How do you know your jars sealed, and what do you do with one that didn't?
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Let processed jars cool upright and undisturbed for 12 to 24 hours, then take off the bands and test each lid: a sealed lid curves slightly down in the center, does not flex when you press it, and gives a clear ringing sound when tapped with a spoon [1][2]. A jar that fails has two good options within 24 hours: put it in the refrigerator and use it first, or reprocess it with a new lid [2][3]. Sealed jars get washed, labeled with the date and stored without bands [4]. This page takes you through the cooling period, the seal tests, the reasons lids fail and what to do with each failed jar.
The first day, hour by hour
| When | What to do | Why |
|---|---|---|
| Out of the canner | Set jars upright on a towel, rack or board, at least 1 inch apart, away from cold surfaces and drafts [5] | UGA's bulletins both say to keep hot jars off cold surfaces and out of drafts [2][5] |
| First hour | Leave them alone. Do not tighten bands, tip jars, or press lids [5][1] | Retightening a hot lid can cut through the gasket and cause the seal to fail [1] |
| Over the next hours | Listen for the snap of lids pulling down | The lid's compound seals as the jar cools and a vacuum forms [6] |
| 12 to 24 hours | Leave jars in place | UGA: cool upright while the vacuum seal is drawn and the jam sets up [2] |
| At 12 to 24 hours | Remove bands and test every seal [1] | A band can hold an unsealed lid down and hide the failure |
Some jams take longer than a day to reach their final set. A soft jar at 24 hours is not yet a failed batch; jam that didn't set says how long to wait before deciding.
Three ways to test a seal
NCHFP gives three tests [1]. Use at least two on every jar.
- Press. Press the middle of the lid with a finger. If it springs up when you release it, the lid is not sealed.
- Tap. Tap the lid with the bottom of a teaspoon. A sealed jar gives a ringing, high-pitched sound. A dull sound can mean the lid is not sealed, or that jam is touching the underside of the lid; check that jar with the other tests.
- Look. Hold the jar at eye level and look across the lid. A sealed lid is concave, curving slightly down in the center. A flat or bulging center may mean no seal.
SDSU adds a fourth check once the band is off: lift the jar gently by the edge of the lid, just a little off the counter, with your other hand underneath [7]. A sealed lid holds. Do this low over a towel.
Write the result in your batch record: how many jars, how many sealed, and anything unusual. Patterns across batches are how you find a cause. The batch log template has a column for it.
A jar didn't seal: your options
| Option | How | When to choose it |
|---|---|---|
| Refrigerate and use first | Put the jar in the refrigerator and treat it as an opened jar [2] | One or two jars; the easiest choice and the best for quality |
| Reprocess within 24 hours | Check the rim for nicks, use a clean jar if needed, heat the jam to boiling, refill hot, put on a new lid and process again for the full time [1][3] | A whole batch failed, from a cause you have fixed |
| Freeze | Move to a freezer container or adjust the headspace for freezing; NCHFP gives up to a year frozen [3] | You have more jars than you can eat soon |
| Discard | Only if the original recipe was not a tested one [3], or the jar shows spoilage | See how to tell a jar has gone bad |
How long a refrigerated unsealed jar keeps
The sources give different figures. NCHFP's general canning advice, written for every kind of home-canned food, is to refrigerate an unsealed jar and eat it within several days to a week [1][3]. UGA's jam and jelly bulletin says simply to refrigerate unsealed jars of jam and use them first [2], and NCHFP's jam storage guidance gives an opened jar of regular full-sugar cooked jam about 1 month in the refrigerator at 40°F or below [8]. This page follows the jam-specific guidance, because an unsealed jar of full-sugar jam that was hot-filled and processed is in the same position as one you have just opened. Use reduced-sugar jam sooner, and discard any jar that shows mold, a yeasty or alcohol smell, or bubbling [8].
Reprocessing jam, and what it costs you
NCHFP permits reprocessing only within 24 hours, and only when the first attempt followed a research-tested recipe [3]. It also says reprocessing "is not recommended" because of the quality loss [3]. For jam, a second boil and a second process add exactly the kind of extra heat that UGA warns can weaken the gel of a fruit low in pectin [2]. That is why refrigerating a single failed jar is usually the better choice, and why reprocessing makes sense mostly when a whole batch failed for a reason you can see and fix.
People ask whether they can reboil runny jam right in its jars. No: a sealed jar of jam that set too soft is a different problem from a failed seal. To fix the set, the jam comes out of the jars and is remade with added pectin or more cooking, then processed with new lids. The method is on jam that didn't set.
