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Cone 6 Porcelain Mugs, Microwave "pings"

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Hello! This is my first post after reading so much fabulous and useful advice on these forums the last couple of years. I'm still a bit of a newbie potter and I've been selling for just over a year. So far I've been really pleased and no complaints or negative comments (exception: why don't you make in blue?) LOL.

So I sent off a pair of mugs (Etsy sale) and the new owner is really happy with them, except that he just tried to microwave one to reheat his beverage, and he heard some "pinging" noises. He was alarmed and took the mug out and didn't try again; he's just wondering if I know what is going on.

The details: I use Tuckers Pottery 6-50 cone 6 porcelain clay. I fire to cone 6 based on witness cones. (I use cone 7 in the kiln sitter to get there). Glaze: I am making my own glazes. The liner glaze in these mugs was an Assad Opalescent (basic high percentage frit based glaze with no added oxides). Outside was a floating blue glaze (from John Britt's book of Cone 6 glazes: Floating Blue #2) I've used both of these before and have been very happy, and I've personally have had no problems with anything in my home. Unfortunately, I have sold everything at this moment that has this exact glaze combo. Also not sure if a mug done the same isn't necessarily going to be a perfect "test" as glaze thicknesses, exact kiln position, exact top temp, etc may be slightly different?

Any advice would be really appreciated! Mostly I'd like to know if in *your* opinion I should tell this buyer not to try to nuke these mugs? I know I've been a bit "reckless" in claiming my things are microwave safe and I'm sorry now I did without being more sure. I've read the forums about the astm standards and I plan to at least test more carefully than just a few things I have at home.

Thanks in advance!

Cheers, Lorraine.

 

 

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Door #1  The clay body is absorbing heat at a different rate than the glaze: creating a COE issue.

 

Door #2 The clay body has micro fissures, and the heat is causing them to expand.

 

Answer: does the customer see any crazing rings in the glaze now?  Let some coffee or tea sit in there for a day- it will highlight crazing rings.

              Best not advertise micro friendly until further testing is complete.

 

Nerd

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Hello! This is my first post after reading so much fabulous and useful advice on these forums the last couple of years. I'm still a bit of a newbie potter and I've been selling for just over a year. So far I've been really pleased and no complaints or negative comments (exception: why don't you make in blue?) LOL. So I sent off a pair of mugs (Etsy sale) and the new owner is really happy with them, except that he just tried to microwave one to reheat his beverage, and he heard some "pinging" noises. He was alarmed and took the mug out and didn't try again; he's just wondering if I know what is going on. The details: I use Tuckers Pottery 6-50 cone 6 porcelain clay. I fire to cone 6 based on witness cones. (I use cone 7 in the kiln sitter to get there). Glaze: I am making my own glazes. The liner glaze in these mugs was an Assad Opalescent (basic high percentage frit based glaze with no added oxides). Outside was a floating blue glaze (from John Britt's book of Cone 6 glazes: Floating Blue #2) I've used both of these before and have been very happy, and I've personally have had no problems with anything in my home. Unfortunately, I have sold everything at this moment that has this exact glaze combo. Also not sure if a mug done the same isn't necessarily going to be a perfect "test" as glaze thicknesses, exact kiln position, exact top temp, etc may be slightly different? Any advice would be really appreciated! Mostly I'd like to know if in *your* opinion I should tell this buyer not to try to nuke these mugs? I know I've been a bit "reckless" in claiming my things are microwave safe and I'm sorry now I did without being more sure. I've read the forums about the astm standards and I plan to at least test more carefully than just a few things I have at home. Thanks in advance! Cheers, Lorraine.

The floating Blue #2 is suspect, and is nowhere near food safe limits.  You have to be careful about what you say and at least try some of your mugs in the microwave first.  Make some leak tests. try some lemon peals on the surface etc.

David

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Thanks for the information! I guess I have made the assumption that if the inside of a mug has a safe "liner" glaze, the fact that the more questionable Floating Blue #2 glaze is only on the exterior/ no food contact that it should be okay. I did test these glazes in to microwave before, just not this specific combo of glazes together. That's aside from the glaze fit- COE issue that I'll be trying to figure out. I will change my microwave safe claim. Was too bold in thinking since none of my pots has issues for me so far that it was safe to proclaim : not! ;) Thanks again!

