Custard Fireballs

Explanation

Burning is just a chemical reaction of the fuel with oxygen.  They can only react where the oxygen meets the fuel. So the more places this can happen at once the faster the fuel can burn.

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The log can only burn at the surface where the oxygen in the air can get to the fuel, so it burns slowly.	The same amount of wood in smaller pieces will have a far larger area for the air to get to it, so will burn in more places at the same time so it will burn much more quickly.

A large object like a log has a large amount of fuel inside it but quite a small amount of surface for it to react with oxygen and burn on. Every time you chop the log in half you create more surface for it to burn on so it will burn quicker, so small twigs have much more surface and will burn more vigorously. Paper has much more surface again so burns very quickly, but the custard powder has a ridiculous surface area. The particles are about a hundredth of a mm across so a gram of custard powder has a surface area of about 600 square metres. so it can burn almost everywhere at once incredibly quickly forming an impressive fireball.

The results of a dust Explosion in a factory in Kentucky © US Chemical Safety and Hazard Board

This has caused many dangerous explosions in flour mills factories etc. when a flammable powder is mixed with air and then meets a source of ignition. Because people don't think of powders as explosive they often don't consider them as dangerous with explosive results.