Microwave Safe

[Kelly in AK]

Microwave safe. What does that mean?

I’ve been puzzling over this a while, a few years, at least. Lately an event, which I’ll get to, pushed me into posting the question here.

My understanding was that vitrified clay wouldn’t get particularly hot in the microwave, but there was some question when it came to high iron clay bodies. Experience in my kitchen over twenty years bears this out, the semi-vitreous pots could be counted on to get hot, the really porous earthenware could get dangerously hot while the food remained cold. High iron clays don’t seem to behave differently, but I remain open to the possibility they can. The other experience I’ve noted is that any dish fresh from the dishwasher, even cooled to room temperature, will get hot in the microwave. My assumption is adsorption (water that intimately sticks to the surface), not just absorption, plays a role.

The event that gets me seriously wondering centers on a diner plate. My dad was in the catering business and when he retired I snagged a few restaurant dishes. They’ve been in service for over twenty years in my home, unbelievably perfect ceramic engineering. Plain white, no chips, cutlery marks, or cracks, despite regular use and abuse (They’re diner plates, I don’t treat them like handmade pottery!). They don’t get hot in the microwave. My son burned his finger pulling one out of the microwave the other day. That didn’t make sense. Tonight he tells me he observed the plate closely and saw a craze and discoloration, and that it looked like a glaze crack, not a clay crack. I’m proud as a dad, he nailed it, though I doubted until I saw for myself. That discoloration is at about 11:00 in the photo, poorly visible I’m afraid. There is another blotch around 2:15. 

So finally, after all these years, one failed. Time for destructive testing. I threw that plate in the microwave, one minute. At 50 seconds, I added another minute. At one minute and fifteen seconds it shattered cleanly into three pieces.

What did I learn? These plates, tough as hell, never got hot in the microwave for twenty years, never chipped, are in fact made of porous clay. I put a drop of water on the newly bared ceramic and it soaked right in. This rocks my world a bit. I honestly didn’t imagine it was possible to fit a glaze that well to non vitreous ware or that non vitrified ceramic could be so tough. 

The ASTM has a standard and a test for “microwave safe,” I’m not shelling out sixty bucks to read it. I’m not sure my pots would even pass the prerequisites to perform the test. I figured “vitrified”, as in 0.5%, was a prerequisite. 

What does microwave safe mean? 

 

[PeterH]

Some ideas on the causes & avoidance of problems in:
Microwave safe
https://community.ceramicartsdaily.org/forum/23-clay-and-glaze-chemistry/

PS Pedantically in the USA:

... although I expect that -- like many stress tests -- this only implies safe at the point of test rather than lifelong safety. [Unless you re-test after representative "wear and tear" such as crazing and chips.]

Edited May 11 by PeterH

[Hulk]

Good question.
ASTM article (formerly $48) https://www.astm.org/c1607-12r20.html
Mr. Hansen's article Microwave Safe (digitalfire.com)
A Forum thread on the subject: Microwave safe. - Clay and Glaze Chemistry - Ceramic Arts Daily Community

The ASTM article may include a standard/industry definition, idk.

This is where I am with it: "...not melt or otherwise lose form, not leach bad stuff, not get too hot to touch after one minute in the waver [microwave oven] ...anything that gets more than pleasantly warm (empty, clean) is not microwave safe...!"

I still have microwave test clean dry wares on my "to do" list.
I have an inexpensive infrared thermometer now...

Edited May 11 by Hulk
actual

[Min]

In a nutshell to pass the ASTM for "Microwave Safe for Reheating" there is mention of "no metallic surfaces" but no threshold limit  for porosity or absorption of the claybody. Ware must show no degradation after the test nor exceed specific temperatures after a set amount of time at full power.

I would hazard a guess that those discoloured areas on the plate have let moisture into the body which resulted in the plate getting hot when your son removed it from the microwave. Agree with you, that is one well fitting glaze to stand up for catering use then your home use for 20 years.

[Kelly in AK]

Thank you all for the insight. And thank you @Min, I was very curious as to the nature of that test. 

[davidh4976]

I tried an experiment. I microwaved five unglazed items made with five different clay bodies along with a beaker of water. None of these have ever been exposed to water. Humidity exposure is minimal (kept inside and in New Mexico where it is a fairly dry weather).  Readings using a laser thermometer after 1 minute in the microwave:

• Cashmere and Ochre 100F
• Marilyn's BOD and Chocolate 120F
• IronStone 210F

These are all cone 5/6 clay bodies by New Mexico Clay.  I am guessing, based on their appearance, that Cashmere (a white porcelain/stoneware mix like B-mix) and Ochre (a light tan)  have the lowest iron content.  Marilyn's BOD (a red-brown stoneware) and Chocolate (Dark Hershey brown smooth clay, almost black) have a bit more, again based on appearance. Ironstone is advertised as having so much iron that a magnet will stick to it which I tested and it's true! Fortunately, it wasn't so much iron as to fry my microwave!

 I did not do the presoak as prescribed by ASTM because I was more interested in the effects of the iron in the clay body. Even without the presoak, per ATSM C1607-06, the Ironstone would NOT be considered microwave safe.  Maybe someday, I'll make  mugs out of Ironstone and the others and then run the mug test complete with presoak.

[neilestrick]

I just ran a 1 minute microwave test with 4 pots with the following clay bodies:

• Standard 365 cone 6 English porcelain
• Standard 266 black clay
• A medium-brown cone 10 stoneware with grog
• A red body, fired around cone 3.

After microwaving for 1 minute and measuring the temperature of the piece in the middle, using a laser pyrometer:

1. The porcelain and the cone 10 body both came out around 140F. Kinda hot, but I could get them out of the microwave.
2. The 266 black hit 170F, too hot to handle.
3. The red body was over 220F.

