Swamp,
Some nice comments; I think your idea that the mag block is way hotter is correct.
I too have been playing with "natural spark" fires. Everytime I go off in the woods I bring the char cloth and try different rock combos to try and get an ember on the char cloth. Mostly you need hard rocks. Here in the Bay area there is not a lot to work with but I keep searching.
Bill

LW,
Ideally the char cloth would get a glowing area that would quickly grow. Once it catches a spark it glows hot and spreads fast but if you only use a small piece to ignite a tinder bundle a 4 inch X 4 inch piece could last for dozens of fires. But char cloth is not reusable in the sense that you can reuse the area that actually catches the spark. The hot area burns up fsat so you need to use a little piece of the cloth and get it into a tinder bundle, then start blowing.
Bill

Sounds like ignited magnesium shavings and char cloth equals FIRE!

Pyros unite! We are not sociopaths were are the keepers of the lost fire knowledge. Go burn something today!