I've been home canning for over 15 years. I think that's a while, though of course my great-grandmother, whose cellar looked like that Library of Congress photo on Wikipedia that I use as a background image on my computer sometimes, would call me a mere novice. But whatever my great-grandmother would call my skill level, in those 15-plus years I've lost only a single jar in my home canners.
There are four things that can cause a canning jar to break: thermal shock, impact shock, internal overpressure, or a manufacturing flaw. The last is very rare! Anecdotally, I hear complaints that jars aren't made as well as they used to be. But I think the real problems are unintentional mis-use related to the first three problems. Keep reading!
Thermal shock happens when a cold jar meets hot, or vice versa. Examples: a jar is not thoroughly heated before hot product is put into it; jars are pre-heated in an oven before filling; jars are removed from a canner after processing and placed next to an open window, or on a bare counter rather than atop a towel or wooden cutting board.
Impact shock happens when a jar hits a counter, sink, canner wall, or other surface too hard to withstand the strike. This is a straightforward problem, and it can happen to the most careful of us because home canning is a steamy, liquidy endeavor where the potential for spills and slips is everywhere.
Internal overpressure happens when a jar is closed too tightly before processing. This is why "fingertip tight" is an important thing to understand. Unfortunately, it's an area of home canning that can be more art than science. Sharon at Simply Canning describes it as not "crank[ing] down hard, snug is fine." Why does she mention cranking? Because in the olden days, you would take your jars out of the canner and wrench the paraffin-lined caps closed super tightly -- literally with a special wrench tool. Old habits die hard, and on YouTube videos you'll see people over-tightening their jars before placing them in the canner. Don't you believe it! While the jars are in the canner, the product and air inside them expands. The jars must vent that air (and, sometimes, a portion of the product). If you over-tighten the jars, they will fail to vent, but the expanding air and product have to go somewhere, so boom! The glass will break.
I suspect the vast majority of "my jars had a manufacturing defect" anecdotes are actually problems of internal overpressure. I think it's especially the case when you hear about someone losing multiple new jars in a single canning project. In computer software terms, it's not a bug (a flaw in the software), but a user error. Learn to love and embrace "fingertip tight"!
As I say, I have lost only one jar in my years of home canning. How have I avoided breaking my jars? By running through this checklist every single time I embark on a canning project:
- Do not mix canning and food-storage jars. I've discussed that question previously.
- Check the state of the jar. Flaws on the rim will prevent the lid from sealing. Look at the bottom, sides, shoulders, and neck for non-catastrophic cracks that the jar may have developed (unnoticed) last time it was used, or while in storage. The smallest flaw will cause a jar to not withstand the canning process. At this point, it's best to toss or recycle a jar with any crack; don't use it for dry food storage.
- Pre-heat the jar gently and completely. Are your jars in a cold basement or cellar? Bring them up to room temperature before starting. In any event, heat them by submerging them in your boiling-water bath canner and then turning on the flame. Or fill them with water, place them in a few inches of water in the pressure canner, and turn on the flame. When the water is boiling, the jars are ready to fill. Lift them out carefully, dump the water inside them into the sink, fill the jars, and replace them in the canner. Do not heat them in the oven. Opening and closing the oven door, as well as the difference between the oven's dry heat and the wet heat of the product, will raise your risk of thermal shock on the jars. Why? Keep in mind the temperature of the product you're canning. Now, pressure-canned product is going to be about the same temperature as boiling water, so the jars can more easily thermally withstand filling for pressure canning. But water-bath canned product is usually very sugary. Whether it's grape jelly, strawberry jam, peach butter, or even unsweetened applesauce, it will be hotter than boiling water. You need the jars to be as close to the product's temperature as possible.
- Fill jars and get them back into the canner quickly. Minimize the time a jar sits (and cools) while it waits to be filled. My method is to remove and fill only one or two jars at a time. I do understand that would be cumbersome if you're, say, canning a batch of 19 pints; but that kind of canning is mostly beyond the scope of this blog.
- Remove jars from the canner smoothly and gently. After water-bath canning, lift up the rack, remove the jars, and place them on a towel or a wooden cutting board. If you're pressure canning, follow the canner manufacturer's instructions to the letter. Don't follow the Ball Blue Book. Don't follow the steps on the website where you got the recipe, even if it's a county extension service. Follow the canner's user manual, which will tell you exactly how to let your particular canner vent and cool gradually. Most of the time, if you don't follow the instructions, your main risk is losing a lot of product that vents out of the jar. But there's also a chance that the cooler air rushing into the canner too early will cause a thermal shock to a jar.
- Let the jars sit, untouched, out of drafts, overnight. Then remove the bands, wipe the jars down, and store them.
- Empty jars carefully. No metal utensils! But you knew that.
- Handle with care while cleaning and storing. Food residue comes off glass easily if you let it soak overnight. Certainly avoid metal scrubbers and the like when cleaning a jar! Then store in the pantry or some other at least relatively temperature-controlled area until next season.
So, after all these rules, what caused my one jar to break that time? It was a failure of a pint jar in my pressure canner. Because I'm not sure whether the jar was one I'd bought new, or one I'd picked up at a thrift store, there are two possibilities. One, I over-tightened it; the internal pressure built up; and with nowhere for the food and air to go, the jar blew. Or two, the jar had been mistreated before I got it, and there was a tiny crack or weakness in the jar that I didn't detect ahead of using it. I'll be honest: I think it's more likely that I over-tightened it. It happens to the best of us, especially us "novices."