Canning Basics: Busting more Mason jar myths 

MYTH:  Canning jars can be sterilized in the oven.  Canning jars can even be used for baking.  No and no. Canning jars are made from annealed glass and are not designed for the dry, fluctuating heat of an oven. These conditions can cause thermal shock, resulting in cracked or even exploding jars. And no, putting a pan of water in the oven along with the jars doesn’t work. Sterilize jars in a boiling water or steam canner, and never bake in canning jars.

MYTH:  Headspace should be increased at higher altitudes, or when using smaller jars.  If processing at altitudes over 1000 feet, adjust processing time or pressure, not headspace. Headspace should not be changed from the amount specified in the recipe – regardless of jar size – as doing so could affect the seal or cause the lid to buckle. (There is one exception to adjusting headspace offered by University of Wisconsin Cooperative Extension that applies only to canning fruit pie fillings made with Clearjel^® in quart-size jars; see their website for details.)

MYTH:  Half gallon canning jars can be used to can anything, including homemade chili or big batches of tomatoes, simply by increasing the processing time.  Nooooooooooooooo. Half gallon canning jars are currently recommended only for apple and grape juices. At one time (decades and decades ago) adjustments for canning in half gallon jars existed, but that is no longer the case. Safe processing times have not yet been developed for foods other than apple and grape juices; at present neither the jar manufacturer nor the USDA recommend their use for other canning purposes. 

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

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Last updated July 2025