>>27254long time Sigg user here.
They can handle getting filled with boiling water.
But not actually boiling water in it:
They are aluminium and have a coating of Paint on the Outside as well as a poating of some kind of Resin on the inside.
So putting into a fire is a noGo, unless you are happy with all kinds of chemicals in your water and havin a molten bottle.
If you want to boil water in your canteen, get a Tatonka bottle in stainless steel.
That's what i did
