The guest of honor grins at the flickering candles upon the birthday cake, internally makes a wish, and extinguishes the flames in one breath. For a moment, everyone is cheering, the guest of honor smiles at the trailing wisps of smoke, and the flames seem to have vanished for good. But suddenly, a flash of light reappears - before long, the ring of candles is alight once more, the flames dancing obstinately. The guest of honor, bewildered, continues to blow, and is soon out of breath from battling the seemingly inextinguishable flickers of light.
Of course, birthday candles and trick candles, the stubborn twists on the classics, are both variations on one of our oldest sources of light--the candle. Now used primarily for ornamental or religious purposes, candles are made of a wick and a fuel source. The candle “fuel” that allows the wick to burn and produce light is wax, which is made of paraffin hydrocarbons, or chemicals that only have carbon-carbon and carbon-hydrogen single bonds, such as those in CH4 (methane). Paraffin hydrocarbons are fairly large, with more than 20 carbons per molecule. Paraffin wax is composed of variations of paraffin hydrocarbons, the majority of which are C25H52. If they are melted, these hydrocarbons can combine with oxygen and ignite in a process called combustion. The paraffin is the component that reignites in trick candles and thus makes multiple burnings possible.
The second component of candles is the wick. The wick is made of tightly wound cotton or nylon fibers and is treated with a flame-retardant solution, such as a chemical salt solution (the details of which are kept secret by candle manufacturers), in a process called mordanting to prevent destruction of the wick by the flames. As soon as it meets heat, the wick conducts heat towards the surface of the wax to commence melting. The molten wax allows the candle to combust continuously; the wax is melted so that it can travel upward through the wick using capillary action and burn. The wick acts like a delivery system that carries the molten wax to the oxygen at the top of the candle.
What makes tricks candles so special and different from plain old candles? The secret is the tiny flecks of magnesium powder in the wick. Magnesium flecks are highly reactive and flammable, causing it to burn even at temperatures as low as 430oC. This means that after the flame has been blown out, the glowing, hot ember remaining in the wick can ignite the magnesium and produce tiny sparks, which then reignite the paraffin vapors. The cotton and nylon fibers of a regular wick lack magnesium powder and thus cannot reignite so dependably and rapidly after the flame has been blown out. Trick candles repeatedly reignite because the magnesium in the lower part of the wick is protected from burning prematurely by the paraffin wax.
However, regular candles actually also have the ability to reignite. Unburned wax particles in the smoke can be lit again after a candle is blown out, allowing the flame to return. This surprising property, however, also sheds light on the danger of candles. Candles can be carelessly thrown in the trash, only to reignite and set the trash on fire. Since all candles can reignite, they should be considered hazardous and doused with water after use to cut off the oxygen supply and prevent the paraffin vapors from reigniting.
As entertaining as they are doggedly persistent, trick candles are also a neat and simple marvel of science. Their trick is only a result of magnesium and paraffin wax - mystery solved, no magic involved, or perhaps the chemical properties and processes that contribute to trick candle’s quirks are magical in themselves. Either way, these sneaky candles, invariably followed by cake consumption, will be sure to dazzle, confuse, entertain, and scientifically intrigue at the typical birthday celebration.
Rohrig, Brian. “The Captivating Chemistry of Candles”. Chem Matters. December 2007. http://portal.acs.org/preview/fileFetch/C/WPCP_007150/pdf/WPCP_007150.pdf
Wang, Linda. “Trick Candles”. Chemical&Engineering News. August 2009. http://pubs.acs.org/cen/science/88/8832sci2.html