A short history of glass jars.
Glass jars and bottles have been used as food container material dates
from ancient times. The Egyptians made pottery for cookware and used pots
for storing foods for short times. The glaze on pottery is similar to
glass. The use of glass as a container for foods is however quite recent,
originating with the innovation of preserving otherwise perishable foods,
such as meats and vegetables, by the French confectioner Nicolas Appert
in the early years of the nineteenth century. Glass bottles used for champagne
became Appert's receptacles for food which he heated at the boiling point
of water for long periods to stabilize the foods against spoilage. Appert
published recipes for more than 50 foods , winning the prize established
by Napoleon and became the father of modern food preservation some half
a century before Pasteur's discoveries of the causes of food spoilage.
The properties of glass that make it especially attractive in packaging
and storage of foods are its inertness , impermeability , transparency
and heat resistance, all of which are called into play in the majority
of food applications for glass jars.
Glass jars are usually composed of soda lime glass manufactured by melting
together sand, limestone, soda ash and minor additives in a gas fired
furnace. The molten glass is then fed through machines where it is finally
blown into finished jar shape using a mold. Within seconds the temperature
of the glass has dropped several hundred degrees, generating thermal and
shrinkage stresses which must be removed or reduced to achieve optimum
strength in the finished container. Annealing in a tunnel oven provides
the requisite strain relief, during which temperatures are reduced in
several increments from 600C to ambient resulting in a finished glass
jar with the strength and durability we expect for consumer use.
Some jars are made using different glass compounds or further heat treated
(tempered) to increase mechanical strength placed on the edges of the
jars or to increase their resistance to temperature extremes.
Metal ring and seal canning jars were developed because they are cheaper
to make and they are less susceptible to damage when opening canned foods
than the wire bail jars. During canning you create a vacuum in the jar
and occasionally the glass would be damaged by prying the lid up with
a tool like a knife. Hi-Fi jars and gaskets were developed to eliminate
this damage. You just cut the tab and the vacuum is released. |
