Microwave safe.

[Babs]

What clay body r glazes would you avoid using in a microwave oven?

[Bill Kielb]

For me basically anything that has too much porosity and can trap enough water to make it overheat.

[neilestrick]

Most vitrified bodies work just fine in the microwave. Some get hotter than others, but I've never heard a definitive answer on which/why. No lusters in the microwave.

[Babs]

So iron rich clays and glazes ok?

[Bill Kielb]

No issues I have had. Hard to get sparks out of  a microwave these days without sharp metal edges.

[Babs]

1 hour ago, Bill Kielb said:

No issues I have had. Hard to get sparks out of  a microwave these days without sharp metal edges.

Thanks Bill. Just asked recently and I had forgotten what I learned a fair number of years ago. Thought maybe was an issue with iron rich etc in the past. Hadn't been asked for many years...

[Pyewackette]

I just read somewhere that it is low fire earthenwares that are the ones that get uber uber hot in the microwave.  I have a set of cheap mugs I got at Costco that I am getting rid of because of this.  I never use mugs except for hot stuff and these get so hot in the microwave that the handle  will burn your hand. 

Years ago I threw away an entire set of dinnerware that my dad got from my stepmom because they got so hot in the microwave it scared me.  The cheap-o mugs are very light weight and sort of "fluffy" if you can apply that to something made of clay, I'm guessing they were slip cast despite the slip-trail look on the surface.  But the dinnerware was very solid.  I wish I'd broken one just to see what the clay looked like under the glaze, but they were NOT keepers.  I didn't even want to donate that stuff in this day and age of the ubiquitous microwave.  I could just see some kid burning her hand trying to take a plate out of the microwave.  

[Hulk]

Curious, having assumed the culprit to be iron in the clay, am not finding specifics right away, interesting buzz though:
"...not microwave safe..."
"heavy metals"
"design issue"
"dangerous metals"
"wrong dish"

Porous, hence water soaked in the clay getting hot, ok, that's specific.
Empty clean ware doesn't get hot from a minute in the microwave, as a definition, ok, that's somewhat specific.
Material that heats contains "polar molecules" - which glass doesn't have, ok.

Tony Hansen mentions iron, and water infiltration.
Microwave Safe (digitalfire.com)
Ah, elsewhere Mr Hansen has link to astm article, hence, a definition (American Society for Testing Materials)Standard Test Method for Determination of “Microwave Safe for Reheating” for Ceramicware (astm.org) for $48, hrrm.
So there are standards; the above is for reheating (up to a time limit), hence there may be a standard for cooking (longer than reheating) as well. Let me know if you can find them.

I'll have to go with not melt or otherwise lose form, not leach bad stuff, not get too hot to touch after one minute in the waver, retest the clays and glazes I use, and go from there; anything that gets more than pleasantly warm (empty, clean) is not microwave safe, thanks for the question!

For clays that don't achieve less than two percent absorption, the glaze has to fit uber well, plus handwash and then dry, don't soak. For me, those clays were also red or red-ish and they get warm. I may continue with lower absorption clay, using red slip when I want the color, and glazed over, not exposed.
Now I'm curious how warm the slipped ware gets. Looking forward to getting back in the studio - three weeks in, looks like another two weeks here. 

[Babs]

Not asking re porosity of ware but the metal content. Thanks Hulk but not buying info on safety issues, should be free for all in the ideal world eh?

[Min]

I agree with what Neil said, haven’t seen a definitive answer to the question of high transition metals and microwave use. I would want to isolate each of the variables (even figuring out each of the variables would probably be controversial), and go from there. Absorption figure of less than X %, clear glaze or (micro)crystalline, pot thickness, claybody composition and so forth.

I think this is one of those areas that if in doubt run your own microwave tests and see what happens. I have a mug from a really well know local potter who works in earthenware. I went to a workshop he was giving and someone asked him about his pots getting hot in the microwave. (which they certainly do) His reply was along the lines of “have some common sense and use a potholder”. Personally, I don’t want to use a potholder to get a hot mug out the microwave.

