Are Glass Jars Microwave Safe? Plus, How To Heat Them Safely

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Glass jars are the indie darling of the food storage world. Hailing from all over Instagram and Pinterest, these jars are hip and sustainable and give your pantry an apothecary vibe. But are they actually safe to heat up in the microwave?

Most glass jars are microwave safe. You can confirm this by looking for a microwave-safe label or symbol on the bottom of the jar. If there is no such symbol, do a quick trial run. For the best results, heat the jar in short intervals, keep the lid off, and never microwave a frozen jar.

From storing cookies to making homemade preserves to growing garden herbs, we’ve probably experimented with every possible way to use a glass jar. But when it comes to microwaving them, things get a little trickier. So let’s take a closer look at what makes microwaving glass jars safe (or not), plus how to do it safely if you decide to give it a go.

Are Glass Jars Microwave Safe?

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’Glass jars’ is a broad term that can refer to many types made from different kinds of glass. Ordinary glass, like the kind used to make drinking glasses, ice cream bowls, or mayonnaise jars, is not microwave-safe because these things are not meant to be heated up.

On the other hand, some glass jars are made from tempered or borosilicate glass designed to withstand high temperatures. Canning or “Mason” jars (on Amazon) can be used for, you guessed it, canning.

These glass jars are usually safe to microwave as long as they have n metal trim, lids, or decorations. To make things easy, manufacturers clearly label or mark their products ’microwave-safe’ if they can be used in the microwave.

So, if your mason jar has a microwave-safe label or symbol (a rectangle with wavy lines inside or above it) on the bottom, it should be safe to heat up in the microwave.

If your jar has no markings, it’s best to play it safe and avoid microwaving it.

What Differentiates Microwave-Safe Glass?

Usually, there is no visual difference between microwave-safe and non-microwave-safe glass. Both look the same to the naked eye unless the glass is of terrible quality, in which case you shouldn’t be using it anyway.

The only safest way to differentiate a microwave-safe glass is to check for the microwave-safe symbol—a rectangle or microwave with wavy lines.

The symbol is usually stamped, etched, or printed on the jar. Sometimes manufacturers explicitly state ’microwave-safe’ in the product description and packaging.

The inherent difference between the two types of glass is in their manufacturing. In addition to the boron oxide, microwave-safe glass is much purer than regular glass.

As a result, it doesn’t contain any dyes, air bubbles, or other contaminants that would decrease its ability to withstand heat.

How Can You Test if Glass is Microwave Safe?

If your glass jar doesn’t have any safety indications, but you’re positive it’s good quality, this small test can give you the answer.

  1. Find a microwave-safe cup or bowl and fill it with water.
  1. Place the water-filled cup with the glass jar in the microwave. It’s important because you should never heat an empty container in the microwave.
  1. If the jar is too big and doesn’t leave space for the cup, you can place the cup inside the jar.
  1. Microwave on high power for 1 minute.
  1. Carefully remove the bowl and glass jar from the microwave.
  1. Touch the outside of the glass jar. If it’s slightly warm or at room temperature, it’s safe to microwave. If it feels too hot, that means the glass is not microwave-safe.

How to Microwave Glass Jars Safely?

Even microwave-safe glass jars should not be thrown in the microwave without preparation. Instead, follow these tips to microwave glass jars safely:

  • Do Not Overheat: Do not heat the glass jar for more than 3 minutes at a time. If you need to heat the floor for longer, do it in smaller intervals and let the jar cool down for a minute in between.
  • No Metals at All: We all know it, but it’s worth repeating—do not place any dish with metal in the microwave, no matter how small the amount is. This includes rings around the opening, decorations, or a latch for the lid.
  • Keep The Lid Off: Whether the lid is plastic, metal, or glass, it’s best to take it off while microwaving because you don’t know if it is also microwave-safe. Plus, the steam may cause excessive pressure build-up inside the jar.
  • Let the Jar Thaw First: If your glass jar is just out of the fridge, let it thaw out a bit at room temperature before microwaving. Sudden temperature changes can crack or break the glass.

How to Microwave Food in Glass Containers?

Container with food in the microwave
  1. Look for a microwave-safe symbol or label on the jar to ensure it can go in the microwave. If there’s no indication, do the water test described above.
  1. Make sure the container doesn’t have any metal parts. Plus, if it has chips or cracks, don’t use it.
  1. Take the lid off and add the food to the jar, but don’t stuff it too full. Instead, leave space for the food to expand as it heats up.
  1. If you’re heating food frozen inside the glass jar, let it thaw at room temperature first. Or put it in a water bath to speed up the process.
  1. If your glass container is at normal temperature, but the food is frozen, you can directly put it in the microwave—just make sure you run it on defrost for the first few minutes.
  1. Microwave for small intervals according to the recipe or package instructions. Keep stirring in between to help the heat distribute evenly and prevent hotspots.
  1. Once the food is heated, take it out of the microwave and enjoy! However, even microwave-safe glass containers can heat up quite a bit if you leave them in the microwave for too long. So be careful when taking out the food and use oven gloves if necessary.

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