It’s time for a reader question! Bay emailed to ask, “Help! My jars keep breaking when I put the wire basket in the boiling water. What am I doing wrong?”
Thanks for the question, Bay. Let me guess – pickles? It’s almost always pickles.
I’ve heard from several readers with this same jar breakage problem, and I’ve been there myself. I’ve learned a few tricks to help keep my jars crack-free, so I think I can help you out.
The quick answer is, what’s happening to cause your jars to break is called thermal shock. Basically, the glass is being asked to accommodate a too-rapid temperature change.
As your jar is lowered into boiling water, the glass attempts to expand slightly. When this expansion happens unevenly, stress builds up, and the brittle glass simply can’t take it.
Result? Cracked jars.
Thermal Shock 101
Imagine a hot cup of tea. Now imagine an ice cube. The tea is too hot, so you drop in the ice cube. What happens?
Pop! You can hear the ice cube cracking, right? And if you look you might see fractures form along the surface of the ice cube.
That’s thermal shock. The outside of the ice cube heats up quickly, but the inside stays cold. The uneven pressure across the ice causes the cracking.
Three things make a material more likely to experience thermal shock:
1. Flexibility vs. Brittleness
Materials like rubber respond to movement by stretching and bending. They are flexible. Materials like glass respond to movement by shattering or breaking. They are brittle.
Thermal shock in your jars is all about movement at the microscopic level – the glass is trying to expand in response to changing temperatures, but not all the glass is expanding at the same rate, and is too brittle to accommodate that movement.
2. Rate of Expansion
The more quickly a material expands or contracts in response to temperature changes, the most likely it will be to experience thermal shock.
Most glass has a fairly high rate of expansion. That’s not good news for thermal shock.
Specialty borosilicate glass has a much lower rate of expansion that makes is far less prone to thermal shock than standard soda lime glass. You are most likely to know borosilicate glass as Pyrex – up until 1998 Pyrex baking dishes and glass measuring cups were borosilicate. Then there was a buyout, a cost-cutting measure, and, well, let’s just say that if you have any pre-1998 Pyrex you should keep it forever.
When heat can pass through a material quickly and easily, that material has high conductivity. Copper has incredible heat conductivity, which is why copper is a prefered component in high-end cooking pots.
Glass has terrible conductivity. So terrible, in fact, that glass is used as an insulator – something that prevents the rapid movement of heat. Materials that are slow to conduct excess heat away are more prone to thermal shock.
Thermal Shock and Canning
Typically, jars that experience thermal shock crack at their base, all the way around the bottom. They tend to have a rather clean break that leaves the disk of the jars base separate from the rest of the jar.
→ Why? Because the thickness of the glass changes at that bottom-to-side transition zone, leading to uneven heating of the glass in this area.
Jars that have been scraped clean with a butter knife or metal spatula are more likely to crack from thermal shock.
→ Why? Because microscopic scrapes in the jar left from metal utensils can create weaknesses in the glass.
Thermal shock of jars is more common with vinegar pickles and raw packed fruit.
→ Why? Because pickles and raw fruit take a while to fit into jars, so even if the jars start pre-warmed, they’ve often cooled down by the time they get loaded into a canner. If cool jars are loaded into boiling water, they are being asked to expand rapidly, and often can’t. Result – cracked jars.
How To Stop Thermal Shock In Canning
Okay, enough background – here’s the important stuff. How do you stop your jars from cracking?
1. Inspect your jars carefully for hairline scratches or cracks
If a jar looks even a little suspect – a scratch on the inside, say, or a tiny ding on the lip of the jar – don’t process your preserve in that jar. The jar might be totally fine for dry or refrigerated storage, or for using as a drinking glass or cocktail shaker, but it’s not worth the risk to can in.
2. Heat your jars before filling
Keep your jars warm until you fill them. There’s a few ways to do this:
Clean jars should then be kept warm prior to filling. You can leave them in the closed dishwasher after the cycle, or use your canner as it is preheating, or create a separate water bath that will keep the jars both clean and warm.
Only pull out as many jars as you can fill in a timely manner. Let’s say you have some epic mega pot that can accommodate 12 quarts at a go. Well that’s only great if you can quickly fill 12 jars in a calm, methodical way. Stick with batch sizes you’re comfortable with.
3. Use piping hot brine or syrup
With jam, jelly and tomato sauce the product you are loading into the jars is often as hot as the boiling point of water anyway, so these preserves tend to help keep jars hot through the filling process.
With pickles and raw packed fruit, jars tend to cool off quickly as you arrange your produce in the jar.
Make sure any brine or syrup you pour over pickled vegetables or fruit is boiling, or at least piping hot – I’d say as hot as you can safely pour into your jars.
Remember, when brine or syrup is poured over raw cucumbers or peaches, the average temperature of the jar doesn’t come up to the temperature of the brine or syrup because the preserved vegetable or fruit absorbs some of that heat.
4. Load your jars into simmering – not boiling – water
This is the number one thing I do to avoid cracked jars – and it really works.
Instead of loading jars into water that’s at a full, rolling boil, load into water that’s at a bare simmer – say, 190 degrees. After all your jars are loaded, turn the heat up under your canning kettle and bring the water and the jars up to that boiling temp together.
Important! Start your processing time only once the water is at a full, rolling boil.
Some pickle recipes can be lower-temp pasteurized instead of being processed at a full boiling water. Here’s more info. Important! Only pasteurize pickles when an approved recipe gives this as a safe option.
5. Give your jars 5 minutes of cool down time
Typically, jars crack just as they go into the canning kettle. But, thermal shock can happen when jars cool down too rapidly or unevenly, too. It goes in both directions. A little cool-down for your jars after processing can help.
After the full processing time for your preserve has elapsed, turn off the heat under your canning kettle, and remove the lid from the kettle. Allow the jars to sit undisturbed for 5 minutes, then remove them to a draft-free area to set their seals.
This technique will help prevent cool-down thermal shock, and I also find it improves the percentage and reliability of my seals.
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