A Mason jar is a molded glass container used for home canning. It is made of thicker glass to withstand the heat and pressure needed to kill organisms which could cause spoilage of the food inside the jar. The mouth of the jar has screw threads on the outside. A two-piece metal lid with a rubber ring underneath adhers to the lip of the jar under heat and pressure creating a hermetic seal.

John Landis Mason, a Philadelphia tinsmith, received his patent for an improvement on screw-neck bottles on November 30, 1858. The process of forming the lip area of a glass container to be smooth, even and strong enough for a lid to be screwed on was difficult. The first jars were made at the Atlantic Glass Works in Washington Township, New Jersey.

Through the following decades of the nineteenth century many companies produced canning jars with Mason's patent date embossed on them. Today's jar collectors have a difficult time identifying authentic containers. Modern reproductions with Mason's patent date come from China and Taiwan. They are used for a variety of craft and decorative projects. They also come in bright colors which nineteenth century manufacturers could never have achieved.

After food is packed in the jars, leaving some empty space at the top of the jar to allow for expansion during processing, the lids are screwed on, but not too tightly. Low-acid foods such as green beans and corn are processed in a pressure canner. Acid foods such as pickles and fruits are processed in a water-bath canner. The pressure canner is necessary to kill microbes which produce botulism, a deadly poison.

Today, jams and jellies are also processed in the water-bath canner, even though they contain large amounts of sugar which acts as a preservative. The old method of covering jams and jellies with melted paraffin is no longer a recommended procedure because mold can grow under the paraffin.

We do not know if Carl Sandburg's mother canned food for the family. Clara Sandburg had a large family and was a careful homemaker. It is likely she, with the help of her daughters Esther and Martha, canned food in season We do know that she stored cabbages and potatoes in the basement of their Berrien Street home. Rutabagas were dug from the frozen garden soil, as needed, by the Sandburg boys Carl and Martin.