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People have been burning candles for centuries and the idea that candles may cause smoke is not a new one. When the European church community raised this issue, scientist Michael Faraday rose to the call and lectured on combustion of a candle in 1861. “Ghosting stains” on walls may be a haunting mystery to home builders and heating, ventilation and air conditioning (HVAC) installers in the 90's, but those who have made and used candles throughout the centuries know if a candle is not burned properly, it may smoke. And, as in the case of the church, if you burn enough candles, you can expect to paint your walls from time to time.

With the skyrocketing rate of candle usage by homeowners – up to 70% according to a study by Smith and Kline – it is easy to shift the blame of ghosting stains to the candle industry. Candles burn; burning is combustion, incomplete combustion can produce smoke. It's that simple, right? Maybe not. Frank Vigil, of Advanced Energy, a not-for-profit organization in Raleigh, North Carolina and author of "Black Stains in Houses: Soot, Dust, or Ghosts?", agrees that “candles are not the only source of soot ⁱ.” Then what is the problem? Where’s all this soot coming from? Let’s start at the very beginning . . .

SMOKE - What is it?

Smoke is a natural by-product of incomplete combustion. Combustion is triggered when an initiator, such as a spark, sets off a reaction that separates hydrogen and carbon atoms. In complete or stoichiometric combustion all of the carbon atoms combine with oxygen to form carbon dioxide (CO²) and all of the hydrogen atoms combine with oxygen to form water (H²O). Carbon atoms are emitted when oxygen does not attach to the carbon atoms. When the combustion process is incomplete or inefficient, the carbon particles can escape the flame and agglomerate (stick together). Those carbon particles are commonly thought of as smoke or soot. Eventually these particles settle on surfaces such as walls and floors. Several of Michael Faraday’s experiments show that the level of carbon or soot increases as the amount of oxygen introduced to the combustion reaction decreases.

Where does it come from?

Soot deposition has numerous sources. A homeowner’s lifestyle and environment can help shed light on the increase in soot damage. For example, a household with smokers is at a higher risk for soot damage compared to a household with nonsmokers. Other common sources of soot deposition include fireplaces, stoves, furnaces, natural gas, oil, gas water heaters and candles. Vigil even saw a house where “soot staining has been found…and the only source of combustion was a standing pilot light in a gas log fireplace ⁱ.”

Though we have identified several sources of soot, we cannot stop here. As Frank Vigil noted in his article, “…there are always two culprits at fault. Any time deposition of soot, dust, or carbon appears, first there must be a source of a material, and second, there must be a driving force to cause the material to deposit on a surface. ⁱⁱ” Vigil went on to discuss three driving forces: impaction, gravity, and attraction. A possible attraction force behind soot deposition that warrants further investigation is the heating ventilation (HVAC) system. [Quoting from 1998 article, this is no longer an issue, could not be replicated in a study by Advanced Energy.] HVAC systems can increase soot deposition because of improper installation or because of static charging.

According to another study by David Krause, a researcher in Florida, “Long runs of ductwork and high fan velocity essentially charge the particles the same way that a Van de Graaff generator charges the large dome on its top. ^xi” In HVAC systems, the particles may be dragged along the ductwork the same way a brush is dragged along the leather belt in the Van de Graaff generator. The generator positively charges the ball on top and ductwork positively charges the particles. It should make sense then that if you increase the speed of the belt or the speed of the air velocity you would get more static charging.

According to Vigil’s article on ghosting stains, the Air Conditioning Contractors of America’s (ACCA), Manual D for residential duct systems has a recommended air velocity of 600-900 feet per minute (FPM). In many houses evaluated by Vigil for soot deposition problems, Vigil noted an average velocity of 1,500-2,000 FPM ⁱⁱⁱ. With the aforementioned explanation of air velocity; damage to these houses is no surprise. While Vigil does not attribute soot deposition to these velocities, David Krause does.