[Pyewackette]

Well I put one of my bowls in the microwave for the first time today, 2 mins with soup in the bowl, and it got so hot I had to get a potholder.  It is fairly thin for that clay (the generic white stoneware).  I have no idea why it got so hot.  It's certainly not earthenware and it wasn't low fired.

BTW it was first use and thoroughly dry.  Except for giving it a good swipe with soapy water and a rinse weeks ago, it hasn't been near water since it came out of the glaze kiln.

[Bill Kielb]

19 minutes ago, Pyewackette said:

Well I put one of my bowls in the microwave for the first time today, 2 mins with soup in the bowl, and it got so hot I had to get a potholder.  It is fairly thin for that clay (the generic white stoneware).  I have no idea why it got so hot.  It's certainly not earthenware and it wasn't low fired

Was the soup very hot? Just guessing, it ought to be about the temperature of the soup.

[Pyewackette]

@Bill Kielb Nope. And it was hot all the way to the rim of the bowl.  It was only about half full of soup.

[Callie Beller Diesel]

I put my mature red stoneware mugs in the microwave. They don’t appear to get hotter than the couple of commercial mugs still in the house do. The one at least has been in use for about 3-4 years so far, and I put them in the dishwasher. I’m inclined to agree with Bill about the porosity being a factor on this one.

[Bill Kielb]

6 hours ago, Pyewackette said:

@Bill Kielb Nope. And it was hot all the way to the rim of the bowl.  It was only about half full of soup

If it was relatively thin, heat does travel through ceramic pretty speedy and heat does rise. Maybe try with an empty bowl, then again you might just have a body not good for the microwave after all.

[Pyewackette]

@Bill KielbThat was from the "good" batches of studio clay, before the changing of the guard.  I could throw that stuff thin.  The new stuff - not so much.  Too wet. (And I'm not good enough LOL!)

I'll keep an eye on it.  I'll have to throw a couple of bowls with the new stuff and see how that works out.  It'll just about have to be thicker, at least until they get it in hand.

Currently all my yunomi are from the good stuff and I reheat tea in the thin one fairly often.  It gets hot, but no hotter than you would expect.  I also have a thicker one and one that's sort of in-between.  Haven't tried those yet (one is ugly and one is being refired due to pinholes in the liner glaze)

[Pyewackette]

@Callie Beller Diesel My too-hot soup bowl was a cone 6 white stoneware though.  One wouldn't expect that to be high porosity. No way to test the porosity on that clay body as it has changed due to changing personnel and different talc.  I'm not sure how  much the recipe has changed but it must have, since the old talc is a goner.

[neilestrick]

I've got small bowls from many different potters and many different clay bodies, ranging from dark brown stoneware to porcelain, but they're all vitrified, no porous low fire bodies. They all get pretty hot after 2-3 minutes in the microwave, but the heat is mostly in the area where the contents are touching the bowl. I can pick them up by the rim, but would have to use a potholder in the middle. I heat a porcelain mug of water to boiling every morning for tea, 2 minutes in my small microwave, and the handle is always very comfortable to touch. So I think it's more a case of heat transfer from the contents rather than the clay itself getting heated by the microwave.

[Min]

On 4/26/2022 at 12:04 PM, Pyewackette said:

My too-hot soup bowl was a cone 6 white stoneware though.  One wouldn't expect that to be high porosity

Why not? There are definitely some cone 6 white stoneware bodies with absorption figures above 2% or so. I wouldn’t just take the studios word for the clay being okay for functional ware that will be going in a microwave. It’s a good idea to run your own tests when using a new claybody. Link to how to do an absorption test here if you need it. https://ceramicartsnetwork.org/daily/article/Testing-1-2-3-How-to-Test-Clay-Bodies-to-Find-the-Right-Sculpture-or-Pottery-Clay-for-Your-Work I’ld recommend the proper test of soaking a clay bar and working out the actual absorption amount rather than the quick test with the coloured ink in that article.

Another good test that isn’t in that article s to make up some simple flat bottomed cylinders and fire them without glaze, try and scatter them throughout the glaze kiln. After firing put some water in them and sit them on some smooth unwrinkled newsprint. Leave them sit for a week without moving them then check the paper. Any wrinkles or dampness and the porosity is too high for functional pots that might go in the microwave (or vases).