Marty Morgan on mon 3 apr 06
I had a longtime customer come in and tellme that two of her medium
size, 8 1/2", plates had cracked when put in the microwave. These
were made from Laguna (Miller) 900 stoneware clay fired to cone 10 in
reduction with a dark rutile glaze. Mugs and bowls with the same
clay/glaze combination have been fine in the microwave. Anybody have
an idea why this would happen?
Marty Morgan
in Gloucester, MA, where spring has truly arrived at last
Vince Pitelka on tue 4 apr 06
Marty wrote:
">I had a longtime customer come in and tellme that two of her medium
> size, 8 1/2", plates had cracked when put in the microwave. These
> were made from Laguna (Miller) 900 stoneware clay fired to cone 10 in
> reduction with a dark rutile glaze. Mugs and bowls with the same
> clay/glaze combination have been fine in the microwave. Anybody have
> an idea why this would happen?"
Marty -
Flat ceramic items are least able to accommodate uneven thermal stresses.
It is possible that they just heat the food in the center of a cold plate,
so that the center became very hot and expanded. The flat form was unable
to accomodate that uneven thermal stress, and cracked across the center. I
would warn people not to heat up food in microwave ovens on your plates. I
don't see any way to avoid the likelihood of the plate cracking in that
situation. It's an unfortunate reality, and undoubtably, some claybodies
are more susceptible to this than others.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
skiasonaranthropos@FSMAIL.NET on tue 4 apr 06
Hi Marty,
Impossible to be definite but a couple of thought:
1. thermal shock sufficient to lead to failure, or
2. underlaying making flaw that only revealed itself following expansion of
the piece due to thermal expansion
Regards,
Antony
David Hendley on tue 4 apr 06
Plates can crack in a microwave because of uneven heating.
This is caused by food, in only one area of the plate, heating and,
by conduction, causing a concentration of heat in one spot.
When I sell plate, I include a card with the purchase:
"This stoneware pottery is durable and will last for many years with proper
care. It is dishwasher and microwave safe. However, be careful when using
plates in the microwave - food on the plate can cause uneven heating of the
plate, which can cause it to crack. Likewise, very hot food should not be
placed on a cold plate (pre-heat in a warm oven). Uneven heating is not a
problem with mugs and bowls, since any liquid in them will be evenly in
contact with the entire surface."
You can also advise people who want to use plates in the microwave
to simply turn down the power. I think most plates can do fine in
a microwave at half-power, even if cold food is in contact with
one area of the plate.
David Hendley
Old Farmhouse Pottery
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
>I had a longtime customer come in and tellme that two of her medium
> size, 8 1/2", plates had cracked when put in the microwave. Mugs and bowls
> with the same
> clay/glaze combination have been fine in the microwave. Anybody have
> an idea why this would happen?
Jennifer Boyer on wed 5 apr 06
Hi Marty
Did they have anything on them? I've had customers report a plate
breaking when l part of the plate had cheese or meat touching it, but
not touching the whole plate. One was nachos, with random touching
cheese. One was meat loaf down the middle of the dish, but the sides
were bare. This customer was probably cooking the same type thing on
each plate. If they were just being pre-heated with nothing on them,
she may have been doing it too fast....
Jennifer, watching the SNOW that fell yesterday melt!! We're ready
for spring.....
On Apr 3, 2006, at 10:36 PM, Marty Morgan wrote:
> I had a longtime customer come in and tellme that two of her medium
> size, 8 1/2", plates had cracked when put in the microwave. These
> were made from Laguna (Miller) 900 stoneware clay fired to cone 10 in
> reduction with a dark rutile glaze. Mugs and bowls with the same
> clay/glaze combination have been fine in the microwave. Anybody have
> an idea why this would happen?
> Marty Morgan
> in Gloucester, MA, where spring has truly arrived at last
>
> ______________________________________________________________________
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*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************
Ron Roy on wed 5 apr 06
Hi Marty,
I am not disagreeing with what others have said but - this can also be due
to an imbalance in fit between clay and glaze.
It is well know that a low expansion glaze - when only on one side of a
piece - like some plates - will result in that type of cracking under
uneven heat stress. It is also well known that if the ware has the same low
expansion glaze inside and out - the stress is equalized and the ware
becomes stronger - up to a point.
I'll be glaze to give you my opinion about the expansion of your glaze so
that you know that the answer in not liked to that.
It is also possible that the clay has some cristobalite after firing - I
would be happy to measure it for you - making a sample is easy - let me
know and I'll send instructions.
There are many clay/glaze combinations that will survive microwave heating
- finding out why some don't work leads to the solution of the problem.
RR
>I had a longtime customer come in and tellme that two of her medium
>size, 8 1/2", plates had cracked when put in the microwave. These
>were made from Laguna (Miller) 900 stoneware clay fired to cone 10 in
>reduction with a dark rutile glaze. Mugs and bowls with the same
>clay/glaze combination have been fine in the microwave. Anybody have
>an idea why this would happen?
>Marty Morgan
>in Gloucester, MA, where spring has truly arrived at last
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Linda Arbuckle on wed 5 apr 06
I've had porcelain slab-built plates w/a thrown foot crack in the
microwave when heating high sugar or fat things in a localized place.
E.g. 2 fake bacon strips on the plate cracked the nice porcelain plate
in 2. In theory the porcelain body should be vitreous, and so water in
the body not an issue. Putting larger, more evenly spaced food items on
the same plates has not been an issue, and doing the fake bacon strips
on plates that are less tight, dense bodies (and better able to take
thermal shock) has not been an issue. So, I'm guessing it was too much
uneven, localized heating for a dense body.
You may want to research what your customer was microwaving and how it
was arranged to gather further info.
Linda
http://lindaarbuckle.com
Eleanora Eden on wed 5 apr 06
The classic one I've heard is a piece of bacon on a plate. The narrow area
of hot grease down the middle of a plate......so this is the example I drag
out of what not to do when people ask me about micro-waving.
Eleanora
>
>On Apr 3, 2006, at 10:36 PM, Marty Morgan wrote:
>
>>I had a longtime customer come in and tellme that two of her medium
>>size, 8 1/2", plates had cracked when put in the microwave. These
>>were made from Laguna (Miller) 900 stoneware clay fired to cone 10 in
>>reduction with a dark rutile glaze. Mugs and bowls with the same
>>clay/glaze combination have been fine in the microwave. Anybody have
>>an idea why this would happen?
>>Marty Morgan
>>in Gloucester, MA, where spring has truly arrived at last
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