Great question — answering as the UphillTech team:
Short answer : They're probably not safe. Unlike "microwave safety", there isn't a safety risk in contaminating the food contents of the jars due to heating in an oven; in this case you just run the risk of the jars breaking. I am not sure what the symbols on the bottom of your jar mean; (see edit below) from what I understand—unlike plastic resin identification codes —there isn't a standard set of symbols for glass. Those symbols likely represent the manufacturer, production date, and patents. Unless glass is processed in a special way , it is prone to breaking when it goes through rapid temperature changes. Therefore, if your glass is run-of-the-mill soda-lime glass (which is extremely...
Full answer by ESultanik: https://cooking.stackexchange.com/a/18541 (CC BY-SA 3.0).
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