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Jars not sealing is a problem for many new home canners. Once you get the hang of it, you should rarely have a jar fail to seal. My own failure rate is less than 1%.
So, here are typical causes for jars not sealing and what to do to prevent it - and save the batch that did not seal!
Using nonstandard jars. It's best to use official canning jars and/or lids. (like Ball, Kerr, Golden Harvest, etc)
Chipped or uneven rim on jar.
Screw rings (also called bands) are rusty or bent, causing poor contact.
Bands not screwed down tightly enough before processing. (Turn until you meet resistance, then turn it one-quarter turn.)
Rim on jar not clean. (Wipe rim well before putting lid on.)
Liquid leaks out of jar during processing, leaving food particles on the sealing edge.
Insufficient heat during processing - air not removed from jar so a vacuum seal never forms. (Remove air by inserting a rubber spatula or plastic knife inside the jar gently lift food to remove any trapped air.)
Lids were improperly prepared before placing them on rims. (Follow manufacturer's directions to prepare lids.Basically, this means to keep them in a pot of steam hot - but not boiling water to keep them clean and soften the gasket)
Pressure canners only: Rapid, forced cooling of a pressure canner can cause a rapid pressure and temperature change inside the canner, causing the liquid to "boil" out of the jars, leaving particles on the sealing rim and unsealing the jars. (Canners should not be forced into cooling rapidly by submerging them in water.)
Insufficient processing of raw-packed food - the air may not have been completely driven out of the food, leaving residual air in the jar so the seal does not form.
Use of canning procedures which are not recommended, such as open-kettle canning, inversion canning, dishwasher canning, microwave canning and oven canning.
Incorrect amount of headspace. (I always maintain at least 1.5 inches of water above the top of the jars.
If a lid fails to seal on a jar, you have 3 different options:
1. Remove the lid and check the jar-sealing surface for tiny nicks. If necessary, change the jar, add a new, properly prepared lid, and reprocess within 24 hours using the same processing time as recommended in the recipes..
2. Adjust headspace in unsealed jars to 1½ inch (to allow for expansion) and put it in the freezer, upright, instead of reprocessing.
3. Refrigerate unsealed jars and eat canned product within seven days of refrigeration.