What Is a Full Melt Pool, and How Deep Should It Be?

A full melt pool means the top layer of wax is liquid from one side of the jar to the other. Suppliers give a depth of about 1/4 to 3/4 inch (6 to 19 mm) and a time of roughly 2 to 4 hours, but the sources differ, so I lay them out below. On a first burn, the National Candle Association (NCA) says to burn one hour for each inch of jar width, and in general no longer than four hours. Stay in the room, and test in your own jar with your own wax and wick.

What is a full melt pool, and why does it matter?

It is wax melted edge to edge across the top. The NCA says that if the wax does not melt "from edge to edge," a "memory ring" forms and the candle keeps tunneling. CandleScience says "a full melt pool is fundamental to a strong hot throw." Our tunneling guide covers the fix.

How wide, how deep, and when?

CandleScience's own pages differ. Its wick-size page says a pool deeper than 1/2 inch (13 mm) is a sign of an over-wicked candle, while its wicking guide allows up to 3/4 inch. If you are unsure, use the shallower limit, and use the same number every time. A deep pool is also a heat sign, so see our guide on wicks that are too big.

One note on timing: CandleScience says pools form slower in the first burns and faster when the wax level is lower. It says to wait for the fourth cycle before judging.

NorthWood makes one exception: with coconut wax blends, it says you ideally do not get a full pool on the first burn, because that may mean the wick burns too hot later. I found that only on its page.

None of the sources I opened gives a method for measuring pool depth. Do not poke hot wax. Measure only when you can do it safely.

How long should I burn a candle the first time?

The NCA says one hour for every 1 inch in diameter, so a 2 inch candle gets 2 hours, and that in general a candle should not burn longer than four hours. Let it cool at least two hours before you relight. Makesy and Lone Star (in its tunneling fix) give the same one-hour-per-inch rule for a first burn. Makesy's table lists about 2 to 2.5 hours for 2 to 2.5 inches wide, about 3 to 3.5 hours for 3 to 3.5 inches, and about 4 hours for 4 inches and up, with a check at 3 hours.

For testing, the rule is different. CandleScience and Craftybase burn in 4-hour sessions with at least 5 hours of cooling. CandleScience says not to burn past four hours, because it can overheat and break the container and burn off the fragrance too fast. Never leave a burning candle out of sight. The NCA says to stop when 1/2 inch of wax remains in a jar.

Worked example: a first-burn plan

Rule: one hour per inch of width, capped at 4 hours (NCA). All jar sizes are examples.

For the wide jars, the two rules cannot both hold, because 4.5 and 6 hours are over the 4 hour cap. The NCA does not say what to do there, so keep the 4 hour cap. Suppliers suggest two or three wicks for wide jars. See our two or three wicks guide.

Now the test clock. In a 3.5 inch jar, CandleScience's within-1/4-inch rule means a pool at least 3.5 - 2 x 0.25 = 3.0 inches wide at 3 to 4 hours. Judging after four 4-hour cycles is 4 x 4 = 16 burn hours. Between four cycles there are three cool-downs of at least 5 hours, which is 3 x 5 = 15 hours. So the test takes at least 16 + 15 = 31 hours from first light. With two sessions a day, that is about two days.

Common mistakes

• Putting the candle out after an hour on the first burn and creating a memory ring.
• Judging the pool at the first cycle, when CandleScience says wait for the fourth.
• Mixing the customer rule (one hour per inch) with the test rule (4-hour sessions).
• Burning past four hours to hurry the pool.
• Comparing pool depth between candles measured at different times.

Questions

Is a deeper pool always better for scent?

A bigger pool helps. CandleScience says the larger the melt pool, the stronger the hot throw. But a depth past what your source allows is an over-wicking sign and a heat risk, so test both.

What if the pool reaches the edge in an hour?

The sources I read do not give a rule for that. Check jar heat and flame height, and see whether the wick is too big for the jar. NorthWood's note on coconut blends may apply if you use one.

Should a pillar have a full pool?

CandleScience says many makers leave about 1/4 inch (6 mm) of side wall on pillars so wax does not spill. That is for pillars, not jars.