I went googling for shaved magnesium, and ran across this first, from Wikipedia:
"Solid magnesium bars are impossible to ignite under normal conditions (you need a welding torch, and even then it's quite difficult), and are thus very safe to carry. Magnesium powder and shavings are pyrophoric (they oxidise rapidly when exposed to the air). It is dangerous to carry pre-shaved magnesium — at best, it loses potency, at worst, it can spontaneously catch fire and is then nearly unquenchable."
Is it simply mixing with oxygen that makes them catch fire spontaneously? So... pour out of bottle into cleared fire ring, and wait? I wouldn't think there was too much chance of spontaneous combustion within the bottle...
Sue