Jim:

This article is very good. I tried a dozen different scrapers this evening based on the idea that "anything will work as long as it is 'hard enough' to scrape shavings from the ferrocerium rod".

Turns out that hardness and sharpness are indeed key. Items that are sharp but not hard will slide down the rod as you described. Items that are hard but not sharp will do the same. Sharpening something soft gave one or two passes of feeble sparks.

The quality of the ferrocerium rods themselves didn't seem to matter as much (the sweedish ones seemed slightly better than the cheap ones attached to the walmart magnesium blocks).

At the end of my test, the toothed side of a hacksaw blade did indeed perform best. Note that carbon steel isn't required and that the likely problem with your can openers is that they're too soft (i.e. steel, but not hardened). Sharpening may give you a few strikes, but will be unreliable for field use.

As far as speed goes, with the right combination of sharpness and hardness, it'll be nearly impossible to not have a major spark-fest.

Of course, as with anything, I could be full of sheep dip... or your rods are impotent. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />