What’s A Melt Pool?
There’s a piece of advice floating around the candle making community about testing and melt pool design that says,
“Candles should reach full melt pool in ‘X’ hours, where ‘X’ is the diameter of the vessel in inches.”
The Melt Pool is just a description for the melted wax in a burning candle. A Full Melt Pool is when the melted wax fills the entire top layer of the candle. Another part of this theory is that Melt Pool depths should stay between 1/8″ and 1/4″ when fully formed.
The advice suggests a properly sized wick is expected to melt wax at a specific rate to provide optimal scent performance (hot throw).
This idea isn’t entirely wrong, but candles should consider a lot of factors besides the Melt Pool in testing and evaluation. Candles are complicated. They rely on the laws of chemistry to vaporize melted wax drawn into a wick where it combusts into the air to create carbon dioxide, water, heat, and light. Good candles have a wick that isn’t too big or small, but just right to balance the wax fuel with the fire above.
Many beginner candle makers fall into the trap that if their candle creates a melt pool in a particular time frame the wick is ready, but there’s more to the story.
The Melt Pool is really just a measure of how hot the fire is. It’s not fair to simplify the success of a candle based on how slow or fast it melts wax. The best candles pass a plethora of other criteria as well before their Maker is satisfied that it’s safe and smells good!
So what’s a young candle maker to do? Consider the following.
Size wicks according to industry standards for performance and safety
Melted wax travels through the wick until the flame vaporizes it before eventually combusting. That’s how candles work. Think of the wick as a pipe.
Larger “pipes” carry more fuel, and more fuel generates more heat. Too much fuel (or too big of a wick) overloads the flame and air which causes mushrooming (carbon balls). Wicks that are too large actually burn so hot the candle smells more like fuel than fragrance sometimes.
On the other hand, if the wick is too small, there isn’t enough fuel to get hot enough and thoroughly melt the wax – the candle begins tunneling. It never reaches Full Melt Pool.
A wick that is “just right” feeds just enough fuel to eventually melt all the wax without mushrooming or tunneling. It’ll also provide the optimal scent throw!
It’s okay to size your wick to create a Full Melt Pool, but don’t forget to check if it also passes industry standards for safety and general recommendations for performance, which also considers:
- Container temperature
- Carbon formation (mushrooming)
- Flame height
- Black smoke (soot) emission
- Secondary ignition risks
- Tunneling
- Scent throw (hot & cold)
Candles that fail to meet standards but have an impressive melt pool are still failed candles.
Decide how you want the candle to generate the greatest hot throw
The other half of the Full Melt Pool theory says hot throw is optimal when a full melt pool is present, but that’s not entirely clear. Remember, there’s a flame at the top of the wick burning through wax blended with fragrance oil, constantly releasing carbon dioxide, water, heat, light and aroma into the air.
While there’s certainly fragrance coming from the pool (after all, wax melts count on it), there’s anecdotal evidence saying the active melting of wax makes up part of the hot throw too. That is to say, it seems that fragrance emits strongly from melting wax as much as melted wax.
If a candle slowly melts wax over a four hour period, it may smell stronger than if a pool of melted wax formed in one hour and sat for three more.
Candle makers subscribing to this theory size wicks to melt wax over the longest period of time instead of forcing a Full Melt Pool in a specific time frame (though, there is certainly overlap of the two theories).
Size for a full melt pool, but not right away
The largest risk for sizing a candle to melt that fast is heat. Throughout a candle’s life, the average temperature of the container continuously increases. First, the heat that used to disperse into the air near the top of the candle begins to absorb into the container walls instead. Second, the flame is fed more oxygen due to local air currents excited by a warmer atmosphere.
A 3-inch diameter candle that reaches full melt pool in 3 hours near the beginning of it’s life will almost certainly reach full melt pool substantially faster in the later half of its life. Candles like this almost always fail industry safety standards in the last half of their burn life (even if they pass near the top).
Do not assume that candles are okay if they pass tests in the first half of their burn life.
Experienced candle makers harness this phenomenon to an advantage.
Their candles may never reach full melt pool until late in the burn life, leaving a thin layer of wax after most burns. The additional heat in the jar near the end actually “cleans” the walls and melts all the wax eventually, even if it’s not right away.
Designs like this count on constant melting throughout every burn to optimize hot throw created by melting wax in addition to maintaining a sizeable, not necessarily full, melt pool.
The best candle is safe, performs well, and uses as much wax as possible regardless of how fast, deep, or wide the melt pool may be.
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