What is the chemistry behind the reaction of KNO₃, K₂CO₃ and Sulfur?

I'm preparing a chemistry show where I end the show with yellow powder bang.

The powder is a mixture of:

$$\ce{K2CO3 + KNO3 + S}$$ My question is, what's the reaction between these chemicals?

• Not sure about this, but the nitrate might oxidise the sulphur to sulphur dioxide or trioxide. – Binary Geek Mar 18 '15 at 13:28
• It seems that nobody really knows about the role of the carbonate, but there's a nice video in the Free Range Chemistry series of the RSC. – Klaus-Dieter Warzecha Mar 18 '15 at 16:09
• It looks like a variation on a gunpowder mixture which is usually sulfur, KNO3 and charcoal – bon Mar 18 '15 at 18:34
• Yellow powder is said to be unpredictable, so it is probably better to use a different bang. – permeakra Mar 19 '15 at 12:19

$\ce{KNO3 + S -> K2SO4 + K2SO3 + K2S + K2O + SO2 + N2}$
$\ce{K2CO3 ->[\Delta] K2O + CO2}$