Before they are used, they should be tested in the manner
shown in Fig. 5. Good jar rubbers will return to their original shape
after being stretched. Such rubbers should be rather soft and elastic,
and they should fit the jars perfectly and lie down flat when adjusted.
A new supply of rubbers should be purchased each canning season, because
rubber deteriorates as it grows old. Rubbers of good quality will stand
boiling for 5 hours without being affected, but when they have become
stiff and hard from age it is sometimes impossible to make jars
air-tight. Occasionally, two old rubbers that are comparatively soft may
be used in place of a new one, and sometimes old rubbers are dipped in
paraffin and then used. However, if there is any difficulty in sealing
jars properly with rubbers so treated, they should be discarded and good
ones used.
25. TIN CANS.--For household canning, tin cans are not so convenient as
glass jars, but in spite of this they are coming into extensive use. The
kind that may be used without any special equipment has a tin lid that
fits into a groove and is fastened in place with rosin or sealing wax.
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