A Guide to Proper Use of Mason Canning Jars | What Are They Safe For?

Whether you are a food-preserving aficionado, or you use canning jars to store your seeds and nuts, safety should always come first. While canning jars might seem indestructible, nicks and cracks can happen if they are not handled properly.

What canning jars are safe and not safe for:

• Safe for freezing if there is enough place between the content and the lid. Avoid temperature shocks.
• Dishwasher safe, a great way to sterilize the jar
• Not oven safe, not tempered for such temperatures
• Placing a mason jar in the microwave is not recommendable unless labeled as “microwave safe.” 

Before heading to your jars and placing them all in the freezer, there is a little more to know about proper storage methods. Below, we’ll look into the details of what you can do with mason jars.

Are Canning Jars Microwave Safe?

While many users tend to heat their canned vegetables or stews in the microwave, it is not wise to assume that a mason jar is microwave safe. Indeed, the glass should be tempered for such high temperatures and labeled as “microwave safe” for microwave use. 

Depending on how old your mason jar is, it might not have such a label. Moreover, generic canning jars produced by famous manufacturers, such as Ball and Kerr, might not bear the “microwave safe” sign at all. In this case, it is recommendable to avoid placing them in the appliance altogether.

Indeed, while you might heat your food successfully once or twice, this practice will end up reducing the life span of otherwise long-lived jars. Even worse, the glass can shatter and crack, containing the food and becoming a significant health and safety hazard.

Even in the case of jars labeled as “microwave safe,” there are some safety guidelines you should follow before placing your preserved jam in the appliance. Here is what you should do:

• Test them in the microwave while empty for a few seconds before proceeding with storing your food in them.

• Remove both the lid and the band from the jar. The metal contained in these parts can create damage to the microwave. Indeed, metal tends to reflect microwaves instead of absorbing them, which can harm the appliance and reduce the level of safety. If the jars bear a metal tag or label, remove this part as well.

• Do not put frozen mason jars directly into the microwave. Such a drastic temperature change can crack or break the glass. Instead, opt to cool down the pot beforehand under lukewarm water or at room temperature. Microwave only when defrosted. 

• Follow the manufacturer’s instructions. Even if your canning jar can stand microwaves, it is recommendable to follow the instructions given by the manufacturer regarding timings and temperatures.

If you are unsure whether your jars will survive a round in the microwave or not, you should get in touch with the manufacturer’s customer service. In any case, if you are using a vintage mason jar, you should avoid placing it in the microwave altogether.

Storing jars correctly is important. I wrote an article on how to do it correctly, called How to Store Empty Canning Jars | The Best Way. You may want to check it out.

Are Canning Jars Freezer Safe?

Canning jars, as well as their content, are safe for freezing. However, there are some safety measures to take to avoid seeing your jars shattering to pieces while in your freezer.

First, you should ensure that the food or liquid inside the container has space to expand. Liquids can expand and react to freezing differently.

So, it is recommendable to leave at least 2 to 3 inches (5 to 7cm) from the top ridge of the jar empty. This headspace will allow the food in the pot to extend and contract without resistance from the glass.

Moreover, when thinking of freezing mason jars, reach out for the “wide mouth” ones. Regular ones have shoulders below the neck that can suffer the pressure and make the pot more prone to breakages.

Another vital factor to consider is that, while canning jars are optimal for freezing, they don’t stand well against hits and crashes.

Ideally, you would use a cardboard box for mason jars, which effortlessly keeps the jars separated from one another. However, the chances of being enough space in your freezer for a whole try might be slim. You can efficiently resolve this problem by allowing enough space between the jars or wrap them in a cloth.

Lastly, as mentioned above, you should do anything you can to avoid temperature shocks. To do so, avoid placing boiling content straight in the freezer and vice-versa.

In any case, food preserved in mason jars can last up to a year. Freezing can increase this lifespan, but you might not need to do so as often.

Are Canning Pots Dishwasher Safe?

Similarly to your other glasses and cups, mason jars are generally dishwasher-safe. Moreover, if you are planning to preserve your homemade jam or sauce in them, you could place them in the dishwasher beforehand for sterilization. 

All parts of a canning jar – the glass, the metal band, and the lid – are dishwasher safe. However, you should ensure to place the glass away from other objects it can get in contact with while in the machine. 

Instead, you should place both the jar and the metal components in the top compartment or rack of the dishwasher. Indeed, the heating element of the appliance is usually at the bottom of it (check the specifications of your dishwasher before proceeding). This trick reduces the exposure of your container to high temperatures. 

In any case, like all your other glasses, you should avoid filling up the newly-washed and still-hot jars with cold food. Since glass naturally tends to expand and contract depending on temperatures, such a shock can cause it to crack.

Instead, allow your jars to cool in a compartment at room temperature before using or storing them.

Are Canning Jars Oven-Safe?

Unless tempered, standard canning jars are not over-safe. The material used in the making of preserving jars is usually annealed glass. Unlike PYREX, annealed glass is susceptible to thermal change, and, if broken, it shatters into several uneven shards that can easily contaminate its content. 

Unless explicitly specified, you should avoid placing your jars in the oven altogether. If you are dealing with a vintage or older jar, it will not be oven-safe. However, more recent models or tempered canning jars can stand temperatures better than older ones.

Nonetheless, if you have ever been preserving food in a canning jar before, you know that bathing them in boiling water does not create an issue. At the same time, sterilization can happen in an oven, too, although under controlled circumstances. 

Indeed, to sterilize your empty jars in an oven, place them on parchment and turn the oven on at 225℉ (105℃) for 20 minutes. Do not increase the temperatures, and take them out only when you are ready to fill them with food or liquid.

Additional Reading

• The Best Jars for Canning | Which Size to Use by Food Type
• How to Store Empty Canning Jars | The Best Way
• Are Canning Jars Airtight? (How to Tell if It’s Sealed)
• How Long Canning Jars Take to Seal (And How to Tell)
• Can You Reuse Canning Lids? | A 50+ Year Canner Weighs In)
• Reusable Canning Lids | Should You Use Them?
• Can You Use One Piece Lids for Canning? (And Should You)

Final Thoughts

Canning jars, like these found on Amazon, are highly versatile and long-lived when adequately maintained. They are cheap and safe options to store and preserve your food. However, you should keep an eye out when these jars are entering into contact with a cooling or heating home appliance. 

While glass jars are freezing and dishwasher safe, you should not place them in an oven or microwave unless specified on the label.

Lastly, remember that the glass of canning jars is rarely tempered. Therefore, it can be susceptible to thermal changes and breakages. Check the manufacturer’s instructions before placing your pot in a home appliance.

For more, don’t miss The Best Jars for Canning | Which Size to Use by Food Type.