chornedsnorkack
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Autoignition of gunpowder
What is the real autoignition temperature of gunpowder?
Your wiki:
http://www.sciencemadness.org/smwiki/index.php/Black_powder
claims:
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The standard black powder has an autoignition temperature of 464°C. |
However, the problem is that one component is sulphur:
https://www.teck.com/media/Sulfur.pdf
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Auto-ignition Temperature:232°C |
If gunpowder is heated without contact to sparks and flame, the sulphur should melt and separate at 115...120 Celsius. And the vapours of sulphur
should autoignite already at +232, not at +464.
Or what really happens?
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Herr Haber
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Read Urbanski !
He explains it a thousand times better that I ever would but basically sulfur acts as a tinder for the other two.
You cant assume that a mix of several chemicals will ignite at the lowest decomposition temperature (or highest for that matter) of said chemicals.
The spirit of adventure was upon me. Having nitric acid and copper, I had only to learn what the words 'act upon' meant. - Ira Remsen
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Junk_Enginerd
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Maybe the answer lies in that black powder might have two different autoignition temperatures? Consider blackpowder encased in a mostly oxygen free
environment, such as a tightly packed casing. The sulfur would not be able to burn without oxygen even if it was heated to its autoignition
temperature. I don't think KNO3 decomposes significantly at 232°C, so it would not be providing any oxygen for the sulfur.
I think pin pointing an exact autoignition temperature would be impossible. If the temperature was slowly increased I think it would be a matter of
probability and increasing instability. At some point enough KNO3 will have decomposed to allow one spot of sulfur to start reacting with the oxygen,
and the chain reaction will start.
If there's oxygen available from the start, like if it was a pile of black powder in open air, then the autoignition point would likely be closer to
that of sulfur's.
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Herr Haber
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I thought Conkling and wrote Urbanski.
There's a short discussion there:
https://rec.pyrotechnics.narkive.com/yh2c2Oa1/tamman-tempera...
With most of the thoughts and answers revolving around OP's question.
Certainly worth a read if you are into molten compositions.
That being said, a Red Bull can perfectly houses a pound of molten KNSO. Do not repeat with table sugar 
The spirit of adventure was upon me. Having nitric acid and copper, I had only to learn what the words 'act upon' meant. - Ira Remsen
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chornedsnorkack
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Interesting. It suggests that gunpowder autoignites slightly above melting point of potassium nitrate because melting promotes "intimate mixing".
Does it mean that sulphur is appreciably soluble in molten potassium nitrate?
Because otherwise melting should allow sulphur (low polarity, less sense) and nitrate (more polar, denser) to separate.
Potassium nitrate melts at 334 Celsius.
Sodium nitrate is regarded as worse than potassium nitrate for gunpowder because it is more hygroscopic. Nevertheless, the inferior sodium nitrate
gunpowder with its limitations is commonly used.
Sodium nitrate melts at 308 Celsius.
Eutectic mixture of potassium and sodium nitrates melts at 238 Celsius. 96 degrees lower than potassium nitrate.
How does the autoignition behaviour of gunpowder rich in both potassium and sodium nitrate compare to gunpowder containing only one of those?
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