A Mason jar is a glass jar used in canning to preserve food. They were invented and patented by John Landis Mason, a Philadelphia tinsmith[1] in 1858.[2] They are also called Ball jars, after Ball Corp., a popular and early manufacturer of the jars; fruit jars because they are often used to store fruit; "jam jars" or generically glass canning jars. While largely supplanted by other methods for commercial mass-production, they are still commonly used in home canning.
Today, the terms often refer to jars featuring a two-piece cover: an inner, flat, metal or glass lid, covered by a screw-on ring. The ring holds the lid in place during the canning process, which creates a partial vacuum, sealing the lid until opened. Because lids are sold separately, the jars and rings can be reused.
Mason jars are made of soda-lime glass, and come in a variety of sizes including pint, quart, half-gallon, and cup, as well as in wide-mouth (3 inch diameter) and standard mouth (2 3/8 inch) openings.[3]
The most common U.S. brands of Mason jars are Ball and Kerr. Both are now part of the Jarden corporation.
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In home canning, food is packed into the jar, and the steel lid is placed on top of the jar with the integral rubber seal resting on the rim of the jar. The band is screwed loosely over the lid, which will allow air and steam to escape. The jar is heat sterilized in boiling water, if Boiling-Water-Bath-canned (BWB-canned), or steam, if pressure-canned using a cooking utensil called a canner. When the jar is removed from the canner it is placed on the counter where it is allowed to cool to room temperature. The cooling of the contents creates a vacuum in the jar and the lids seals. Once cooled, the band is removed to prevent residual water between the jar threads and the lid from rusting the band. If the jar seal is properly formed, internal partial vacuum will keep the lid tightly on the jar. Some metal lids are slightly domed, much like the bottom of a mechanic's oil can. On a properly sealed Mason jar, the lid will be slightly concave. A "popped up" (flat or convex) lid may be a sign of microbial growth, including potential botulism, or seal failure. Some old Mason jars used glass lids with separate rubber rings, instead of steel, and are now considered unsafe for canning.
The earliest glass jars were called wax sealers, because they used sealing wax, which was poured into a channel around the lip that held on a tin lid. This process was complicated and error-prone, but was largely the only one available for a long time, and widely used even into the early 1900s.
By far, though, the most popular form of seal was the screw-on zinc cap, the precursor to today's screw-on lids. The earliest successful application of this was discovered by Mason and patented on November 30, 1858, a date embossed on thousands of jars. Jars with "Patent Nov 30th 1858" were made in many shapes, sizes and colors well into the 1900s. Since they were made in such quantity and used for such long periods, many of them have survived to the present day.
Another popular closure was known as the Lightning closure, named after the first jar to use it, which was embossed with "Lightning" on the side. More commonly, this is often known as a bail closure, or French Kilner — it consists of a metal wire that leverages a glass lid down when pressed against the side of the jar. While these jars are still sold for storage, they are now rarely used for canning.
The heyday for jars was probably 1860-1900, when an explosion of patents for various closures were issued, ranging from the effective to the absurd. The more absurd closures were quickly abandoned, but often fetch high prices in today's antique market.
Antique mason jars are eagerly sought by collectors, and are bought and sold not only through antique stores, but also on auction sites such as eBay. While most jars sell for only a few dollars, some have sold for as high as $30,000 . The value of a jar is related to its age, rarity, and condition.
The age and rarity of a jar can be determined by its color, shape, mold and production marks, and closure. Most antique jars that aren't colorless are in some shade of aqua (in particular "Ball blue," named for the company that made them). Colored jars were considered better for canning use, as they block some light from reaching the food, which helps to retain flavor and nutritional value longer. More rarely, jars will turn up in amber, and occasionally in darker shades of green. Rarer still are cobalt blues, blacks, and milk glass jars. Some unscrupulous dealers will irradiate jars to bring out colors not original to the jar.