Why Did My Candle Jar Crack While Burning?

A jar cracks when one part of the glass gets hotter than the part next to it. Lone Star Candle Supply puts it this way: "Even good glass can crack, and the reason is almost always heat that is uneven rather than heat that is high overall." The causes you can control are the glass you pick, the wick, and when you stop burning. This is a safety topic. I am a candle maker, not an engineer, so test in your own jar with your own wick and wax, and never leave a burning candle out of sight.

Why does glass crack from a candle?

According to Lone Star, a temperature gap between neighboring parts of the glass builds stress inside it. Once that stress is more than the glass can take, it fractures. Things they name that put more heat on one part of the glass:

• An off-center wick
• An oversized wick
• Burning down to the bottom of the jar

CandleScience adds that "power burning" (burning far past the recommended session) "can cause the container to overheat and potentially break."

Which jars are the problem?

• Glass made for something else. Lone Star says "thin glass, drinking glasses, and thrifted decorative jars are made for cool contents." It says mason jars "are made for canning, not for the sustained heat of a burning wick."
• Glass with flaws. Lone Star says to "set aside any container with a chip, crack, internal bubble, or scratch," because "a flaw is a stress point where a crack starts once the glass heats."
• Candle-rated glass. Lone Star says candle glass "is annealed to take repeated heating and cooling, which ordinary jars are not," and that choosing glass rated as a candle container "removes the guesswork." Annealing is a slow cooling step in glass making that relieves stress.

Is there a standard for candle glass?

Yes. ASTM F2179 covers annealed glass containers made for use as filled candle containers. ASTM's store page says it excludes votive holders, hurricanes and glass holders for free-standing candles, and that the glass maker or processor is responsible for compliance and test records. The National Candle Association describes it as setting scratch test and temper specifications, with thermal shock requirements. I could not open the paid text, so I am not quoting its limits. Ask your jar supplier whether the jar meets F2179 and get that in writing. CPSC lists F2179 among the voluntary candle standards. Voluntary means no law forces you to follow it, but it is a fair thing for a shop or insurer to ask about.

How hot should the glass get?

Lone Star says "warm to the touch is normal; too hot to hold comfortably is a warning sign." If a test jar is too hot to hold, treat it as a stop sign and see our guide on jar heat. CandleScience says at three to four hours the melt pool should be no more than 1/4 inch (6 mm) from the edge. It is a good habit to measure it, not guess.

When should I stop burning?

Lone Star and the NCA both say to stop when about half an inch (13 mm) of wax remains. Lone Star lists burning down to the bottom as one of the ways heat ends up concentrated on one spot of the glass.

Worked example: how much wax is left?

Say a straight-sided jar holds a 3 inch (7.6 cm) depth of wax, and the candle holds 9 oz (255 g) of wax. These are example numbers.

• Wax you stop at: 0.5 inch
• Share left: 0.5 / 3 = 0.167, or about 17 percent
• Weight left: 9 x 0.167 = 1.5 oz (43 g)

This is geometry for a straight-sided jar, not a measured result. It shows why customers may see about 1.5 oz of wax left in the jar and feel short-changed. A line on your care card explaining the stop point helps.

What does a cracked jar look like in the real world?

On February 5, 2026, CPSC announced a recall of about 1,140 nine ounce candles in glass jars sold at New Seasons Market and Town and Country stores. The notice says "the glass jar holding the candle can crack or break during use, posing fire, burn and laceration hazards." It reported two reports of jars breaking and no injuries. The remedy was a refund, and owners were told to stop using the candles. One recall is not the whole picture, but it shows that a jar cracking is treated as a fire and cut hazard, not just a cosmetic problem.

What do I do if a jar cracks?

Stop using that candle. Keep the jar and note the batch date, the jar supplier and the wick, then contact your jar supplier. This is my own practice, not something I found in a source. If a customer reports it, take it seriously, ask for a photo, and consider whether other jars from the same lot are at risk.

Common mistakes

• Using a jar that was not sold as a candle container.
• Reusing a chipped or scratched jar.
• Burning down to the last quarter inch.
• Skipping the burn test because the wick "worked last time" in a different jar.
• Using a wick too big for the jar, which concentrates heat.
• Moving a lit candle.

Questions

Can I use mason jars?

Lone Star says they are made for canning, not sustained heat from a burning wick. If you still want to try one, ask the jar maker first and run your own burn tests.

Do I need ASTM F2179 jars?

It is voluntary. If a supplier says its jars are made for candles, ask for the standard they meet. See our container guide.

Does preheating jars stop cracking?

Pouring into warm jars helps wax cool evenly, as in our frosting guide. The sources I opened do not say preheating prevents cracks during a burn, so I would not rely on it for that.