Can you microwave mason jars?

Mason and Kilner jars are made to survive a slow water bath, not the steep local temperature gradients a microwave produces. They are soda-lime glass rather than borosilicate, so a hot spot next to a cool region can crack them, and the risk is highest when the jar comes out of the fridge. The two-piece metal lid is a second problem, since it will spark. A few brands now sell jars specifically marked microwave-safe; those are the exception and they say so on the glass.

What goes wrong

• Thermal shock cracks the jar, sometimes while you are holding it
• The metal lid and band arc against the cavity
• A sealed jar builds steam pressure with no way to vent

What to do instead

• Decant into a bowl or a mug
• Stand the sealed jar in a bowl of hot tap water for a few minutes to take the chill off

Other "can you microwave" questions

• Can you microwave styrofoam? It depends — Only if the container is labeled microwave-safe, and most takeout foam is not.
• Can you microwave aluminum foil? No — No — foil reflects microwaves and the edges arc, which can start a fire.
• Can you microwave plastic containers? It depends — Only ones labeled microwave-safe — cold-storage tubs are not among them.
• Can you microwave eggs? It depends — Beaten or pricked eggs, yes. A whole egg in its shell will explode.
• Can you microwave Tupperware? It depends — Yes, if that specific piece carries the microwave-safe symbol on its base.
• Can you microwave paper plates? It depends — Plain white paper plates are fine; anything with metallic trim or a foam layer is not.
• Can you microwave metal? No — No — metal reflects microwaves, arcs at edges, and can damage the oven or start a fire.
• Can you microwave breast milk? No — No — microwaves create hot spots that scald a baby even when the bottle feels cool.
• Can you microwave takeout containers? It depends — Only if the container is marked microwave-safe — most are not, and metal-handled ones never are.
• Can you microwave cardboard? It depends — Plain dry cardboard is usually fine briefly; anything with glue, staples, wax or foil is not.
• Can you microwave glass? It depends — Yes for ordinary heatproof glass; no for leaded crystal, and never straight from the freezer.
• Can you microwave water? It depends — Yes, but very clean water can superheat and erupt when you move the cup.