Soft, elastic rubbers should be chosen. It is poor economy to use old rubbers. Rubber after usage becomes hard and inelastic; it may cause imperfect sealing and hence decay of the fruit.
In certain processes of canning, it is necessary to subject the jars provided with rubber rings and covers to long periods of boiling or to the intense heat of a pressure or steam cooker. When such a method is followed it is especially necessary that rubber rings of good quality be used. To meet this requirement, the United States Department of Agriculture advises that rubber rings conform to the following:
1. Inside diameter of 2 1/4 inches (for the jar of standard size).
2. Width of ring or flange from 1/4 to 12/32 of an inch.
3. Thickness of 1/12 of an inch.
4. Tensile strength sufficient to "stretch considerably and return promptly to place without changing the inside diameter."
5. Firm enough so that no crease or break shows after it has been tightly folded.