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Hi, so I looked up the calculated expansion of the liner glaze in this set of mugs, and from Insight Live = Assad Opalescent = 5.7

The Tuckers 6-50 clay appears to have an COE of 7.71

 

 

So, I am still learning (and enjoying this!) but I understand that the calculated expansion values are not necessarily predictive for risk of crazing (or shivering) BUT these two are quite extremely different, especially compared to some of the other glaze recipes I use. For example, the Floating Blue #2 Has a COE of 7.8, and the liner clear I usually use, Hansen's 20x5, is 7.0 .

So, am I right in thinking: the Assad is not the best choice of recipe for this clay based on the COE? That the Assad, as a liner, may be starting to craze in these mugs, causing the pings in the microwave?

 

And, is it theoretically possible, that the crazing of *any* 'at risk' glaze occurs more frequently if the pot/mug is heated too quickly/ to high heat vs slow heating ?

 

Thanks again, Lorraine.

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The Assad being a much lower coe than the FB#2 is not likely going to be the one that is crazing. In my previous post I made that assumption, apologies. Clear glazes are usually more prone to crazing than ones with colouring oxides added to them.

 

Having a wide mismatch of coe glazes on the same pot can lead to the pot being torn apart (dunting), this can happen months after the pot is taken from the kiln. If the liner Assad glaze is putting the clay under compression and the FB#2 putting the clay under tension there could be a problem. 

 

Can you make a test cylinder (thrown thinly) with the Assad clear applied thickly on the inside and the FB#2 on the outside. Fire it then put it in the coldest part of your freezer overnight then fill it with boiling water (in the sink). If the mismatch of coe’s is great enough the pot will split in 2 or at least shiver glaze off the rim.

 

Have you done stress tests on the clay with each of the glazes and checked for crazing? Like you said the calculated coe values are not always indicative of possible crazing.

 

"And, is it theoretically possible, that the crazing of *any* 'at risk' glaze occurs more frequently if the pot/mug is heated too quickly/ to high heat vs slow heating ?"

I think this depends on your definition of too quickly. You should be able to pour boiling water into your mug/pot, or heat a beverage up in the microwave in it without it crazing. 

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Thanks again! I think I'm getting it..

Yes! I will make a test cylinder and craze test these glazes.

I've tested many of them with pots that I keep, but not this combination.

One thing I'm still wondering about is the lower COE of the Assad having less chance of crazing..i thought that the COE should be close to the COE of the the clay body. Or is it just lower COE means less crazing in general?

I do understand that these 2 glazes inside vs outside would cause stresses.

 

Lorraine

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Hi Lorraine,

 

“Or is it just lower COE means less crazing in general?â€

This. but….

 

The COE figures from glaze calc programs don’t really tell the whole story. Yes in general lowering the COE means lowering the risk of crazing but more recently glazes have been looked at another way too. I’m just going to quote Tony Hansen (DigitalFire) here, he says it better than I would “Thermal expansion values predicted by calculation are relative (not absolute) and apply within 'systems'. Thus, if a glaze calculates to a higher expansion than another, and is in the same system, then it is more likely to craze. For example, if you have a dolomite, whiting, feldspar, kaolin, silica glaze and you try a bunch of variations, the calculated expansions will give you an indication of which variations have higher and lower expansions. But if you introduce lithium carbonate, or boron frit, or zinc, for example, now you have a different system.† 

 

So, if you have a glaze that crazes you can usually swap out the high expansion fluxes for ones with lower expansion, rebalance boron, silica and alumina and go from there with the testing. I have a relatively high COE glaze (7.18) high in sodium and alumina glaze that doesn’t craze on my clay, also have a high in magnesium, alumina and silica with a COE of 5.76 that doesn’t craze either. On paper the former glaze should theoretically craze but it doesn’t and the low COE glaze does not shiver. They both fit really well. So, I’m starting to try and look more at the family of fluxes rather than just COE figures when predicting if a glaze will fit my clay(s). The glaze calc software is brilliant at what it does but you still have to do actual clay/glaze fit tests.

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Perfect! Thanks so much for the great explanation :D

The person who bought my mugs said (even though I had said he may not want to microwave these mugs again) he did microwave in them several times and after the couple of "tinks" the first time, there has been none since, and no crazing (I told him how to check)! So, he knows to be watchful, but I'm pleased that they seem to be ok *so far*. He said he is happy to update me to let me know longterm.

Thanks so much everyone!

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