How Hot Should a Candle Jar Get?

There is no single jar temperature that every source agrees is "safe". The number you will see most often is 140 degrees F (60 C) measured on the outside of the jar, and makers treat a reading above it as a warning that the wick is too big or the oil load is too high. I could not open the paid ASTM text to confirm where that number comes from, so treat it as a working line. Test in your own jar with your own wick and wax.

Where does the 140 degree figure come from?

It shows up in maker guides, but its source is unclear. Here is what the pages I opened say:

• Craftybase's burn test guide lists "container temperature exceeds 140 F" as a sign the wick is too large or the fragrance load too high, and suggests a smaller wick or less oil. It does not say where the number comes from.
• Armatage's guide uses "outer container temperature remains at or below 140 F (60 C)" as one of its checks. It says plainly that this is not an ASTM marker, and that ASTM F2417-17 "doesn't mention anything for temperature control or carbon formation". ASTM's store now lists F2417-26 as the active edition, and its public page gives no temperature limit; the full text is paid.
• I also saw other pages in search results that credit older ASTM F2417 editions with a 140 F glass limit and a 125 F metal limit. I could not open those editions to check, so I am not repeating it as fact.

So the honest summary: 140 F is a widely used maker rule of thumb, not something I can point you to in the standard. If a shop, insurer or lab asks for a temperature limit, get it from them in writing and check it against the current ASTM F2417 text.

What is the real danger of a hot jar?

Heat makes three problems. A jar that is too hot to touch can burn a customer. Glass can crack from uneven heat, and the ASTM-style test in Armatage's guide fails a candle whose container does not stay intact. An over-wicked candle also tends to have a tall flame and a deep pool, and CandleScience says an over-wicked container can get very hot and may crack. The jar itself matters as well. CPSC lists ASTM F2179, a specification for annealed soda-lime-silicate glass containers made for candles, so buy jars sold for candle use.

How do I measure it?

An infrared thermometer pointed at the outside of the jar works well, and it is my own practice rather than a method from a standard. Measure at the hottest spot, which is usually near the wax line, and take a reading every hour. Do not touch a jar that may be hot to check it.

Worked example

Say a 3 inch (7.6 cm) jar in a 72 F (22 C) room reads like this at each hourly check (all numbers are examples):

The jar passes through 140 F at hour 4. By the 140 F rule of thumb, this wick is a candidate to move down one size. The rise is 142 - 98 = 44 F over three hours, and it is still climbing, so waiting longer would only make it hotter. You would pour the next size down and run the same test side by side.

If the next candle peaks at 126 F (52 C) but leaves wax on the sides, the wick may now be too small. The right size lives between the two, which is why you test three sizes, not one.

What else can I do if the jar runs hot?

• Go down one wick size, the usual fix.
• Lower the fragrance load a little. Craftybase lists both fixes.
• Trim the wick to about 1/4 inch (6 mm) before every burn, so the flame stays small.
• Try a thicker or different jar. Test any new jar from scratch.
• Compare a hot candle against the melt pool target. CandleScience gives a pool depth target of about 1/4 to 3/4 inch (6 to 19 mm). A pool much deeper than that means the wick is doing too much.

Common mistakes

• Quoting one number as the safe temperature, with no source and no testing.
• Measuring only once, at the start.
• Testing only the jar's cool side, or on a table with the wind blowing on one side.
• Fixing heat by shortening burn time on the label instead of fixing the wick.
• Using a glass jar not made for candles.

Questions

Is 140 degrees F an ASTM requirement?

I cannot confirm that. One of the guides I opened says the 2017 edition, F2417-17, does not mention container temperature. ASTM now lists F2417-26 as the active edition, and I could not open the paid text of either. Check the current ASTM F2417 text before you claim it as one.

Should I use a different limit for ceramic or metal containers?

I did not find a source that gives one for ceramic or metal. Test the container you plan to sell, record the hottest reading, and ask the container supplier whether it has heat guidance.

What if the jar passes but the candle still smokes or cracks?

Treat that as a fail. A cracked container fails the container check whatever the thermometer says. Smoke and soot point to too much fuel for the wick.