I found out about this from a chemistry master at school, who used a long rod to poke his fireplace. It occasionally sputtered and sparked for his entertainment.
It was the magnesium anode from an old water-heater.
If you can break the retaining nut loose (it tends to corrode pretty heavily), you can pull the anode out of an old unit. The material isn't all that pure (certainly not reagent grade), but it's high-quality enough to ignite.
