by 
The Curious Bag
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Pressure canning saves money by allowing a gardener to preserve foods from her garden. Jars can break, though, if the proper precautions aren't taken.
Old Jars
Although canning jars can be reused, after time they can become weak. If the jars are too weak, they will shatter in a pressure canner. Canning jars can be reused as long as there is no wearing, scratches, chips or other blemishes.
Overfilled Jars
Canned foods need head room. When the jars are filled, there should be a 1/4 to 1 inch of room at the top of the jar, depending on the food. This allows the food to expand while under pressure.
Cracked Jars
Check jars thoroughly before filling them. Look closely for any cracks, nicks or hairline fractures. Even if a crack doesn't go all the way through the jar, it can still cause the jar to break when under pressure.
Not Enough Water
Read the canner's directions thoroughly before canning to find out how much water is to be used during the process. If there isn't enough water, pressure can become too high and shatter the jars.
Movement
Never use a pressure canner without using the insert. If jars sit on the bottom of the pressure canner, they can move around and bump into each other. This can cause the jars to break.
Source:
Paul and Bernice Noll's Window on the World: Canning Headspace Guidelines
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