Here's the really interesting thing, though: in all cases, the unglazed areas were a lot hotter than the glazed areas. When I measured down the side of the piece, they were all cooler at the top than they were at the bottom, presumably because they all have unglazed bottoms. The red clay pot was glazed on the inside and about 3/4 of the way down on the outside, and at the very bottom of the outside where it was unglazed, it measured over 260F. In all cases, the bottom inside was at least 30 degrees hotter than further up the walls.

So glaze on both sides seems to have a pretty big effect on how hot the pot gets. @Kelly in AK if that plate was glazed on the bottom like most commercial plates are, that may explain why it stayed cool in the microwave.

Another odd thing:  If I microwaved several at a time, they didn't get as hot- just like when you have more food in the microwave you have to run it longer. But the unglazed areas heated up almost as much as when I ran them alone. Strange stuff.

So iron in the body, glaze, vitrification? I don't know if the red body got hotter because of the iron in it or because it's not fully vitrified, or both? The Standard 266 has a lot of iron, maybe more than the red body, but it's more vitrified than the red, so is that why it didn't get as hot? Or did the red get hotter because it has more unglazed area? Lots of mysteries here.

Having something in the pot that has to get heated up definitely slows down the heating of the pot. I microwave a mug of water 3-4 times a week in my studio for tea. It's made of Standard 365 cone 6 English porcelain, and every day I pull it out of the microwave by the handle with no problem.

 

[Min]

Without going into too specific details to follow the ASTM... for cups and mugs put a cup of cool water in the corner of a microwave then with the empty test sample mug run it for 2 minutes at full power.  Handle temp must not exceed 60C/140F and  any of the other surfaces 121C/250F. Before commencing test the test pieces must be submerged in room temp water for 12 hours and also pass the ASTM 325F oven to water test. (which I don't have) Slightly different times and temps for plates etc.

Edited May 13 by Min

[davidh4976]

3 hours ago, neilestrick said:

In all cases, the unglazed areas were a lot hotter than the glazed areas. When I measured down the side of the piece, they were all cooler at the top than they were at the bottom, presumably because they all have unglazed bottoms. 

I also saw this on other test pieces I have microwaved. I believe it was because water had absorbed through the foot of the cup and was present in the lower part of the cup but had not been drawn up into the upper parts of the cup. The water heats up pretty fast. 

[davidh4976]

3 hours ago, Min said:

... and also pass the ASTM 325F oven to water test.

Maybe you mean ASTM 554? It calls for three passes of going from 250F to room temperature water without crazing. Repeat at +25F increments up to 450F. Crazing fails the test. Too much work for me!  I just rely on three passes of boiling water to ice water to determine whether it's going to craze or not. It is a little bit more of a cheat for me because boiling water at my altitude is only 199F. Although the microwave test calls for doing ASTM 554 first, to me, the two tests seem mostly independent.  If you are going to determine whether a piece is microwave safe, microwaving it and seeing if it gets too hot seems to be enough for me.

[Kelly in AK]

Yes, @davidh4976, ASTM C554 is the “prerequisite” test for test C1607.  And I agree, putting pots through the worst I can imagine a reasonable person doing in their kitchen gives sufficient information as to how they’ll hold up.

I believe it’s important to use your own work, it helps you understand what you’re expecting of people. 

Edited May 14 by Kelly in AK

[Min]

1 hour ago, davidh4976 said:

Maybe you mean ASTM 554? It calls for three passes of going from 250F to room temperature water without crazing. Repeat at +25F increments up to 450F. Crazing fails the test. Too much work for me!  I just rely on three passes of boiling water to ice water to determine whether it's going to craze or not. It is a little bit more of a cheat for me because boiling water at my altitude is only 199F. Although the microwave test calls for doing ASTM 554 first, to me, the two tests seem mostly independent.  If you are going to determine whether a piece is microwave safe, microwaving it and seeing if it gets too hot seems to be enough for me.

I question the testing using the same piece for each subsequent heat increment as each cycle will stress/shock the glaze. I know that's the point but I agree with Hansen's take on this. I sometimes go 300F/ice water, then 315F/ice water then 325F/ice water. If a glaze doesn't stress craze from this I don't think it will with everyday normal (sane) usage. 450F to ice water IMHO is far too extreme! We are after all making ceramics, not metal bakeware. It does seem logical to soak the test piece for 12 hours after a reasonable stress/shock testing though.

edit: In the past I did try the boiling water / ice water testing, had a glaze that passed it so went ahead and glazed a load of pots with it which later went on to craze. I'm sticking with the oven/ice water test, greater difference in temp therefore a harsher test, either the one above or just 3 sets of samples at 315F. 

Edited May 14 by Min

[neilestrick]

11 hours ago, davidh4976 said:

I also saw this on other test pieces I have microwaved. I believe it was because water had absorbed through the foot of the cup and was present in the lower part of the cup but had not been drawn up into the upper parts of the cup. The water heats up pretty fast. 

These pieces were all dry, right out of the cabinet.

[Min]

16 hours ago, neilestrick said:

Here's the really interesting thing, though: in all cases, the unglazed areas were a lot hotter than the glazed areas. When I measured down the side of the piece, they were all cooler at the top than they were at the bottom, presumably because they all have unglazed bottoms.

My guess would be it's because of different reflected indexes between the glazed and unglazed surfaces while in the microwave. 

[Min]

@davidh4976, I was thinking more about the 450F temp and crazing. Is the ASTM554 test still valid if the body cracks but the glaze doesn't?

[neilestrick]

I just made my morning tea- porcelain mug with water in it, microwaved for 2 minutes. The water is too hot to drink, but the mug handle is very cool, like maybe 85 degrees instead of room temp. I'm going to call that microwave safe.