Material?

[LRB]

  Easy pard. I should have said seems to make a good spring. My lead pot was an old Saeco that could have been off on the temp control. It did finally quit on me. It certainly would do no harm to go at a higher temp, then see if the spring was as strong as you expected. If not, it could be HTed again and tempered at a lower temp. The springs I made were very good, and are still working well on a pretty regular basis, and no one called you a fool.

[Dphariss]

I temper 1075/1095/O1 in oil, usually, the carbon steel can sometimes use water OK.
Then I use molten saltpeter to temper.
When I get a good blue I let it cool. Rate of breakage for FORGED springs is low. well under 10% I suppose I only have one broken mainspring in the tool box. The only ones I have had break recently were castings and they showed flaws.
I have read the British tempered to blue grey, a little past blue and I sometimes get them this hot as well.
With saltpeter I put the heavy parts of the spring in first to soak them longer or the thinner sections will tend to overheat.
Small springs I usually heat over a flame to get the color.
I have used springs in locks that were likely made during the American Civil War, probably Civil War Surplus. I suspect these were all tempered by eyeball. Wonderful springs, strong, fast and never had one break. Though I likely only used 10 or 20 of them.

There is a wide variety of ways that people here do things. Just about everyone has found a way that works for them and while is different than what someone else may do for the most part  the END RESULT is the same.
I for example would only use molten lead as a last resort. Others won't use anything else. So what? So long as the springs don't break how the spring is made is up to the maker.
A good spring is a marvelous thing no matter how its tempered.

Dan