I have about eleven of these. After buying them, in 1999, (guess why?) discovered that while they work well, they seem to have been designed to work indoors. This page has some cautions and precautions. Paul.
Quote from, “Fire – A Survivor’s Best Friend – Fire starters, Tinder, Stoves..”
“The magnesium block by Doan (1 x 3 x 3/8 in.) with an integral 3/16 in. flint rod glued on top (also manufactured for other companies and for the military) is still popular, at least in part because it is widely available at the consumer level.
Unfortunately, the magnesium is not necessarily as effective a tinder as you may be led to believe. While the flame from the magnesium shavings is extremely hot, this flame is also relatively short lived.
Additionally, the magnesium scraping/ shavings can be difficult to use as tinder in some circumstances because they are so light they tend to blow away with the slightest breeze.
Moreover, scraping together a small pile can be difficult at times. So, the bottom line is that they work, but the magnesium tinder has some limitations that must be understood and dealt with. ” …..
Quote from Old Jimbo– http://www.oldjimbo.com/survival/rob.html