As more cases of soot damage pop up, different factors need to be considered. Are the heating/cooling ducts leaking? Is air properly circulated through a duct system and then properly vented out? Is the water heater defective? And if the building is new, are synthetic materials emitting soot-producing gases? Is the ventilation system of new buildings too tight?

Do not be too quick to dismiss the issue of proper ventilation in houses. Faraday experimented with the importance of airflow in Chemical History of a Candle. He confirmed that the less air flow and “fresh air” a combustion reaction had, the more soot it would produce ^iv. Wayne Johnson also touched on the subject with “Candle, candle burning bright may cause soot marks on walls” published in Air Conditioning, Heating, & Refrigeration News. “Homeowners who are using candles as “aromatherapy” are not likely to accept the word of an HVAC mechanic who looks too anxious to limit his own liability. [He] may be expected to prove [his] allegations, which won’t be easy since not even the experts agree that candles are the main cause of modern soot. And whatever the original source, the ventilation system can deliver and deposit soot throughout the house, making detection and remediation more difficult ^v.”

THE MORE THINGS CHANGE . . . THE MORE THEY STAY THE SAME

It would be naïve to think that candle products made prior to the past five or ten years were not capable of causing smoke. Why are we now seeing a rise in claims of soot deposition? What has changed?

The candle industry has more than doubled in the past five years, so it makes sense that we would see an increase in the number of soot cases since soot is a naturally occurring product. It is important to understand that soot has always existed, and always will. In fact, soot particles are what make a flame burn yellow. NASA has performed experiments in microgravity and experimented with the laminar soot processes. The experiments show that, in reduced gravity, the flame is round and not distorted as we see it on earth. Gravity elongates the flame cone and causes the flame to smoke given the right circumstances ^vi. The writer feels such circumstances include wick length and too much air circulation [drafts].

Another factor in soot deposition is consumer misuse. It seems that many consumers have lost the “art of burning candles”. In the past consumers made their own candles, and knew how to burn them. Faraday notes that any candle burned improperly may produce soot. For example, the longer the wick on a candle, the more soot it is likely to produce. Also, placing candles in rooms with drafts may increase soot deposition. Air circulation is needed to evenly distribute heating and cooling throughout the house. However, drafts produced by a ceiling fan or an open window will contribute to the possibility of soot emission from a burning candle.

Today’s construction practices on new buildings may also contribute to ghosting stains. The preponderance of complaints about ghosting stains come from owners of houses five years old or newer. Vigil stated that “There have even been reports of the problem in newly built, still unoccupied, model homes ^vii.” He reports only one such complaint. The writer believes one possible explanation of this anomaly is the use of more synthetic materials than in the past. These materials may produce gases that, along with a driving force, cause soot to accumulate.

Faraday told of the impropriety of the poor having to live in houses with “poor air” over a century ago. The row houses of London had no ventilation and Faraday warned of combusting materials without sufficient air. Those row houses were most likely very well ventilated by today’s standards.

The air tightness of new construction is an issue of concern for indoor air quality professionals throughout the nation. Many studies today indicate that new homes have at best poor ventilation. Studies of newly built houses in Florida, North Carolina, Tennessee, and Washington indicated average air exchange rates of between .1 and .2 air changes per hour or only changing 10% to 20% of the air in the structure per hour. Few homeowners understand that to reduce energy costs homebuilders and designers greatly reduced the amount of fresh air permitted to enter the home. The American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) suggests .35 air changes per hour [or, 35 % of indoor air] to maintain adequate indoor air quality. Faraday’s London row houses would have been “windy” by this standard.

WHAT DO WE DO?

If we know the sources of soot, and we know some of the reasons why soot accumulates, then what can we do to minimize the effects of soot?

To prevent smoking, candle wicks should be trimmed to ¼” before lighting and re-trimmed to ¼” upon re-lighting. If a candle begins to smoke, consumers need to take measures to ensure proper burning procedures of their candles. Wicks should be trimmed at least every two to four hours, if needed. Keep in mind longer wicks and higher flames may produce soot. Consumers should always follow the manufacturer’s burning instructions for their particular candle.