Why lids fail to seal
| What you find | Likely cause | Prevention |
|---|---|---|
| Nick or chip on the rim | Damaged jar [3] | Run a finger around every rim before filling |
| Jam residue on the rim or under the lid | Rim not wiped, or too little headspace so jam bubbled out [9] | Wipe every rim; leave 1/4 inch of headspace |
| Lid buckled or creased at the edge | Band too tight, so air could not vent; or lids preheated when the maker says not to [10] | Fingertip tight; follow the lid box |
| Many failures in one batch from one box of lids | Possible lid defect, or very old lids | Lids work for at least 5 years from manufacture [6]; contact the maker [11] |
| Failures after you pressed lids down to help them | Pressing lids to force a seal | Leave lids alone until tested [11] |
| Failures across a batch, clean rims, correct bands | Processing too short, or the boil stopped and timing was not restarted | Check your time and altitude on how long to process jam |
| Lid sealed, then came loose in storage | A broken seal in storage allows recontamination and spoilage [4] | Discard it; see how to tell a jar has gone bad |
Lid preparation and band tightness are covered on lids and bands; headspace on its own page.
Washing, labeling and putting jars away
- Leave the bands off. Stored bands trap moisture and rust, and they hide a seal that fails later [4][6]. Wash and dry them for the next batch.
- Wash the jars and lids with a damp, clean cloth or warm soapy water to remove any sticky residue, without disturbing the sealed lids [4][5].
- Label and date each jar [4]. At minimum, write the product and the date it was made; add a batch number if you make several batches of one product. Labels go on after processing and cooling; for labels that hold up on glass and in the refrigerator, see labels that stay on.
- Store in a cool, dry, dark place [2]. The temperatures and how long jam keeps are on storing jam and how long it keeps.
How long your sealed jars will keep
A good seal starts the shelf life. Temperature, light and sugar level then decide how long the color and flavor last. Go next to storing jam and how long it keeps.
References
- National Center for Home Food Preservation, Cooling Jars and Testing Jar Seals (adapted from USDA Agriculture Information Bulletin 539, revised 2015), viewed October 2026. https://nchfp.uga.edu/how/can/general-information/cooling-jars-and-testing-jar-seals
- University of Georgia Extension, Andress, Preserving Food: Processing Jams and Jellies, FDNS-E-37-1, revised June 2011. https://openscholar.uga.edu/record/23661/files/FDNS-E-37-1.pdf
- National Center for Home Food Preservation, May canned food be reprocessed if the lid does not seal?, undated, viewed October 2026. https://nchfp.uga.edu/newsflash/may-canned-food-be-reprocessed-if-the-lid-does-not-seal
- National Center for Home Food Preservation, Storing Home Canned Foods, viewed October 2026. https://nchfp.uga.edu/how/can/general-information/storing-home-canned-foods
- University of Georgia Cooperative Extension, Schwan, Beltran and Andress, Preserving Food Series: Using Boiling Water Canners, Circular 1344-01, May 2025. https://nchfp.uga.edu/papers/factsheets/Preserving_Food__Using_Boiling_Water_Canners.pdf
- National Center for Home Food Preservation, Recommended Jars and Lids (from USDA Complete Guide to Home Canning, Agriculture Information Bulletin 539, revised 2015), viewed October 2026. https://nchfp.uga.edu/how/can/general-information/recommended-jars-and-lids/
- South Dakota State University Extension, Kleine and Botzet, A Guide to Water Bath Canning (P-00255), 2022. https://extension.sdstate.edu/sites/default/files/2022-10/P-00255-v2.pdf
- National Center for Home Food Preservation, Storing Home-Canned Jams and Jellies, October 2005. https://nchfp.uga.edu/how/make-jam-jelly/jams-jellies-general-information/storing-home-canned-jams-and-jellies/
- Kansas State University Research and Extension, Blakeslee, The Importance of Headspace in Canning, You Asked It!, August 13, 2020. https://www.k-state.edu/news/you-asked-it/articles/2020/08/the-importance-of-headspace-in-canning.html
- Kansas State University Research and Extension, Blakeslee, What Causes Canning Lids to Buckle?, You Asked It!, June 25, 2026. https://www.k-state.edu/news/you-asked-it/articles/2026/06/what-causes-canning-lids-to-buckle.html
- University of Wisconsin-Madison Extension, Winnebago County, Ingham, Canning Update: Successful Jar Sealing, August 14, 2020, updated September 4, 2020. https://winnebago.extension.wisc.edu/2020/08/14/canning-update-successful-jar-sealing
