The first thing you need to know—and I wish someone had yelled this at me—is that shape matters. Regular mason jars with that iconic shoulder (the curve near the neck) are not your freezer friends. When liquid freezes, it expands, and that shoulder creates a pressure point where glass loves to crack.
The heroes of the story are straight-sided mason jars. Think of the ones used for jelly or the wide-mouth pint jars. They have a uniform cylinder shape from top to bottom, giving that expanding ice plenty of room to push upward without shattering everything in sight.
Look for the word “freezer safe” directly on the box or the jar’s side. Ball and Kerr make specific lines—Ball Freeze Smart is a game-changer—with thicker glass and a special shoulder design that handles the chill.
