When chemicals burn, it is really a chemical reaction between the substance and the oxygen in the air. When substances do not get enough oxygen, they don’t burn completely. Below is a video of 30 ml of flour exposed to a flame. Only a small fraction of the flour burns, while the rest does not. The surface area of the flour is quite small and needs to be spread out.
If the flour particles spread farther out from each other, more oxygen will be able to react with them. The following demo does just that.
The three pictures below show the required materials.
Place the pie tin with candles on a chair so that the pie tin sticks out a few feet. The meter stick can be kept in place by placing a few books on it. Measure out about 50 ml of flour and place into the sifter. Stand on a chair and sift the flour so that it falls onto the candles. Caution: Wear goggles, lab coat (or apron), and keep flammable materials away. Have a fire extinguisher close by just in case. Do not use too much flour!