per set the flame) With your right hand, take your torch and burn off the flux with a quick brush of the flame, then using a hot small flame ( not overly intense) I’d bounce the flame at the joint until the solder melts. The direction of the flame is away from the clasp. Good luck.
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21mm not 21cm 
The padlock clasp is quite literally a padlock in design , there is a button on the bottom that causes the spring loaded mechanism to release the padlock like clasp. Pretty cool.
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Could you provide a picture of the clasp, it would help see how it works.
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my bad ~ Ive been working a little bigger pieces lately ~
You might look at the clasp and see if you can somehow remove the spring and then solder on the jump ring, returning the spring when you are done. It will lose its hardness from soldering the jump ring. Otherwise, you might look for someone with a laser welder or pulse arc welder. The only experience that I have with a steel clasp spring is finding one in a silver melt that I had missed when I was preparing the scrap to be recycled. It reinforced in me the habit of stirring the melt with a graphite rod. Good luck…Rob
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I want to solder the jump ring on the bottom next to the button