A Kilner jar is a screw-topped jar with a rubber seal in its lid (which creates an airtight seal to prevent the ingress of air-borne bacteria) used for the storage of food, and which was invented by the Kilner family and produced by John Kilner & Co, Yorkshire, England. Classically, it was a glass plug with a rubber seal attached to it in the top, with the whole being secured with a metal screw-top lid.
Contemporary "Kilner-style" jars usually have a lid made entirely of metal. Kilner jars are used for storing and preserving homemade jams, marmalades and other relishes. They are also used for pickling food such as eggs, onions and garlic.
Kilner jars should always be sterilised before using to store food for long periods of time.
Spare rubber rings / seals for new clip-top Kilner jars
Lakeland Plastics can now supply replacement rubber rings/ seals for the new type clip-top Kilner jars: