Inspect every canning jar before each use by running a fingertip around the whole rim to feel for chips and rough spots, holding the glass to the light to look for cracks, and checking that the threads are undamaged. A jar with any rim defect cannot seal and a jar with a crack can burst in the canner, so retire it from canning and keep it for dry storage.
Part of Canning & Pressure Preservation
Jar inspection is the pre-use check of the sealing rim, the glass body and the threads of a reused canning jar.
The Rim Is The Seal
The lid’s compound seals against the top edge of the jar. A chip or nick there, even one too small to see, leaves a leak path that the vacuum cannot overcome. That is why the parent article says to run a finger around the rim: touch finds what the eye misses. The glass body matters as well. A jar is heated and cooled many times in its life, and a crack or deep scratch is a stress concentration where it can burst in a pressure canner, scattering glass and hot food.
What You Need
- Good light and a clean cloth
- Fingertips, and a second person to check what the first examined
- A sorting rule and containers, one for jars passed for canning, one for jars passed for dry storage only, and one for scrap
Method: Four Checks Per Jar
Step 1: Rim. Run a fingertip all the way round the top edge, slowly. Any catch, chip, nick or rough spot fails the jar for canning.
Step 2: Body. Hold the jar up to the light and turn it. Look for hairline cracks, particularly near the base and shoulder, stress marks, and cloudy or etched patches that point to damage.
Step 3: Threads. The screw band must turn cleanly on the threads. Damaged threads fail the jar for canning.
Step 4: Cleanliness. The jar must be washed in hot soapy water before filling, as the parent article directs; hard-water scale or residue inside the rim can prevent a seal.
Sort the jar into “pass for canning”, “dry storage only” or “scrap”. Keep a failing jar: it is a valuable container for flour, beans or salt, only unfit for canning.
Which Jars To Trust
Purpose-made canning jars are made of tempered glass for heat cycling. Thinner commercial food jars, the mayonnaise and sauce jars of the parent article, are made for a single heat cycle and are not recommended. If you must use them, use them for water bath only, with the same inspection, and retire them at the first defect.
Common Mistakes
- Passing a jar by looking only. The fingertip check is the test.
- Washing hot jars in cold water, or cold jars in boiling water. Thermal shock cracks glass; keep jars hot until filling.
- Using a jar that has been frozen full. Frozen contents can crack a jar invisibly.
- Using a jar with a chip “just once, in the water bath”. A chip does not stay in the water bath; it ends up on the shelf unsealed.
Safety
Broken glass in food
A jar that bursts in a canner or on the counter can scatter fine glass fragments into food. If a jar breaks during processing, discard the whole batch in that canner load unless the food can be examined and the fragments removed with certainty, and clean the canner thoroughly. Do not use chipped jars in any heating process.
What’s Next
- Learn next: Reusable lid alternatives (Tattler-style) — what to close the jars with when lids run out
- Combine with: Jar types, lids, and seal mechanics — why the rim matters