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Sorry, when I mentioned the pic I was referring to your pic of the spring.
You could test in the shim stock with a piece of wire or material you’re making the spring out of to see if you get the hardness you’re after. It’s certainly worth a try. Brass has a slightly lower melting point than copper so may burn through that .01 shim if not careful. I usually wrap my copper boxes with iron binding wire and watch the color of the wire as well as the copper. Generally I bring the box and wire to a cherry red (shade depends on past results with type of steel) and keep it in the flame long enough for the heat to transfer into the part. If the box or your shim wrapping is just slightly larger than the part inside, the heat will transfer into and throughout the part very quickly. Extra space inside isn’t a good idea as cooling may not be quick enough for the part when quenched.
Motor oil works well in most cases for watch parts. Also thinner vegetable oils or olive oil is good. When heat treating parts by hand the color and heat transfer time is more or less a judgement call based on experience with the steel and technique you’re using. It appears rather primitive to machinists/toolmakers who heat treat steels using ovens and industry quenching oils. An oven can be set to the exact temp that is recommended for a particular steel and the length of time to leave it in the oven can be determined by the area of the part or parts being treated. Whereas making watch parts is much more ‘hands on’ and timing is everything. That’s the fun part though and once you’ve done it a few times the success rate really gets up there!
Again would be a good idea to test a few pieces first. Test, test, test…
A few tests should reveal the best way to go about it.
Please let us know how you do it and the results before and after you’ve tempered it Paul,
Good luck and have fun!