Though many have an appearance similar to that of canning or Mason Jars, commercial tomato sauce and pickle jars, to name a few, are not made using the same, stringent process used for making canning jars.
Incidentally, the canning jar “look” is, in part, a marketing-based design. The canning jar is such an icon of the modern kitchen that even younger generations recognize it and associate it with the idea of “fresh” and “homemade.” This is not bad in and of itself, but it definitely does not lend to the jar the right properties to sustain the canning process. In addition to this, the Mason jar shape is a very sound one for a glass container that must resist to shock during transportation and handling.
Bill Lindsey, a renowned collector who owns over a thousand glass jars and bottles and writes about them for the Society for Historical Archeology had this to say about the legacy of the Mason jar when asked about its effect on the glass jar industry: “Mason jars are the glass-jar industry,” he said. “The first machine that was successful for making glass containers was making Mason jars.”
Jars designed for commercially storing sauces and foods for transportation and display purposes are not designed to resist the repeated heating and cooling process inherent to canning. The price of a jar of Prego sauce would probably be higher if the container were made for canning, because it would require a higher density of glass to produce it. This is simple not necessary given this jar’s purpose.
If it sounds like I am trying to make a strong point for NOT using commercial sauce jars and the like for canning, well, you got the point! We do not sell canning jars, so you can rest assured that this Granny is simply trying to make sure you use caution. Remember… listen to your Granny! She is about to tell you how canning jars are made.
Soda-lime-silica glass is used in the manufacturing of canning jars and other glass product that must be highly breakage resistant such as window panes and windshield. Cookware like pie plates and roasting pans are made with soda-lime-silica glass as well. It is composed of about 70% silicon dioxide, 15% sodium oxide, 9% calcium oxide and traces of other compounds that contribute to its strength and workability.
These ingredients are melted together in a blast furnace that can reach temperatures up to 1,600 degrees Celsius. From the furnace, the molten glass product then goes to the manufacturing process where it is injected into metal molds with a high force blower. A specific quantity of the molten glass product is used for each jar size.
A blower is used in the injection process in order to add the pressure necessary to obtain the right shape and density, as well as perfect cohesion so that all parts of the jar are sound and smooth, including markings and lid threads. Once properly filled, molds are allowed to cool for several minutes before the jars are released.
After this, the jars are allowed to cool for several hours in an annealing room. This room allows for temperature control such that the temperature near the surface of the jar is never very different from that of the interior. This reduces the strain on the glass so that it does not crack or weaken in any way during the cooling process. Such glass is said to be annealed. Anneal is a term used in metallurgy and glass making and is defined as follows: To subject glass or metal to a process of heating and slow cooling in order to toughen it and reduce brittleness.
You might find it interesting that as it cools, freshly molded glass remains in a semi-malleable state similar to that of a soft plastic until it reaches a temperature of 730 degrees Celsius.
So how should you recycle those store-bought tomato sauce jars? Use them for direct storage of leftovers to be refrigerated or to make refrigerator pickles… but, and Granny insists, be mindful that store-bought sauce and pickle jars will react promptly to changes in temperature by cracking (or altogether breaking). Handle with care.