This section is from the book "Everywoman's Canning Book", by Mary B. Hughes. Also available from Amazon: Everywomans canning book; the A B C of safe home canning and preserving.
This one problem seems to give more anxiety than any other to the beginner in canning. It is not necessary to boil either jars or rubbers before using them. Wash thoroughly and rinse in hot water. They are sterilized at the same time the contents of the jars are processed or sterilized. Any previous boiling is superfluous. Much unnecessary drudgery is eliminated when housekeepers fully appreciate the fact that in cold-pack canning it is not necessary to handle the jars out of boiling water. To have them clean is enough.
It does no harm if the jars touch each other or the side of the container in the processing. Arranging hay, cardboard, or wrapping the jars in cloth to keep them from touching, is unnecessary labor.
It is unwise to place the jars on top of each other in processing, for the air in the lower jars cannot expand. This will crack the jars. If a second tier is needed, have a rack fitted in the boiler, which will act as a second shelf.
If a rubber bulges when it is taken from the boiler, press it back immediately with the finger, set the jar in the sterilizing bath, and leave for ten minutes.
It is important to have the water come over the top of the jars to a depth of two inches or more. In processing vegetables which require a long period of sterilizing, such as corn, beans, and peas, have the water three or four inches over the top of the jars. If the boiler is not air-tight, the escaping steam will greatly reduce the water around the jars. Plenty of water in the boiler to start with eliminates the possibility of having the boiler get dry.
By no means. Bubbles usually take care of themselves in a few days. If there is any uncertainty about the jar, set it one side for a few days; and if the bubbles are still present, reprocess for one-half the original period. If a frothy white collection of bubbles appears at the top, usually the jar is defective. In this case use the product at once on the table, or put into a new jar and resterilize for one-half the original period.
When jars break in the sterilizing bath, first see if the second clamp was put down by mistake, or if the rack was forgotten in the boiler. If the jars are set in water of a higher temperature than the liquid in the jar, they will crack. Some breakage is due to defective glass. When the bottom of the jar drops out and leaves a clear, straight cut around the jar, the fault is with the glass. If jars are carefully handled, the percentage of breakage is very small.
Some shrinkage is expected, but when the product shrinks to any great extent, it means careless blanching and a poor pack. Jars do not have to be full to keep. Therefore it is unnecessary to open the jar and fill it up with boiling water. If the jar is opened (which is inadvisable) and boiling water or hot syrup added, it must be processed for one-half the original time. By opening the jar and adding water, the housekeeper does not in any way add to the food value of the jar, and gives herself a great deal of unnecessary work.