I tried to use the mirror in that manner, but I don't think the magnification is quite strong enough. It may work, and in a moment of desparation, I might sit there long enough to try it, but with the other means of starting a fire at my disposal, using the mirror would be pretty far down the list. I honestly would rather used the good ol' rub 2 sticks together method before trying the mirror.
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If only closed minds came with closed mouths.