What Causes Jars to Break During Canning? A Commercial Buyer’s Root-Cause Guide
Why Do Jars Break During Canning? (Main Causes)
Glass jars usually break during canning because the jar experiences more stress than it can handle. Most of the time, this happens when the jar goes through sudden temperature changes or when the jar itself has small flaws. Here are the main reasons your jars may crack or explode during canning:
• Thermal shock
This is the No.1 cause. When a cold jar is placed into boiling water—or a hot jar touches cold surfaces—the sudden temperature jump creates stress. Glass expands and contracts with heat, so any extreme change can cause it to shatter instantly.
• Sudden temperature difference
Even if the jars are not “cold,” a difference of more than 40–50°C can make them break. For example, pouring hot jam into a room-temperature jar or moving hot jars onto a cold countertop can cause cracking.
• Over-tightened lids
If you screw the lids too tight, steam cannot escape during processing. This builds pressure inside the jar, increasing the risk of breakage.
• Overfilled jars
Filling jars past the recommended headspace doesn’t leave room for food expansion. As the contents heat up, pressure increases and the jar can crack from the inside.
• Hidden micro-cracks
Even tiny, invisible cracks or chips around the rim weaken the structure. During heating, these weak points spread and cause the jar to break.
• Low-quality glass bottles
Low-quality jars with uneven wall thickness, poor annealing, or manufacturing defects are more likely to fail during canning. Always choose jars made specifically for high-heat applications.
• Incorrect canning method
Placing jars directly on the bottom of the pot, heating too fast, cooling too fast, or skipping recommended steps can all lead to breakage.
When you understand these causes, you can prevent most canning jar failures—and keep both your products and your time safe.
Thermal Shock: The No.1 Reason Glass Jars Break
Thermal shock is the most common reason glass jars crack or shatter during canning. Glass reacts to heat, and when one part of the jar heats or cools much faster than the rest, the stress becomes too much for the material to handle.
• How heat stress damages glass
Glass expands when exposed to heat and contracts when cooled. If the change happens slowly, the jar can adjust without problems. But when the temperature shifts too quickly—like pouring boiling soup into a cool jar—different parts of the glass expand at different rates. This uneven movement creates internal stress. Once the stress exceeds the glass’s strength, the jar breaks, often suddenly.
• Temperature difference tolerance
Most canning-safe glass jars can only handle a temperature difference of around 40–50°C (104–122°F) at once. If you go beyond that range, the chance of breakage increases sharply. That’s why you should never put a room-temperature jar into a boiling water bath or place a hot jar directly on a cold marble countertop. Even small mistakes like these can cause cracks.
• Real examples
You may have seen jars shatter the moment they touch hot water. This usually happens when the jar is too cool before processing. Another common case is adding hot jam into jars that weren’t preheated. The bottom of the jar heats up quickly while the upper part remains cooler—this uneven heat creates stress and the jar splits. Hot jars placed on cold surfaces can also “ping” and crack instantly due to rapid contraction.
Understanding thermal shock helps you prevent most jar failures. When you warm your jars gradually and avoid sudden temperature swings, you protect your batch—and save yourself from messy cleanup.
Manufacturing Defects That Cause Breakage
Even when you use the correct canning method, some jars may still fail because of hidden manufacturing defects. These flaws weaken the glass structure, making the jar more likely to crack under heat or pressure. Understanding these issues helps you choose safer jars and avoid unnecessary breakage.
• Uneven wall thickness
If the glass walls are not uniform, one side heats and expands faster than the other. This creates internal stress during canning, and the thinner areas often become the weak points where cracks begin.
• Poor annealing
Annealing is the controlled cooling process that removes internal stress after the jar is formed. When annealing is not done properly, the jar retains stress inside the glass. During canning, this trapped stress can react to heat and cause sudden breakage, even if the jar looks perfect from the outside.
• Visible bubbles
Small air bubbles inside the glass are usually harmless, but larger or irregular bubbles can weaken the structure. Under heat, these weak spots expand unevenly and can cause the jar to crack during water-bath or pressure canning.
• Mold seams
All jars have seams from the molding process. However, poorly aligned or overly raised seams indicate low-quality manufacturing. These seams can reduce the jar’s strength and make it more vulnerable to thermal shock or pressure changes.
• Impact damage
Even minor impacts during transport or storage—such as a small chip on the rim or tiny crack near the base—can compromise the jar. When heated, these flaws spread quickly and lead to breakage.
High-quality jars from reliable manufacturers help reduce these risks. When jars are produced with strict quality control, proper annealing, and precise molds, you get consistent strength and far fewer failures during canning.
