Steel with at least .45 of one per cent carbon will show a small but distinct secondary burst. This is considered the minimum for hardening into a useful knife.
Carbon over one per cent will give an intense and multiple bursts.
Moderate silicon makes a spark that is short and ends with a sharp white flash.
Nickel sparks have a small, very intense and bright white color.
Molybdenum gives a spark with a distinct, separate head.
Sparks are best judged by looking at the last third of the trail. Alloys with several additives will confuse the heck out of anyone
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