Are glass jars from the supermarket oven safe?

Question

I recently discovered from the working class foodies video on french onion soup that mugs are oven safe, or at least their mugs and my mugs seem to be by experimentation. I also happen to be one of those people that saves pickle, salsa, etc jars to store food, so I was wondering if these are oven safe too.

The jars in question are trader joes corn and chili salsa, and the bottom has the following symbols and numbers on them, which might hint as to the qualities of the glass.

So, are these jars oven safe? I currently bake my french onion soup at 450°F, but I might want to broil it as well to brown the cheese.

Answer 1

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 likely), and if you were to put it from room temperature directly into a 450°F oven, the shock of that rapid temperature change would likely crack it. Furthermore, even if you were able to gradually heat it up to 450°F without it cracking, it would likely crack even as it naturally cools down. In order to try and ensure that the glass doesn't crack you'd have to both gradually heat the jars up and then very gradually step the oven temperature back down to room temperature.

If you want to use glass, your best bet would be to use something like Pyrex.

Even Pyrex, which is explicitly designed to be oven safe, can't withstand the direct heat of a broiler, though.

Edit: I believe the symbol on the bottom of your jars reads "A.G.C." surrounded by the outline of the state of Arkansas. This implies that your jars were made by the Arkansas Glass Container Company. I believe the numbers indicate the model number, which appears to be this jar. AGC unfortunately don't have anything on their website listing the oven safety of their glass. If you're really interested, you could try contacting the manufacturer.

Answer 2

Did you try it? What happened? I put TJ’s same empty clean jars and lids in an aluminum pan and placed everything in toaster oven cold. Turned on heat and experimented on several settings and times. They were fine. I filled them with solids and liquids and did the same with lids on. They were fine. Note: All were placed in cold and then heat was turned on. Let cool down to almost touchable before removing and opening. Based on sand science, which is what glass is primarily composed of, it’s the temperature changes that cause breaking so the heating up and cooling down is crucial.