How to Prevent Jars from Breaking During Canning
Most jar breakage can be avoided with a few simple habits. Canning puts your jars through high heat, pressure, and quick temperature changes, so giving them the right preparation makes a big difference. Here are practical steps to help you keep every batch safe and intact.
• Preheat jars properly
Always warm your jars before filling them. You don’t need them boiling hot—just warm enough to match the temperature of your food or water bath. A quick rinse with hot tap water or keeping jars in a warm oven (not too hot) works well. The goal is to avoid a big temperature jump.
• Avoid sudden temperature shifts
Don’t move hot jars onto a cold countertop, and don’t pour boiling food into a jar that’s been sitting at room temperature. Sudden changes are the fastest way to trigger thermal shock. A towel, wooden board, or silicone mat is a simple fix that protects your jars during cooling.
• Follow proper headspace
Leaving the right amount of headspace gives the food room to expand. If the jar is packed too full, the pressure inside rises quickly during heating—and the jar may crack from the inside out. Check your recipe’s headspace recommendations and don’t skip this step.
• Use correct lids
Screw bands should be “finger tight,” not overly tight. Over-tightening traps steam inside the jar, creating pressure that can lead to breakage. Good-quality lids also ensure a proper seal, so the jar doesn’t stress or leak during processing.
• Inspect jars before use
Take a moment to check each jar for chips, cracks, or scratches—especially around the rim and base. Even small flaws can spread under heat. If something looks off, set that jar aside. It’s better to lose one empty jar than a whole batch.
• Use canning-approved jars
Not every glass jar is designed for high heat. Recycled jars or jars made for dry foods often have thinner walls or lower heat resistance. Choose jars specifically made for canning or hot-fill processes—they’re engineered to handle temperature changes and pressure safely.
• Storage & handling errors
How you store your jars matters too. Jars stacked too tightly, knocked together, or stored on bare concrete floors may develop micro-cracks without you noticing. Handle them gently, keep them clean and dry, and avoid dropping heavy items around them.
When you follow these steps, you greatly reduce the chances of breakage and keep your canning process smooth and worry-free.
What Types of Glass Jars Are Safe for Canning?
When it comes to canning, not all glass jars are created equal—choosing the right type is one of the simplest ways to avoid breakage and keep your products safe. As a food manufacturer, you need jars that can handle high temperatures, pressure, and the demands of bulk production. Let’s break down the safe options, what makes them reliable, and why you should steer clear of certain jars.
• Heat-resistant soda-lime jars
Most canning-safe jars today are made from high-quality soda-lime glass. When produced with proper wall thickness and good annealing, this type of glass can handle the heat of water-bath canning without breaking. It’s designed to handle moderate thermal shock and is widely used for jams, sauces, pickles, and preserves.
• Mason jars
Mason jars are the classic choice for home canning. They’re thick, sturdy, and made specifically for high-heat processing. Their two-piece lid system also ensures a reliable seal. If you’re new to canning, Mason jars are the safest option to start with.
• Specialized canning jars
Some jars are built specifically for hot-fill or retort processes. These jars are engineered to handle higher temperatures and repeated heating cycles. They usually follow stricter manufacturing standards—making them ideal for small food businesses or anyone producing larger batches.
• Industrial food jars
Food factories and brands often use industrial-grade glass jars that meet strict quality and safety standards. These jars are made with consistent wall thickness, strong thermal shock resistance, and reliable sealing finishes. They’re suitable for hot-fill, pasteurization, and sometimes pressure canning, depending on the design.
Thermal shock resistance standard
Most canning-safe jars are tested to handle about 40–50°C (104–122°F) of sudden temperature change. High-end industrial jars may handle even more, depending on the production process. This standard helps ensure the jar won’t crack during normal canning steps, like moving jars in and out of hot water.
Why some jars are not suitable
Recycled jars, jars from store-bought foods, or jars made for dry storage may look similar but aren’t built for heat. Many of these have:
· Thin walls
· Low heat tolerance
· Uneven glass distribution
· No thermal shock testing
· Decorative shapes that can’t handle pressure
As a result, they break easily when exposed to boiling water or hot food.
Choosing jars designed specifically for canning—or jars tested for thermal shock—is the key to keeping your batches safe. With the right type of jar, you avoid most cracking issues and get a much smoother canning experience.
