Gun Cotton Through a Multispectral Lens
Observing Nitrocellulose in Visible Light, Infrared, and Ultraviolet
Gun cotton – more formally nitrocellulose – is one of those substances that looks utterly unremarkable until it very suddenly isn’t.
To the naked eye, a small piece of gun cotton looks like… well, cotton wool. But touch a flame to it and it vanishes in a brilliant flash, leaving almost no ash behind. It’s dramatic, fast, and a perfect candidate for looking at combustion beyond what our eyes can see.
This time, instead of just watching it burn, I pointed a multispectral camera at the reaction and recorded it in visible light, infrared (IR), and ultraviolet (UV).
🔥 What’s Actually Happening Chemically?
Gun cotton is cellulose where some of the –OH groups have been replaced with –ONO₂ (nitrate ester) groups. That means:
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Oxygen is already built into the fuel
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Combustion doesn’t rely on air diffusing in
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The reaction is extremely fast
In effect, the material carries both the fuel and the oxidiser in the same structure.
👀 What We See in Visible Light
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A sudden white-yellow flash
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Very little smoke
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Almost no solid residue
It’s so fast that high frame-rate video is essential if you want to see anything more than a blink.
🌡️ What Infrared Reveals
Infrared imaging tells a completely different story.
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A rapid temperature spike as the reaction front propagates
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Hot gases expanding outward after the visible flash has gone
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Heat lingering in the air, even though the cotton has vanished
This is a lovely teaching moment:
👉 The reaction is over visually, but thermally it’s still very much happening.
🌌 What UV Shows Us
Ultraviolet is where things get really interesting.
Under UV-sensitive imaging:
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The flame structure appears more extended
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Excited molecular species emit strongly outside the visible spectrum
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The reaction zone looks larger than it appears to the eye
This helps explain why nitrocellulose reactions are so energetic despite appearing “clean” and residue-free.
🧠 Why This Matters for Teaching
This experiment is a brilliant reminder that:
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Our eyes are terrible scientific instruments
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Combustion isn’t just “fire” – it’s light, heat, radiation, and chemistry
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Multispectral imaging turns a simple demo into a rich data source
It’s exactly the sort of thing that makes students stop and say:
“Oh… I didn’t expect that.”
⚠️ Safety Note (Always!)
Gun cotton demonstrations must be:
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Done with very small quantities we used less than 0.25g
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Conducted in a clear, controlled space
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Supervised by someone who understands the chemistry
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Filmed at a safe distance (another win for cameras!)
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