[Clayart] Microwave heating

ronroy at ca.inter.net ronroy at ca.inter.net
Wed Nov 9 12:04:46 EST 2016

Hi Paul,

I don't know at what % absorbed water becomes a problem. That would be  
an interesting project. Seems like you could add increasing amounts of  
water to semi absorbent clay and find out easily enough.

If you care to do some experiments I can do some at this end and  
perhaps we can come up with some useful information. I have an  
extensive collection of mugs and I'm sure some of them will not be  
properly vitrified. Easy to dry them out in a kiln or oven till all  
the water is gone. Then add water and microwave to see when they start  
to heat up.

I'll bet may of us have ware that heats up in a microwave - anyone  
else want to do this?


Quoting Paul Gerhold <gerholdclay at gmail.com>:

> Ron,
> I understand that but if you have a mug say with an absorption rate  
> of say 4% and it sits in an atmosphere that is say thirty percent  
> humidity will the 4% void space in the mug be 100% moisture or will  
> it equilibrate with the atmosphere and the 4% void be only 30% water.
> My guess would be that the partial pressures of water will be  
> equalized and the void space in the mug will be only 30% water.
> Paul
> Sent from my iPad
>> On Nov 8, 2016, at 11:36 AM, ronroy at ca.inter.net wrote:
>> Hi Paul,
>> A not properly vitrified mug would be OK just out of the kiln -  
>> because there was not enough time for water to get into the clay.
>> Depending how vitrified the clay is it may take weeks of constant  
>> use and washing for the water to get into the clay - even faster if  
>> the glaze is crazed. Even moisture from the air will gradually get  
>> in.
>> RR
>> Quoting Paul Gerhold <gerholdclay at gmail.com>:
>>> It is hard for me to imagine that if you take a dry mug , add  
>>> water  and then microwave for a minute or two that enough water  
>>> can be absorbed by the body to explain the heating.  Perhaps  
>>> someone with a mug with this problem can weigh a mug, microwave  
>>> it, remove the water, dry thoroughly and then reweigh to test the  
>>> water absorption hypothesis.
>>> Paul
>>> Sent from my iPad
>>>> On Nov 7, 2016, at 8:10 PM, John Hesselberth  
>>>> <jjhesselberth at gmail.com> wrote:
>>>> Hi Everyone,
>>>> I have been asked off list to post my thoughts on how to assure  
>>>> your pots will not get scalding hot in a microwave.  I have found  
>>>> that getting the water absorption of your fired body down to 2%  
>>>> or less does the job. The procedure for testing that has been  
>>>> posted on the list by Ron Roy several times or it is in our book.  
>>>>  Or bug Ron to post it again—we have let him off kind of light  
>>>> recently.
>>>> There are one or two people on the list who say they have seen  
>>>> the overheating with glazes heavy in iron. I have not seen that  
>>>> with glazes containing up to 9% iron oxide (e.g. Licorice) but  
>>>> maybe it happens in rare circumstances. Clearly minimizing water  
>>>> absorption is the place to start. And don’t rely on the clay  
>>>> manufacturer’s numbers—test it yourself in your kiln firing at  
>>>> your normal conditions, It is pretty easy to do.
>>>> John
>>>> John Hesselberth
>>>> john at frogpondpottery.com <mailto:john at frogpondpottery.com>
>>>> "I love everything that's old: old friends, old times, old  
>>>> manners, old books, old wines." Oliver Goldsmith, "She Stoops to  
>>>> Conquer" (1773)
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>> Ron Roy
>> ronroy at ca.inter.net
>> Web page ronroy.net

Ron Roy
ronroy at ca.inter.net
Web page ronroy.net

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