For Food Brands & Factories: How to Choose Reliable Jars (B2B Insight)
As a food brand or factory, picking the right canning jars isn’t just about looks—it’s about keeping your production smooth, products safe, and costs in check. Here’s what to prioritize when choosing a B2B jar supplier:
First, certifications matter. Look for jars with food-grade certifications (like FDA, LFGB, or ISO) to ensure they’re safe for contact with edible products—no harmful chemicals leach into your food.
Next, supply stability. You can’t afford delays in production—choose a supplier who can meet your bulk needs consistently, even during peak seasons, without stockouts.
Mold design is key for quality. A well-engineered mold means uniform glass thickness, strong rims, and no defects—critical for preventing breakage during canning. Ask suppliers about their mold maintenance and quality checks.
Don’t skip heat-resistant testing. Verify that jars are tested to handle your canning process (e.g., 121°C for pressure canning) to avoid thermal shock cracks.
Bulk production control ensures every batch is the same. Look for suppliers with strict quality control (QC) processes—like 100% visual inspections or automated defect detection—to keep breakage rates low.
Finally, custom branding/embossing helps your product stand out. Many B2B suppliers offer custom logos or embossed designs on jars, so you can reinforce your brand identity without extra cost.
ANT Packaging: Reliable Glass Jars for Canning & Food Packaging (For B2B Buyers)
At ANT Packaging, we’ve been a trusted B2B glass jar manufacturer for over 10 years, specializing in canning jars for food brands and factories worldwide. Our jars are built to solve your biggest pain points—breakage, inconsistency, and slow supply.
Our heat-resistant canning jars are made with food-grade soda-lime glass, tested to withstand thermal shock and high temperatures — perfect for pressure or water bath canning. We offer a range of sizes (100ml to 2000ml) to fit your products, from jams to sauces to preserved fruits.
Need a unique shape or size? Our custom mold service lets you create jars that match your brand—no minimum order for mold development. We also support full OEM/ODM, so you can customize lids, colors, or embossing to make your packaging one-of-a-kind.
With large-volume production capacity, we guarantee supply stability—even for your busiest seasons. Every jar goes through 3 rounds of QC to ensure zero defects, and we offer a breakage replacement policy for peace of mind.
Ready to cut down on canning breakage and elevate your packaging? Contact us today for a free sample of our heat-resistant jars, or request a custom quote tailored to your needs. Our team will work with you to find the perfect solution—let’s grow your business together!
FAQ
Q1: Why do my jars keep breaking in a hot water bath?
Jars usually break because of thermal shock, hidden cracks, or using jars not made for high heat. Sudden temperature changes—like putting a cool jar into boiling water—are the most common cause. Poor-quality glass or over-tightened lids can also lead to breakage.
Q2: How do I prevent thermal shock in canning?
Warm your jars before filling them, avoid placing hot jars on cold surfaces, and don’t pour boiling food into room-temperature jars. Keep everything—jars, food, and water bath—within a similar temperature range to avoid sudden jumps.
Q3: Can any glass jar be used for canning?
No. Only jars designed for canning or hot-fill processes are safe. Recycled grocery-store jars or jars made for dry food storage may have thin walls or low heat tolerance and can easily crack during processing.
Q4: How hot should jars be before canning?
Jars don’t need to be boiling, but they should be warm—around the same temperature as your hot food or water bath. Many people keep them in hot tap water or a warm oven (not above 120°C / 250°F) before filling.
Q5: What temperature change will break glass jars?
Most canning-safe jars tolerate a temperature difference of about 40–50°C (104–122°F) at once. Bigger jumps—like moving a hot jar to a cold countertop—can cause cracking or shattering.
Q6: Should you preheat jars before canning?
Yes. Preheating helps prevent thermal shock and reduces the risk of breakage when adding hot food. Even a quick rinse with hot water makes a big difference.
Q7: How to prevent Mason jars from exploding?
Handle them gently, warm them before use, avoid sudden temperature changes, leave the correct headspace, and don’t screw lids too tightly. Using jars free of cracks or chips is also essential.
Q8: Why do jars break when pickling?
Most pickling liquids are hot, so if you pour them into a cold jar, the sudden heat causes thermal shock. Some jars also break because of micro-cracks or uneven glass thickness.
Q9: Why are my jars unsealing?
Unsealing often happens when jars are overfilled, the headspace isn’t correct, lids are too tight or too loose, or air wasn’t fully removed during canning. Damaged rims can also prevent a good seal.
Q10: What happens if I leave too much headspace when canning?
Too much headspace traps extra air inside the jar. This can lead to weak seals, spoiled food, or floating contents. It may also affect processing time and overall safety. Keeping the recommended headspace ensures proper sealing and heat penetration.
