If you are using some old Mason jars put a rubber on each jar, fill the jar with hot water, and then put the cover on tight and invert.  This is a sure test for leakage.  Never use a Mason cap twice unless the cover and collar are separate so that both can be completely sterilized.  Fortunately the old-fashioned Mason jar metal cover to which a porcelain cap is fastened is going out of style.

If you still have some of these old covers it will be economy to throw them away.  You will be money ahead in the end.  After these tops have been used once it is impossible to make a fastening between the porcelain and the metal so tight that it is not possible for the liquid to seep through and cause the contents to spoil.  This accounts for many failures when old tops are used.  For this reason never use the old-fashioned, zinc-topped covers.

The new and safe Mason jar covers consist of two parts, the metal collar and the porcelain cap.  They are for sale at all grocery or hardware stores.

If you are using the vacuum-seal jars which have a composition attached to the lacquered tops, carefully examine this rubber composition to see that it is perfect.  This composition should go entirely round the top and should not be cut or broken in any place.  If it is the top must be discarded for a perfect one.

Of course with this type of jar no rubber rings are necessary, as the rubber composition on the lacquered top does the sealing.

It is a wise plan to go round the tops and over the inside of all new glass jars with a heavy and dull knife to scrape off any slivers of glass or bursted blisters that may be still clinging to the jars.  Those on the tops cut through the rubber and cause leakage.  Those in the jars may get into the product.  I often find these splinters, particularly on new straight-sided jars.

It matters not what type of jar you use.  Use what you have at hand, but if you are buying new jars consider the following things before making your selections:  No metal, unless it is enameled or lacquered, should come in contact with the food.  The jars should be of smooth, well-finished glass.  The color of the jar does not affect the keeping qualities of the food.  The top or part of the top that comes in contact with the contents should be all in one piece, so as not to offer a place for the accumulation of organisms and dirt.  The jars which have nearly straight sides and a wide mouth or opening are easier to wash and facilitate better, quicker and easier packing of the product.

Wash the jars in soap and water.  Rinse in boiling water.  Some people temper new jars so they will stand the shock of hot water or hot sirup without breaking.  If you wish to take this extra precaution put the jars in a dishpan or kettle of cold water after they have been washed in soapy water; bring the water slowly to a boil and let it boil fifteen minutes.  After the jars are ready test the rubber rings.  This may seem a useless precaution, but it is a necessary one, for there is no one detail in the business of canning that is more important.  Even in the best boxes of rubbers there is occasionally a black sheep, and one black sheep may cause the loss of a jar.