The long-lived mug: Are my pieces microwave and dishwasher-safe?
Two of the most frequently asked questions I receive from my students at my workshops are: Will I be able to throw my pieces in the dishwasher? and, Can I bring my morning coffee back to life in the microwave after ignoring it for hours? And Oh! How I deeply wish I had a sweet and simple answer to these questions but if you have already embarked yourself in the pottery-making realm, you know by now, that there are many sweet things about this world but very few fall into the “simple” category.
That being said, I will try to be as concise and informative as possible in responding to these questions to bring you peace of mind while caring for your favourite ceramic treasures.
Let’s talk about vitrification and porosity ✨
If you've ever wondered (like my students), and want to understand why some pottery is labelled as dishwasher or microwave-safe, then it’s essential to understand what is vitrification and porosity in clay.
The porosity is determined by the type and stage of clay and it refers to how much space there is between clay particles, the more “empty” space between particles, the more capacity a ceramic piece has to absorb liquids.
Vitrification refers to the process of changing the structure of a pottery piece by closing the spaces between particles, ultimately creating a strong, dense, and watertight finished product. The fewer gaps between particles there are, the less likely it is for water to be absorbed and stored in the walls of the pottery piece.
To have a dishwasher and microwave-safe piece, our goal is to achieve 98-100% vitrification.
When we are building our pieces, either with hand-building techniques or using the wheel, the clay is in its wet state, the clay particles are surrounded by water making it flexible and malleable. At this stage, the clay is at full capacity of water, which means it is not porous.
Wet Clay - Wet Clay Particles Illustrated
After a few days, the water in our clay piece evaporates, leaving the clay particles surrounded by air and quite far apart from each other. This makes the piece very porous, extremely fragile, and easy to dissolve upon contact with water. This stage is known as bone-dry clay.
Bone-dry Clay - Bone-dry Clay Particles Illustrated
After the piece is bone-dry, we do our first firing and we’ll have what we call a bisque-fired piece.
Now, the clay particles are closer together, making the piece much stronger and less likely to break while still retaining some of its porosity. The movement of the clay particles during firing causes a slight shrinkage in the piece. At this stage, the piece can be decorated and glazed.
Bisque Clay - Bisque Clay Particles Illustrated
The final stage comes when firing the piece for the second time. This is the moment when the science that feels like magic happens ✨
Our piece will reach the highest temperature in the entire process. The particles will come as close as possible to each other, tightening and closing gaps, reaching their final maturity state.
Final Piece (Stoneware) - Final Piece (Stoneware) Particles Illustrated
So… Does this mean every piece that goes through this process is dishwasher and microwave-safe?
Not really! 🙃
We still need to consider two more factors. One is the type of clay that we are using. To put it simply, not all clays can handle the high temperatures required to reach 98-100% vitrification. Some pieces will still be porous even after the final glazing and firing.
And two, if the piece has lustres or decals, generally we try to avoid placing anything containing metals in the microwave.
Why is this important to determine whether or not to use the microwave?
In theory, a porous ceramic piece can absorb liquids, even if they are just tiny particles. The piece can have water stored in its walls, and when placed in the dishwasher or microwave, the heat will evaporate the water content inside the piece, which is why it is likely to crack.
So, after learning about vitrification and porosity, it will make more sense why some clay bodies are more prone to break or crack, remember the lower the porosity %, the better. Here is a quick visual guide that will help you get more sense of everything we’ve covered:
*As long as there are no decals or luster involved.
How to test a pottery piece to see if it’s microwave and dishwasher-safe?
If you have a ceramic piece you’re not sure if it is Low-Fire or Mid/High-Fire, there is a simple test you can do to find out if your piece is microwave/dishwasher-safe:
Grab a mug or glass that you know is microwave-safe and fill it about three-quarters full with water.
Repeat the first step with the mug or piece you want to test.
Place both pieces in the microwave.
Set your microwave to its highest setting and let it run for one minute.
After the time’s up, check the temperature of the water. If the water is still cool but the piece feels warm, that means it isn’t microwave-safe. If the water is warm and the pottery stays cool, congratulations—your piece is good to go in the microwave!
Extra care for your ceramic pieces
It’s safe to use Mid or High-fire pieces in the microwave or dishwasher without causing any cracking in your piece. However, here are some extra tips to prolongate the life of your cherished pottery pieces:
Avoid changing the temperature of your dish too abruptly; make sure it reaches room temperature before pouring
hot beverages or foods.
Over time harsh detergents and rapid heating and cooling cycles associated with dishwashers may damage the surface/colour of your handmade pottery, try to do it as little as possible.
Wash your pieces with warm water and soap and then dry them with a soft cloth.
So! In conclusion, while vitrified ceramics are the gold standard for durability and safety, there’s room for personal preference and practicality in the world of pottery. Let me know what clay do you use, and why? I'd love to hear your thoughts! 😊
