A Mason jar is a molded glass jar used in home canning to preserve food. The mouth of the jar has screw threads on its outer perimeter to accept a metal ring (or "band"). The band, when screwed down, presses a separate stamped steel disc-shaped lid against the rim of the jar. An integral rubber ring on the underside of the lid creates a hermetic seal to the jar. The bands and lids usually come with new jars, and bands and lids are also sold separately; while the bands are reusable, the lids are intended for single use when canning.
While largely supplanted by other methods, such as the tin can and plastic containers, for commercial mass production, they are still commonly used in home canning.
The Mason jar was invented and patented in 1858 by Philadelphia tinsmith John Landis Mason (1832–1902). Among other common names for them are Ball jars, after Ball Corporation, an early and prolific manufacturer of the jars; fruit jars for a common content; and simply glass canning jars reflecting their material.
Mason jars are made of soda-lime glass and in the U.S. come in regular mouth 2 3⁄8 in (60 mm) inner [2 3⁄4 in (70 mm) outer] diameter or wide mouth 3 in (76 mm) inner [3 3⁄8 in (86 mm) outer] diameter and a variety of volumes including cup (half-pint), pint, quart, and half-gallon.
The most common U.S. brands of Mason jars are Ball, Kerr and Golden Harvest. All three are now part of the Jarden corporation based in Boca Raton, Florida. In Canada, Bernardin, another division of Jarden, is the most common brand; Jarden's Golden Harvest is also available. Jarden offers its Canadian jars in metric volumes of 125ml, 250ml, 500ml and 1 litre.
In home canning, food is packed into the jar, leaving some empty "head space" between the level of food and the top of the jar, then the lid is placed on top of the jar with the integral rubber seal resting on the rim. The band is screwed loosely over the lid, allowing air and steam to escape. The jar is heat sterilized in boiling water or steam and the lid secured. The jar is then allowed to cool to room temperature.
The cooling of the contents creates a vacuum in the head space, pulling the lid into tight contact with the jar rim and creating a hermetic seal. 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 vacuum will keep the lid tightly on the jar. Most metal lids are slightly domed to serve as a seal status indicator: the vacuum in a properly sealed mason jar pulls the lid down such that the dome is concave, but an improper or failed seal or microbial growth will cause the dome to pop upward.
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 it 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 U.S.-made brand to use it, which was embossed with "Lightning" on the side. This is commonly known as a bail closure or Kilner — it consists of a metal wire arrangement with a lever which, when pivoted downward against the side of the jar, applies leverage to a glass lid, clamping it down over a separate rubber O ring. The bail-style jars are still widely used in Western Europe, particularly France and Italy, where the two largest producers (France's La Parfait and Italy's Bormioli Rocco) produce the La Parfait and Fido brands, respectively. However, while bail-type jars are still widely available in the U.S., they are generally marketed there exclusively for dry storage, and thus are only rarely used there for home canning.
This is due to several factors: the lack of U.S.-based manufacturers of bail jars since the early 1960s after a sharp falloff in popularity of home canning in the 1950s and '60s, consolidation of the U.S. canning jar industry around the Ball Corporation (now Jarden) and the resulting higher cost of imported European-made jars. Perhaps most importantly is the official position of "Not Recommended" by the U.S.-based National Center for Home Food Preservation, the USDA and by University Extension Services: the reason they state is that they feel there is "no definitive way to determine if a vacuum seal is formed." 
Between 1860 and 1900, a great many patents were issued for various jar closures. The more esoteric closures were quickly abandoned, and thus can fetch high prices in today's antique market.
Antique mason jars are eagerly sought by collectors, often sold through antique stores and auction sites such as eBay. 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 are not colorless are a shade of aqua known as "Ball blue," named for the prevalent jar maker. 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.
- "John Landis Mason".
- "Reports of Cases Argued and Determined in the Circuit Court of the United States for the Second Circuit: 1845–1887" By Samuel Blatchford, United States Circuit Court (2nd Circuit). Published by Derby and Miller, 1875. Retrieved October 22, 2008
- "After years of moving its employees south, Jarden Corp. has officially switched its headquarters to Boca Raton to give the city another major corporate player to boast about." Bandell, Brian. Company with $7B in sales now calls Boca Raton home. 13 March 2014. Accessed February 2015 at http://www.bizjournals.com/southflorida/news/2014/03/13/company-with-7b-in-sales-now-calls-boca-raton-home.html
- Jars requiring a zinc cap and jar rubber or jars requiring a glass lid, wire bail, and jar rubber have not been recommended since 1989 because there is no definitive way to determine if a vacuum seal is formed." -- Griffith, Patti. The time is ripe for summer melons. University of Wyoming Cooperative Extension Service. From series "Canner's Corner: Enjoying Summer's Bounty." Issue Two. MP-119-2. Accessed March 2015 at http://www.wyomingextension.org/agpubs/pubs/MP119_2.pdf
- "Bottle Finishes & Closures".
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