Raschella on thu 16 may 96
Hello friends,,,
Here's a technical question for you....
I made a set of 6 mugs, ^10 stoneware propane gas fired, and gave to a
friend about 2 years ago. She says she uses them everyday for hot liquids
mostly. Recently, they began to crack after use....on the body of the mug,
around the bottom,,,in a circular manner around the mug. The crack is
through the mug causing it to leak. Does anyone have any ideas why this
would happen now rather than during first few uses? 3 of the 6 have
cracked, leading me to believe the others will go soon too. I don't
remember anything ususual happening during this particular firing, such as
underfiring or overfiring, etc. Any ideas? Apparently some sort of stress
is present, but I would expect this to happen 2 years ago rather then
now.........
Thanks!!!!
Debbie
(304)788-6167
Rt 2 Box 295 Keyser, WV 26726
srasch@access.mountain.net
Bob Kavanagh on fri 17 may 96
There are certain obvious considerations to which you should direct your
thought: glaze fit, glaze thickness, clay porosity and clay strength. Like
most things, clay objects are subject to moisture and water over time; if
the clay is not adequately fired, or protected, it can take on some water
and change size giving rise to delayed crazing and a slightly weakened
body. If your glaze is a borderline fit to the clay (close to crazing)
then very minor changes in the body will aggravate an already delicate
balance and craze. If the glaze is a borderline glaze and thick, it can
very easily add significant stess to the body and push it toward cracking.
Some clays have their strength increased with the right glaze and others
have their strengths lessened with the wrong glaze. Some clays develop
crystals at high fire (cristobalite) and these crystals are affected by
dramatic changes at more modest temperatures (high oven temperatures): they
can expand and can give rise to cracks. You need to do some testing of
your glaze and its fit to the clay, as well as the porosity of your clay
just for a start. Get in touch off the list and we'll talk more details.
Ellen L Price on sat 18 may 96
I was wondering if these mugs were put in a microwave.
Ellen
Sam Cuttell on sun 19 may 96
At 10:41 AM 5/18/96 EDT, you wrote:
>----------------------------Original message----------------------------
>I was wondering if these mugs were put in a microwave.
>Ellen
>
I would venture to say this shouldn't make a difference. My favourite mug
(no one else touches it!!) gets used *every* day (for the last 5 years). I
have 2-4 cups of tea per day. My method is to make a pot, then "nuke" the
tea once the pot has cooled.
As well, I like my tea _steaming_ hot, so generally nuke the tea 1-5 times
(depending on interuptions while drinking) to re-warm it.
The only results I can give you are a slight crazing (but this particular
glaze tends to craze over time whether used functional or non-functional).
That's it. This mug goes through *every* wash cycle of my dishwasher
(approx 3X a week) and like the energizer bunny, it's still going!
I should also add the following: Plainsman H550 clay fired to ^10 ox with
glazes from CM
sam - alias the cat lady
who now has 3 puppies too!
Raschella on sun 19 may 96
>----------------------------Original message----------------------------
>At 10:41 AM 5/18/96 EDT, you wrote:
>>----------------------------Original message----------------------------
>>I was wondering if these mugs were put in a microwave.
>>Ellen
Ellen, I wanted to check before I responded. I talked to my friend today
and she said these mugs have never been used in the microwave, but I have a
tendency to agree with Sam below. I have used my mugs in the microwave many
times, and it has not been a problem.
Debbie
>
>
>I would venture to say this shouldn't make a difference. My favourite mug
>(no one else touches it!!) gets used *every* day (for the last 5 years). I
>have 2-4 cups of tea per day. My method is to make a pot, then "nuke" the
>tea once the pot has cooled.
>
>As well, I like my tea _steaming_ hot, so generally nuke the tea 1-5 times
>(depending on interuptions while drinking) to re-warm it.
>
>The only results I can give you are a slight crazing (but this particular
>glaze tends to craze over time whether used functional or non-functional).
>That's it. This mug goes through *every* wash cycle of my dishwasher
>(approx 3X a week) and like the energizer bunny, it's still going!
>
>I should also add the following: Plainsman H550 clay fired to ^10 ox with
>glazes from CM
>
>sam - alias the cat lady
>who now has 3 puppies too!
>
>
Marvin Bartel on mon 20 may 96
At 07:31 PM 5/16/96 EDT, Debbie wrote:
> I made a set of 6 mugs, ^10 stoneware. . . She says she uses them
>everyday for hot liquids mostly. Recently, they began to crack after
use....on >the body of the mug, around the bottom causing it to leak.
This sounds similar to problems I once had with oven pieces such as
pie plates. They were fine for a few years, but eventually began to crack
and I ended up replacing them for customers. At the time, I was using a
clay body with not enough feldspar in it and I think the quartz inversion
stresses resulting from free quartz produced a rather sensitive body.
Simply increasing the feldspar didn't work because it fluxed the body and it
bloated and warped.
I have since altered the body by increasing the kaolin so that I
could include 15% feldspar. This seems to be enough to solve the quartz
problem. For pieces intended to be subjected to extraordinary heat
stress, I use a more thermo shock tolerant body using some kyanite in the
mix. Kyanite is high in alumina. This is not flameware, but it serves well
for casseroles, pie plates, and such. One of my students tells me he is
putting his mugs on the stove top. I don't recommend it for burner top use,
but it is good for microwave, oven, and hot liquids poured in a cold vessel.
I've frequently used one of the pie plates for cooking whole chickens in a
domed top barbecue.
for Bartel Ovenware Clay use:
50 Goldart Clay
26 OM4 Ball
16 Custer Feldspar
8 Kyanite
All ingredients are 200 mesh or finer. I avoid grog, sand, or course clay
in ovenware clay because it will inevitably have a different coefficient of
expansion compared to the rest of the body. Most standard stoneware glazes
craze on this body, but I figure the pots are sanitized when they are heated
in an oven, dishwasher, or microwave. Brown slip glazes seem to fit best.
This clay is prone to drying cracks in the bottom centers unless dried
carefully. I welcome any comments regarding your experience with this body.
I also welcome other ideas and clay bodies you have found useful for thermo
shock.
++++++++++++++
Marvin Bartel, Art Dept
Goshen College, Goshen, IN 46526
marvinpb@goshen.edu
http://www.goshen.edu
++++++++++++++
Where an anagama is under construction
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