The science behind candle flames is fascinating and complex, involving a number of physical and chemical processes.
At its most basic level, a candle flame is the result of a chemical reaction between the wax and the wick. When the wax is heated, it melts and travels up the wick, where it is vaporised by the heat of the flame. The vaporised wax then reacts with the oxygen in the air, producing heat, light, and vapour.
One of the most important factors affecting the flame is the wick size. The wick acts as a carrier for the melted wax, directing it to the flame. If the wick is too small, the melted wax will not be able to reach the flame, causing the candle to extinguish. If the wick is too large, the flame will be too big, and the wax will burn too quickly.
The temperature of the flame is another important factor, as it affects the size and brightness of the flame. The temperature is determined by the heat produced in the reaction between the vaporised wax and the oxygen, as well as the heat absorbed by the wick and wax. If there is not enough oxygen around the candle, the flame will be smaller and less intense, which eventually will tunnel the candle.
Finally, the colour of the flame is determined by the chemical composition of the wax. Different waxes produce different colours, ranging from yellow to blue to green. The colour can also be affected by the presence of impurities, such as dirt or carbon, which can cause tint the flame more of a yellow colour.