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blue glazes in the microwave cooker

updated tue 25 may 99

 

Mike Dalgleish on thu 20 may 99

Am I imagining it , or does pottery get hotter in a microwave oven if it has
a blue glaze.
--
Mike d. Dalgleish
Staffordshire,
England
\/\/\/\/\/\/\/\/

Ian Dunbar on sat 22 may 99

Hi Mike,

Don't know if there is a connection, but the terracotta tableware I make
gets too hot to handle in 2 - 3 minutes in the microwave. I checked this
out with "National" the local maker of my appliance and was told that the
microwaves "agitated the iron atoms in the clay" so causing the dish to
heat up. The downside of this inquiry was that the technical rep reckoned
that it was damaging the machine, so I took of the 'ideal for microwave
use' line out of my catalogue.

I still warm up an occasional meal and the microwave is still going after 7
years of 'misuse'.

Regards

Ian Dunbar
Kilncraft Pottery & Gallery,
Lynedoch,
Cape Province,
South Africa.





-----Original Message-----
From: Mike Dalgleish [SMTP:Mike@mdalgleish.clara.co.uk]
Sent: 20 May 1999 02:56
To: CLAYART@LSV.UKY.EDU
Subject: Blue glazes in the Microwave cooker

----------------------------Original message----------------------------
Am I imagining it , or does pottery get hotter in a microwave oven if it
has
a blue glaze.
--
Mike d. Dalgleish
Staffordshire,
England
\/\/\/\/\/\/\/\/

Andrew Buck on mon 24 may 99

Hi All,

All the color producing elements in clay and glaze are metals and metals
tend to absorb microwaves and become hot in the ovens. However, the
metals that color the thin layer of glaze on the surface of a pot are of
so small a percentage that the problem would be more likely due to the
clay body than the glaze. Cobalt, being a very strong color producer, is
used in even smaller percentages in glazes than other metals so it should
not make the pot hotter. The way I have been told to test pots for
microwave safeness is to place an empty pot in the microwave oven along
side a glass of water, turn on the oven for two to three minutes, and
check to see if the empty pot gets hot. The water should absorb the
microwaves and heat up, and the pot should not, for the pot to be sold as
microwave safe. DO NOT forget to put the glass of water into the oven for
this test or damage to the oven, or even a fire, could result.

Andy Buck
Raincreek Pottery
Port Orchard, Washington

On Sat, 22 May 1999, Ian Dunbar wrote:

> ----------------------------Original message----------------------------
> Hi Mike,
>
> Don't know if there is a connection, but the terracotta tableware I make
> gets too hot to handle in 2 - 3 minutes in the microwave. I checked this
> out with "National" the local maker of my appliance and was told that the
> microwaves "agitated the iron atoms in the clay" so causing the dish to
> heat up. The downside of this inquiry was that the technical rep reckoned
> that it was damaging the machine, so I took of the 'ideal for microwave
> use' line out of my catalogue.
>
> I still warm up an occasional meal and the microwave is still going after 7
> years of 'misuse'.
>
> Regards
>
> Ian Dunbar
> Kilncraft Pottery & Gallery,
> Lynedoch,
> Cape Province,
> South Africa.
>
>
>
>
>
> -----Original Message-----
> From: Mike Dalgleish [SMTP:Mike@mdalgleish.clara.co.uk]
> Sent: 20 May 1999 02:56
> To: CLAYART@LSV.UKY.EDU
> Subject: Blue glazes in the Microwave cooker
>
> ----------------------------Original message----------------------------
> Am I imagining it , or does pottery get hotter in a microwave oven if it
> has
> a blue glaze.
> --
> Mike d. Dalgleish
> Staffordshire,
> England
> \/\/\/\/\/\/\/\/>

Michael Banks on mon 24 may 99

I've had this experience too. A few years ago I decorated some white
earthenware coffee mugs with an underglaze slip, heavily loaded with cobalt
carbonate. The mugs were decorated prior to bisc and after low-bisc firing,
glazed with a commercial borosilicate clear glaze and fired to cone 3. The
strap handles were decorated with a wide brush stroke in dense cobalt.

Unfortunately, during home testing of one of the mugs, I discovered that the
deep cobalt-blue outside surface of the handle became phenomenally hot in
the microwave. The white clay body tended to heat a bit too, being slightly
porous, but nothing compared to the handle. The first time I grabbed it,
there was a sizzling sound from my thumb, resulting in an instant surface
burn. The inside of the handle, in contact with my index finger, was hot,
but a hundred or so degrees cooler! There is not doubt that the fired
cobalt compound (cobalt silicate?) in the underglaze had preferentially
absorbed microwave radiation - in a big way.



>----------------------------Original message----------------------------
>Am I imagining it , or does pottery get hotter in a microwave oven if it
has
>a blue glaze.
>--
>Mike d. Dalgleish
>Staffordshire,
>England
>\/\/\/\/\/\/\/\/>