G-Kest, bad comparison. The ash you're talking about is essentially soot - material that has already been burned and won't burn again easily. And if it burns it burns rather clean.
What comes out of the vulcano now is SiO2, which is very akin to glass. It will not burn, but rather melt under high pressure and temperature, and when it cools down it will form a glass-like layer on the inside of your engine.
It's a totally different kind of animal.