[Clayart] Microwave heating

David Hendley david at farmpots.com
Mon Nov 7 23:09:00 EST 2016

I agree. Water absorption is the culprit when a mug gets scalding
hot when heated in a microwave.
Glaze does not seem to matter. I use plenty of tenmoku mugs with
6 or 7 % iron oxide in the glaze, and they are fine in the microwave.
But, glaze CAN matter if the claybody is too absorbent AND the
glaze is crazed - this allows more water to be absorbed into the clay,
and it is this water that heats up and burns your hand when you grab
the handle.

Others have said, and I agree in theory, that running an absorbent
mug through the dishwasher also makes things worse because
it sends jets of hot water all over the mug, kind of injecting it into
the clay.

On a mug made with clay that is not vitrified enough, if the
glaze is not crazed, and the mug is not put in a dishwasher,
some water will still be absorbed through the unglazed bottom.
It may end up being OK and not overheating, or maybe not...
It's best to make sure your claybody is properly vitrified.

David Hendley
david at farmpots.com

----- Original Message ----- 
> 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>

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