Is it microwavable?

That is a new one for me. I use that clay and have no problem sticking it in the microwave. You can actually use metal in the microwave as long as you have enough mass of liquid (so you can have a themometer, spoon, etc. in the dish if it has enough food in it. If however, you see arcing or sparks, turn things off and don't do whatever it was you were doing.

Donna

other potter told me that you can't see the arcing but that she had to replace some expensive part in her oven because of that. I'm thinking that she may just think it was because of that type of clay. Who knows, the part may have been getting ready to go bad anyway and not from the clay. Sandi

I really doubt that this problem came from the pottery. For one thing, if there is sufficient metal in the piece you are gong to notice it when you touch the pot. It will be far hotter than the food it contains. Second, way too many people use 112 for it not to have been noticed by others. A quick check comes up with nothing on this but I will ask around.

Donna

P.S. Does this mean they also don't use high iron glazes in the microwave?

i don't know how current it is today, but years ago the word was to put a piece in the micro for 30 seconds on hi. empty. if it came out basically the same temp as it went it, it's fine. if it comes out distinctly hot, it's not micro safe.

see ya

steve

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The way I prefer to test is to have a second container holding water, next to your dry piece under test. Then run the micro on high until the water boils, and see if the test piece is warm. This avoids the possible danger to the microwave of running it without something to heat, which is a problem for some units (or was... maybe new ones no longer have this problem).

As for the high-iron glazes, I have been unable to verify for myself that they can get hot in the micro. This was a topic on Clayart earlier in the year, and my initial feeling at the time was that this was unlikely, since the iron would be embedded in an insulating glassy matrix and would not be conductive. More likely is that a piece that isn't fully vitrified will absorb some water (which is a good microwave "susceptor") and that would heat up. (I have seen this happen. The proof is that you can cook the piece in a conventional oven for a couple of hours on low heat to slowly drive out the water, after which the microwave heating no longer happens... until the piece is run through the dishwasher again, etc.)

However, as a result of the Clayart exchange, one potter reported a high-iron glaze that reliably caused microwave heating. After close questioning and several control tests on her part, I am more inclined to believe it is a real phenomenon. Alas, I have not been able to duplicate her result. The glaze was Bailey's Red cone 10. I have tried the cone 6 version with no luck, but that may be due to my tiny fast-cooling kiln... I also don't get a decent red. I think the red is caused by iron crystals forming in the slow-cooling glaze, and I suspect that maybe they can get big enough to heat up individually, even if they are insulated from each other by the glassy matrix so they don't form a large conductor. But that's only a working hypothesis until I get a larger kiln (or a better controller than me manually trying to do a forced cool-down.)

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

I was actually asking about the RIO glaze with tongue in cheek... Need an emo for that. I have a great many things with Randy's Red which has 15% RIO and have never had a problem with them heating up. I do think that you need some fluid in the piece when sticking it in the microwave. It is really clear when you have a pot that doesn't belong in there. I had a slip cast mugs from a give away and I have no idea what was in the glaze but I can tell you I never drank out of it after the first time I microwaved it and the cup became way noticeably hotter than what it contained. My first thought was "LEAD!". Probably not but I didn't want to play the game of waiting until my hair started falling out.

Donna

Those slip-cast mug symptoms sound like the ones I experienced, also in a slip-cast mug. The rim in particular would get really hot. But that was just due to trapped moisture in the unvitrified body... drying it out in a 250F oven for a couple of hours ended the problem, at least temporarily. I suspect that even if the mug never gets wet, it will pick up moisture from the air through the porous footring.

I don't think lead can be the problem, because I don't think it can be conductive in a glaze. In order for something to be heated in a microwave, it has to be either:

1) A polar molecule like water with slightly positive and negative ends that the microwaves can grab onto, AND it must be free to be dragged around. (Not bound into a matrix.) This can happen in ceramic glazes, but only at high temperatures where the polar molecules can move a bit/

or

2) A (partial) conductor such as metal or silicon carbide. In this case the microwaves cause heating by inducing circulating currents in the conductor, and the current passing through the resistance of the conductor is what generates the heat, just like in any electric heater. Nothing has to move for this to happen (well, other than conduction electrons), but you need a continuous conductor of some size. I don't know if you had tiny conductive particles, just how small they can be and still heat up. But the heating efficiency would go way down if the size got too much smaller than the size of the microwaves, and despite their name they are not "micro" anything... more like a centimeter. So I'd expect you'd need at least millimeter-sized crystals to get any noticeable heating, as a first guess. Not something I'd expect in a lead glaze.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

Fun to know. Note I didn't say good to know because there goes my made up check for lead in glaze..... I really hate giving up my unreality based miniworlds... :)

Donna