Consumers need to make sure that rooms are properly ventilated and candles are kept out of drafts. Also, under no circumstances should a candle be burned while out of sight. Consumers must understand that the type of candle they purchase affects the amount of soot that is produced. For example, jar candles, among the most popular candles sold, may produce soot because of the inhibited airflow. As the wax burns further down into the jar, less fresh air can enter the container to efficiently burn the candle.

The most basic rule that consumers should follow is... if the candle appears to be emitting soot, do something about it. Extinguish and allow the candle to cool, never move a lit or molten candle. Read and follow the manufacturer’s burning instructions again. If necessary, trim and re-light. If the candle continues to smoke, discontinue its use and call the manufacturer.

The materials used to build new homes have changed rapidly with technology. With the expanded use of synthetic materials, the writer believes that gases may be produced that contribute to soot emission. We need to investigate to determine whether or not this is a problem.

Most houses do not have ventilation systems. Frank Vigil advocates that consumers who are interested in the quality of the air they breathe should demand ventilation and good filtration systems. Fresh air should be accessible from outside of the structure and vents should direct “old air” out of the structure. The velocity of airflow should be controlled and systems must be fitted properly to each building.

Manufacturers must take steps to control the quality of candles and to educate consumers. First and foremost, a candle must be a candle. To be a candle, it must burn properly. Candle makers must understand that anything added to a candle may alter the burning characteristics. Candle fragrances are fuels and must be treated as such. All fragrances should be tested to ensure compatibility with candle wax. Manufacturers should also know that fragrances are format specific. Just because a fragrance works well in a pillar candle, that does not guarantee it will even be appropriate for use in a votive candle. Manufacturers also need to educate their customers and the consumer. Warning labels and public education should be introduced to reinforce proper care and use of candles.

SINCE THE CONCEPT IS NOT NEW . . .

Control of soot deposition will necessitate the cooperation of HVAC installers, builders, retailers, consumers, and candle makers. Soot will never be eliminated, according to Dr. David Urban, a combustion specialist with NASA, but proper burning methods and quality control methods can reduce the effects of soot deposition and the damage it may cause ^viii. Like Vigil said, “Bottom line here is, the cause is not singular. There are multiple factors and no two buildings are the same ^ix.”

THE MAGIC ANSWER IS WE OURSELVES.

In Faraday’s time, churches demanded smokeless candles. Many answered the call with additions of natural and animal based waxes to candle formulations. But even these smoked if burned improperly.

Today we hear of “soot free” candles made of vegetable waxes and other magic stuff guaranteeing that they will never smoke. Have we not heard this before? Most candle manufacturers stopped using the claim of “smokeless” many years ago. Candle makers understood Faraday’s teachings. There is carbon in any laminar flame and the yellow glow of a burning candle proves it is there. If the flame cones Faraday described are disturbed, the carbon will come out. Examples might be: bumping a table holding a burning candle or unleashing uncontrolled air flow to the candle’s environment.

NASA chose to burn a candle in space, on the MIR Space station, because it was the most ideal combustion product they believed they could use ^x. Although the candles used in space were not as we know them, the results were extraordinary. In microgravity the flame was round and burned bright blue and without smoke. The same color of blue flame that one can achieve when methane gas is combusted with an ideal fuel air mixture. When combustion is complete, all the carbon is burned and the flame is not yellow; it is blue.

In Faraday’s time candle burning was an art and there were tools to trim wicks, snuff out, and maintain candles. People understood that even their best candle would smoke when burned improperly and their poorest candle may not smoke if burned well.

We have forgotten the art of burning candles and expect some magic fix to make them perfect and burn free of smoke. NASA has shown that the Earth’s gravity is not likely to allow that to happen.

There is some magic in the timeless glow of a candle flame. The candle’s imperfect light has comforted and inspired kings and peasants alike for thousands of years. In a world that runs on the perfection of computers, is there not some sense of our own humanity in the humble candle that will not yield to the demand for perfection? Candles, like us, perform best when someone takes the time to care for them.