Are Ceramics and Pottery Microwave Safe?

If you’re a coffee drinker, a parent, or both, chances are you’ve needed to reheat your coffee at some point. For me, the need to reheat my cup of joe crops up daily. When we stick our cheapo box store mugs in the microwave, we don’t think twice. What about our favorite, hand crafted pottery? Is it safe to microwave our most cherished ceramic dishes? Are they created for microwave use?

While I try to avoid microwave ovens as much as possible because I don’t want that microwave radiation leaching into my food, there are times where it isn’t avoidable. Reheating my coffee in a pinch means I’ve got to microwave ceramic mugs that I’m greatly attached to. You know, the mugs that fit perfectly in your hand. Maybe they have handles that are just right or you love the glazes used on them. Let’s take a look at whether or not it’s safe to put your ceramic pot in the microwave.

A Background on Ceramics

When determining whether or not ceramic mugs are safe to put in the microwave oven, it is important to somewhat understand the manufacturing process of pottery. After ceramic mugs are thrown and dried, they go into a kiln for a bisque fire at high heat (without glaze). They are glazed and fired a final time at a much higher temperature which allows the elements in the glaze to form that glassy surface on a pot. These temperatures are well over 2000 degrees Fahrenheit. After the glazing process, the clay has undergone a vitrification process meaning it is now sealed.

Most microwave ovens don’t reach temperatures above 500 degrees Fahrenheit, depending on wattage. Your pot has been created in an extremely high temperature in a kiln so it should be able to withstand the much lower temperatures of a microwave, with a few important exceptions.

Metallic Glaze + Crackle Glaze

If your mug has a metallic finish or any gold luster detailing, you should not place it in the microwave. You may be familiar with the fact that silverware or foil shouldn’t be placed in the microwave because it can spark and catch fire. I learned this the hard way as a kid, but that’s a story for another day. The same is true for ceramics. Heavy metals such as actual silver or gold are used for these metallic details on ceramics. It is the same as placing a fork in there and hoping for the best. Bone China also often has a metallic rim and although it is only decorative, it can spark as well. It is best to leave metallic pots of any magnitude out of the microwave oven.

Crackle glazes are also unsafe for microwaves use. When you heat food or drinks in a fully sealed, vitrified pot, there is very low likelihood that any heavy metals from the ceramic glazes will leach into them. When mugs have crackle glazes or even a single crack on the interior of the mug, they should not be used in the microwave. These cracks allow the chance for glaze materials like toxic chemicals to mix into your food during the heating process. In fact, these crackled pots shouldn’t even be deemed food safe because mold can also grow in these cracks. This type of glaze is really only suitable for decorative purposes- microwaved or not.

Thermal Shock

Sometimes pots undergo thermal shock during the firing process that can be exacerbated in the microwave. Thermal shock is when pots go from one temperature extreme to the other. For example, if you are firing pottery and the open the lid (which you definitely shouldn’t do) letting all the heat out. Sometimes in Raku pottery, pots go from being directly in a fire to being submersed in water. Taking a cold ceramic baking dish out of the freezer and throwing it in a hot oven may have the same effect.

Thermal shock may not always be visible, but hairline cracks in the glaze or clay body may form from the rapid temperature transition. When you stick a pot with thermal shock in the microwave, it could worsen the cracks in the pot allowing for leaching and potential breakage of the pot. Always check for thermal shock by looking for cracks in your pottery or listening for strange tinkling or cracking sounds whenever your pot is heated.If you detect either of these things, avoid temperature extremes with that pot.

A Few Safety Precautions:

-While is is often safe to place your ceramic bowl, mug, or dish in the microwave, it is always best to err on the side of caution. Pay attention to what kind of ceramic ware you are putting in your microwave oven. There are several different types of ceramics. Avoid ceramic vessels made of China or porcelain as these clay bodies are very brittle in nature. Stoneware dishes are far more likely to withstand the microwave successfully.

Stoneware is usually a safe material to place in the microwave. This clay body usually feels denser. It can sometimes be a white, light tan, brown, or terracotta color. It can also feel a bit gritty or sandy to the touch. These are usually pretty indicative of stoneware.

-Ceramics can become very hot in the microwave or the oven because they retain heat very well. Use oven mitts or heat resistant cloths when removing pots from the microwave.

-If ceramic surfaces seem porous at all, they do not belong in the microwave and should not be used as eating or food preparation surfaces. I wouldn’t recommend using unglazed pots for food or drink either. Clay must be fully vitrified and sealed to come into safe contact with food. Porous ceramics can trap water molecules that allow mold to grow under the surface of your pot. Low fire clays are usually not food safe either since they aren’t ever fully vitrified. They usually aren’t used to make dinnerware though, so this shouldn’t be much of an issue for you.

-It is always a good idea to check the bottom of the dish for specifications on microwaving and dishwashing. Sometimes potters will stamp the bottoms with instructions or a microwave-safe label. Even after checking the bottom, inspect your pot for any signs of damage, cracking, crazing or thermal shock. Don’t just take the bottom at its word.

-Take proper care of your ceramic products by hand washing. Recheck your ceramic dishes for damage after any extended period of time. If you have a favorite mug for daily use, inspect it from time to time. Sometimes things get bumped in the sink and things get hairline cracks that can expand with the next use.

With these tidbits of knowledge about microwave safety under your cap, don’t be afraid to nuke that cup of coffee. Just use your best judgement and enjoy your pottery! Handmade pottery is meant to be used and sometimes breakage can happen, but don’t let that keep you from getting the most use out of your pot as possible. It is always best practice to use microwave-safe dishes, but sometimes the only way to find out if a ceramic mug is microwave ready (after you’ve inspected) is to to try it. Happy heating and I hope you enjoy your pots!

Blessings,

Samm

Want to try your hand at pottery making? Check out this blog.