Michigan State University Extension
Preserving Food Safely - 01600498


     The  common self-sealing lid consists of a  flat  metal
lid  held in place by a metal screw band during  processing.
The  flat  lid is crimped around its bottom edge to  form  a
trough,  which  is filled with a  colored  gasket  compound.
When  jars are processed, the lid gasket softens  and  flows
slightly to cover the jar-sealing surface, yet allows air to
escape from the jar. The gasket then forms an airtight  seal
as  the jar cools.  Gaskets in unused lids work well for  at
least 5 years from date of manufacture.  The gasket compound
in older unused lids may fail to seal on jars.              

     Buy  only the quantity of lids you will use in a  year.
To  ensure a good seal, carefully follow the  manufacturer's
directions  in  preparing lids for use.  Examine  all  metal
lids  carefully.  Do not use old, dented, or deformed  lids,
or lids with gaps or other defects in the sealing gasket.   

     After  filling jars with food, release air  bubbles  by
inserting  a  flat plastic (not metal) spatula  between  the
food and the jar.  Slowly turn the jar and move the  spatula
up  and  down to allow air bubbles to  escape.   Adjust  the
headspace and then clean the jar rim (sealing surface)  with
a  dampened paper towel.  Place the lid, gasket  down,  onto
the  cleaned  jar-sealing  surface.   Uncleaned  jar-sealing
surfaces may cause seal failures.                           

     Then  fit  the  metal screw band  over  the  flat  lid.
Follow the manufacturer's guidelines enclosed with or on the
box for tightening the jar lids properly.                   

cool,  the contents in the jar contract, pulling  the  self-
sealing lid firmly against the jar to form a high vacuum.   

     *  If screw bands are too loose, liquid may escape     
        from jars during processing, and seals may fail.    

     *  If screw bands are too tight, air cannot vent       
        during processing, and food will discolor           
        during storage.  Overtightening also may cause      
        lids to buckle and jars to break,  especially       
        with raw-packed, pressure-processed food.           

     Screw bands are not needed on stored jars.  They can be
removed easily after jars are cooled.  When removed, washed,
dried   and  stored in a dry area, screw bands may  be  used
many  times.  If left on stored jars, they become  difficult
to remove, often rust and may not work properly again.      

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