(Continued from Part 1. This concludes the article.) #4. “Tempered less than Mason jars.” Nearly every book, website, and blog discussing this subject insists that Mason jars are tempered. They’re definitely, absolutely, NOT tempered. Glass has three basic hardening options: tempering, heat strengthening, and annealing. All commercial and home-canning jars are annealed, not tempered. Annealing is a process where jars are cooled down very slowly after production to make them more consistent and to minimize stresses. Whenever glass breaks into the sharp jagged pieces seen when we break a jar or window it’s merely annealed, not tempered or heat strengthened. …
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