Thickness.  Rubber rings as found on the market may vary from 1/18 to 1/10 of an inch in thickness.  Tests show that 1/12 of an inch in thickness is sufficient to take up the unevenness in the jar and still not so thick as to make it difficult to place the cap or adjust the bail.

Cold-pack and intermittent-canning require a rubber ring that is tough, does not enlarge perceptibly when heated in water or steam, and is not forced out of position between the top and the jar by slight pressure within the jar.  This we call a “blow-out.”

Rubber rings should be capable of withstanding four hours of sterilization in boiling water without blowing out on partially sealed jars, or one hour under ten pounds of steam pressure.  They should be selected with reference to proper inside diameter, width of flange, and thickness.  Good rubber will stretch considerably and return promptly to place without changing the inside diameter.  They should also be reasonably firm and able to stand without breakage.  Color is given to rings by adding coloring matter during the manufacturing process.  The color of the ring is no index to its usefulness in home canning.  Red, white, black or gray may be used.

Always use new can-rubbers with each year’s product of canned goods.  An old rubber may look like a new one but it has lost its elasticity and its use may cause imperfect sealing and thus endanger the keeping quality of the food.  This is always a hard thing to impress upon thrifty penny-saving housekeepers.  The old rubber looks so good, so why not use it?  But be wise in this and remember it is never safe to use old rubbers.  New rubbers are expensive but what about the cost of the product, the loss of your time and fuel!  One jar lost due to an old rubber is so much food, time and fuel lost.

And do not think yourself thrifty to use two old rubber rings instead of one, thereby thinking to obtain a better seal, for you will not.  Two old rubbers are inferior in strength to one new good rubber.  If you use old rubbers and your canned goods spoil, blame the rubbers.

GLASS JARS

Next in importance to the rubbers are the glass jars you use.  There are many kinds of fruit jars on the market.  The question is frequently asked, “Which jars on the market are the best.”  The only answer to that is to choose the jar which is simplest in construction, which will seal perfectly and wash easily, which protects the contained food against contact with metal, which has the fewest parts to lose or misplace and which fits the shelves and receptacles planned to hold it.

FLAT SOUR

Flat-sour often causes annoyance to beginners in canning some vegetables, such as corn, peas, beans and asparagus.  These canned foods may show no signs of spoilage and yet when the can is opened the product may have a sour taste and a disagreeable odor.  This “flat-sour” is not harmful and must not be confused with “botulinus,” which is harmful.  However, the taste and odor are so disagreeable you will have no desire to eat “flat-sour” canned goods.