Why Canning Jars Break

There are many reasons why canning jars break. The good news is that it is rare that they do, and most common reasons for breakage can be easily avoided keeping the following things in mind.  

Jars Break When Adding Food

Hot Food in Cold Jars

The reason why the jars must be warmed before filling them with hot food or liquid is so the temperature change is not such a shock to the glass.

I use my roaster to warm up the jars. I set it on 150 degrees and let the jars sit in there while I'm getting everything else ready. That temperature keeps the jars warm enough to be able to withstand the hot liquid that I'll add later. "Hot liquid" doesn't mean liquid at a hard boil. It's liquid that is in between a slow boil and hot tap water.



The 3" of water in the bottom of the pressure canner should also be kept at generally the same temperature. It's on a low setting; just enough to keep the water hot but not boiling or scalding.

Pressure Canner
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Jars Break During Processing

Lids Over Tightened

The lids need to be tightened to "finger tight," which means snug, but not so tight that it is difficult to remove.   

It's helpful to understand how the pressure in the jar works. The contents of the jar will heat and expand as the processing takes place. The air in the headspace is pushed out of the tiny gap between the lid and the jar. That's why there are two top pieces to canning jars; the lid and the ring. The ring keeps the lid from lifting up too much and allowing water in. That's how the vacuum is created.

When you remove the jars from the canner, the contents cool, contract and suck the lids down. 

Applying the rings too tightly prevents the air from escaping as it should during processing.

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Headspace Wrong

Correct headspace is crucial because there needs to be enough space for the expansion of air and jar contents.
Always follow the recommendations for headspace which are specific to each type of food.

And since some foods expand more than others, it's important also to be sure you're not adding an ingredient that is not safe to can because of the product's density.

Further Reading:  Ingredients Not Safe to Can

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Too Much Pressure Too Fast

The pressure inside the canner and the jars needs to rise slowly to finally get to the height of the processing temperature. Slowly is the key, to avoid thermal shock to the jars.

After the jars are placed in the canner and the lid is securely on, the pressure in the canner needs to vent for ten minutes. This step is needed to ensure that your canner is void of air entirely, and that takes time. The idea is to gradually raise the temperature.



Once a steady stream is being released, it's time to put on the gauge and let the canner go to the correct pounds of pressure, then start the timer.

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Bottom Rack Not Used

Jars placed directly in the bottom of the canner without the rack, a wet towel or some other barrier can cause the jars to break.  Although this is not common, it does happen.

The rack keeps the direct heat of the stove away from the bottoms of the jars. If you don't have a rack, you can use a set of canning rings placed around the bottom of the canner.  

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The Jar Itself

Hairline cracks in jars can be very hard to see and may cause a jar to break under pressure.

Keep in mind that canning jars do eventually weaken with age and the repeated use of heating and cooling. If you're using older jars, or if you're just not sure for any reason that the jar is solid, it would be safer to use it in a water bath to keep it from being over stressed.

Jars Break While Cooling

Another way fast change of temperature of the glass jars can cause a problem is when the hot jars go from the canner to cooler temperatures.

After removing the hot canning jars from the pressure canner, place the jars on top of a dish towel as a barrier between the cold surface of the counter top and the hot glass. Placing another dish towel over the jars will help keep out cold drafts.

Spaghetti Jars as Mason Jars

A Word About Commercial Food Jars

Be wary of using commercial food jars in a pressure canner. Those jars may not properly fit the lids you are using and maybe they haven't been specially tempered to withstand the heat necessary in the home canning process. If in doubt, use them for other purposes.

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Pressure canning can be daunting to the new canner, but if you adhere to safe practices, you're risk of having jars break are very low.  So enjoy it!

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