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doing hot bath for sauerkraut. Cleaning jar rims, adding more salt water to 1 inch of top, putting new lids and rings on jars. Screwing rings on tight. Put jars in canning, filling with cold water and bring to rolling boil for 30 minutes. When we turn off stove and remove lid to canner we are seeing most of the lids have bent in upward position. WHAT IS MAKING THIS HAPPEN?

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    You screwed the lids down tight -- you need to let air be able to escape when you're boiling them. – Joe Sep 1 '15 at 18:04
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You should not put the bands on "tight".

From Fresh Preserving Water Bath Canning

CLEAN mason jar rim and threads of jar using a clean, damp cloth to remove any food residue. Center lid on jar allowing sealing compound to come in contact with the jar rim. Apply band and adjust until fit is fingertip tight. Place filled jars in canner until recipe is used or canner is full. Lower rack with jars into water. Make sure water covers jars by 1 to 2 inches.

Fresh Preserving Problem Solver

Condition: Lid buckles, appearing to warp or bulge upward under the screw band. If spoilage is evident, do not use.

When buckling is apparent immediately after heat processing, cause is overly tight application of screw bands. Using your fingers, screw bands down until resistance is met, then increase to fingertip tight. Do not force. Do not use a lid wrench to apply bands.

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It's a somewhat common problem relating to the lids being screwed down too tight. They need to be tight but certainly not cranked on as hard as possible. I usually tighten to the point where the ring is no longer spinning easily and then give maybe another 1/8 of a turn. I believe Ball also sells a ring tightener that is said to put the rings on "just right".

The buckling itself is caused by the air being forced out during the waterbath (or pressure can) process. If the lid is too tight there is no room for the air to "squeak" out where it normally would under the rubber on on the lid. The pressure builds until it is enough to actually bend the metal of the lid to make a space in which to evacuate the jar.

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