On Sat, 6 Apr 2013 18:00:44 -0700 (PDT), Frank Krygowski
>> But, as with Next, it could be magnesium, which burns
The autoignition temperature of magnesium is 473C. It takes some
effort to get it to that temperature. The problem is that magnesium
is a fair heat conductor, and will therefore try to cool down a hot
spot. What usually happens is that it looks like it's starting to
burn, but then goes out. After a few tries, the same thing happens
over and over. Then, usually out of nowhere, the magnesium
autoignites and burns at about 3000C. Until the magnesium gets to
that temperature, it won't burn.
>Actually, it turns out burning a NeXT chassis was never very easy at
>all. Here's an account of an attempt:
>- Frank Krygowski
Several problems. I already mentioned the heat spreading and
subsequent cooling problem. Cast magnesium is never pure magnesium.
It's usually an alloy of aluminum, zinc and other elements,
where the non-magnesium contents can be as high as 15%. Usually,
Aluminum is 5-12% of the mix, resulting in higher heat conductivity,
which makes autoignition more difficult. That's probably what
happened in the NeXT Cube video.
Someone usually brings up the common magnesium "fire starter".
These are also not pure magnesium. The magnesium content can vary
from 20-80%. The remainder is a mix of pyrophoric (ignites
spontaneously when exposed to air or water) material, designed to make
it easier to ignite. With the fire starter, the sparks ignite the
pyrophic material, which then ignites the magnesium. Casting a
chassis or bike frame out of this stuff would be rather dangerous.
However, there's a problem. Some of the Chinese clones (Coleman)
Doan brand works.
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