Canning Basics: Thermal shock and how to prevent it. 

There are many reasons that canning jars can break during the canning process.  One common cause for jar breakage is thermal shock, caused by a temperature differential between hot and cold. The break is characterized by a crack that runs around the lower part of the jar and sometimes up the side. 

Thermal shock can occur for several reasons. Fortunately, a few good canning practices can help eliminate jar breakage and the resultant loss of product. 

• Jars should always be hot before filling. Keeping jars in the canner (with pre-heated water) until you’re ready to add the food can help keep them hot.
• Don’t use the oven to pre-heat jars. Canning jars are not tempered for the dry heat of an oven, which can cause the jars to break or shatter.
• Pre-heat the water in the canner to 140°F for raw-packed foods or 180°F for hot-packed foods. Once all filled jars are loaded into the canner, cover it and then bring the water in the canner to a full boil/steam or correct pressure.
• Be sure to use a rack of some kind inside the canner and set the jars securely on it. Jars should never sit directly on the bottom of the canner.
• Don’t put a hot jar on a cold surface. When removing jars from the canner after processing, set them on a rack or thick towel. Conversely, don’t put a cold jar (e.g., one you’ve just taken out of the freezing garage) directly into hot water; allow it to first come to room temperature before washing it.
• Allow processed jars to cool naturally away from cold drafts. Try to keep jars away from windows, and never use a fan to try to rush the cooling process.

For further information on canning, visit the National Center for Home Food Preservation (NCHFP) or contact your local Cooperative Extension office. 

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Last updated July 2025