The idea that jars are universally freezer-safe circulates widely, usually alongside photographs of neatly stacked soup. Then somebody opens the freezer to find a jar split down the side and a block of frozen stock wearing a glass collar. The physics behind this is not complicated, and once you know the rule you can freeze in glass confidently for years without a single failure.
The shoulder is the problem
Liquid expands as it freezes and has to go somewhere. In a straight-sided jar it rises freely towards the opening. In a shouldered jar, meaning one that narrows towards the neck, the expanding contents push outwards against the slope and the glass takes the force. That is why the same soup freezes safely in one jar and destroys another that looks superficially similar.
How to freeze in glass properly
Use straight-sided jars only, leave two to three centimetres of empty space at the top, and leave the lid loose or off until the contents are frozen solid, then tighten it. Freeze jars upright and not touching each other, so they cool evenly. If you are choosing jars specifically for this, ranges such as https://netany.org/ list wall shape and capacity, and the straight profile is visible in the photograph if you look at the neck rather than the label.
Thawing without cracking
Move the jar to the fridge and give it several hours or overnight. Do not put a frozen jar under a hot tap, into hot water or into a microwave straight from the freezer, because sudden temperature change is exactly what glass cannot cope with. If you are in a hurry, stand the jar in cool water and change the water as it chills.
The rules in short
- Straight-sided jars only, never shouldered ones.
- Leave a clear gap at the top for expansion, and more for very liquid contents.
- Lid loose until frozen solid, then tightened.
- Freeze upright, spaced apart, on a flat surface.
- Thaw in the fridge, never under hot water.
Another myth: any jar can be used for preserving
Jars sold as drinking glasses or for dry storage are not automatically suitable for home preserving, and a decorative lid does not create a proper vacuum seal. Only use jars and closures the manufacturer states are intended for canning, and follow a tested method from a reliable source rather than a recipe found in passing. This is one area where improvisation carries a genuine food safety risk rather than an aesthetic one.
What freezes well in glass and what does not
Stock, soup, sauces, stewed fruit and cooked pulses all freeze well in straight jars. Anything with a high water content and little structure expands the most and needs the largest gap. Things you plan to reheat quickly are better off in a shallow container, since a tall jar of frozen soup takes a long time to thaw evenly no matter how you do it.
Inspecting jars that have been frozen
Look each jar over when you wash it after use, particularly around the base and the rim. Freezing puts stress into glass, and a fine crack that survived one cycle may not survive the next. Retire anything with a visible line or a chip rather than giving it one more go, and remember that a jar which has been frozen repeatedly has had a harder life than its appearance suggests.