Lowell Barron on tue 17 jan 06
Hi,
For Christmas I made 10 mugs for my family members. Unfortunately two
of the mugs had the bottom crack all around at the level of the floor
and immediately start to leak tea. This happened the first time
boiling water was poured into them. They were sitting on a cold fake-
granite countertop.
All were made in Tuckers Mid Smooth Stone. I glazed them with MC6
Bone on inside and Rosie's Red on outside with a little bone brushed
on top inch of outside which then ran down the outside. Fired to a
soft 6 and slow cooled according to Coneart schedule. Gorgeous look
and a combo I have done on a lot of pots the last while.
Only difference I can see with these two is that they were flat on
the bottom whereas I usually put a foot on everything I make. I
pushed the bottom of one right out and the thicknesses seem very
even....not thick/thin.
Is this likely to be a case of thermal shock or is it something else.
It is embarrassing to have the bottom crack and fall out!
Thanks for any help you can give me.
Lowell Ann
Michael Wendt on tue 17 jan 06
Lowell Ann,
If the turn at the bottom is very sharp. it is likely you have the classic
no no in design, an abrupt change in direction which results in stress
concentration at the sharp turn.
Even if the turn seems gentle to you, hot tea water is near 212 F and the
bottom sees the most intense heat because that is where we pour it in.
As the bottom expands outward, it causes microscopic cracks to form on the
outside which is cooler and being stretched, something brittle materials
like clay do not tolerate well.
Try a rounded transition from bottom to wall and see if that helps.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
You wrote:
Only difference I can see with these two is that they were flat on
the bottom whereas I usually put a foot on everything I make. I
pushed the bottom of one right out and the thicknesses seem very
even....not thick/thin.
Is this likely to be a case of thermal shock or is it something else.
It is embarrassing to have the bottom crack and fall out!
Thanks for any help you can give me.
Lowell Ann
skiasonaranthropos@FSMAIL.NET on wed 18 jan 06
Hello Lowell,
Although diagnosis from your description would be impossible a suggestion
is that the two which cracked had pre-existing flaws, perhaps from the
shaping. Pouring boiling water into a mug will cause thermal shock but an
appropriate body / glaze combination made properly into a suitable shape
will be able to withstand the stress. However if flaws are present the
strength of the system is compromised. This may explain why eight have
survived
Why not test each mug in future? Say chill in a fridge / freezer, remove
and immediately pour in boiling water?
Regards,
Antony
Ron Roy on thu 19 jan 06
Hi Lowell,
I just went through all the dilatometer charts of the Tuckers clay bodies
and find that mid smooth stone is in a group of bodies with higher
expansion rates than some of the others.
For instace the Mid Cal 5 is in the lower expansion group - but the MCS is
in the higher group - and we used both those clays in our book.
The Bone Glaze has one of the lowest expansion rates of all our glazes.
I agree with Michael Wendt - in that if you are combining a low expansion
glaze with a clay body that is on the higher side you need to spread the
stress out - not have it consentrated at any sharp corners.
The type of cracking you are having is almost always due to clay/glaze fit
problems - with the glazes having the lower expansion.
I would select a glaze for the inside (liner glaze) that has a higher
expansion rate. This means - in the case of our glazes - any other than the
two high calcium glazes.
If you had used the bone inside and out the stress would have been probably
been equalized by the way - I suspect the Rosie's Red may be crazed and
that will help weaken the pot.
I'll be happy to help you work through this by the way - altering the form
and keeping the inside glaze thinner may be all that is needed.
RR
>For Christmas I made 10 mugs for my family members. Unfortunately two
>of the mugs had the bottom crack all around at the level of the floor
>and immediately start to leak tea. This happened the first time
>boiling water was poured into them. They were sitting on a cold fake-
>granite countertop.
>
>All were made in Tuckers Mid Smooth Stone. I glazed them with MC6
>Bone on inside and Rosie's Red on outside with a little bone brushed
>on top inch of outside which then ran down the outside. Fired to a
>soft 6 and slow cooled according to Coneart schedule. Gorgeous look
>and a combo I have done on a lot of pots the last while.
>
>Only difference I can see with these two is that they were flat on
>the bottom whereas I usually put a foot on everything I make. I
>pushed the bottom of one right out and the thicknesses seem very
>even....not thick/thin.
>
>Is this likely to be a case of thermal shock or is it something else.
>It is embarrassing to have the bottom crack and fall out!
>
>Thanks for any help you can give me.
>
>Lowell Ann
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Lowell Barron on thu 19 jan 06
Hi Ron, Thanks so much for your reply and thanks too to Michael.
This problem is puzzling as the angle is not the problem. It is a
gentle one. I am a hobby potter but have shown the problem mugs to
Kim Davy, our artist-in-residence here in London and she is baffled.
It is important for us to figure this out. We are in the process of
finding some new glazes for the studio that work with our old tried-
and-true ones. I just lent my MC6G book to Kim to read as the guild
copy is out. Kim was impressed with some of the glaze results I
showed her from MC6G. I told her that the glazes are formulated to
work with 90% of clay bodies especially Tuckers. In my ignorance I
have perhaps stumbled on a combo that doesn't work!
At the LPG studio we use a lot of Rosies and Mid Stone Smooth clay.
Bone is something that I have been using at home but is not in use at
the studio. If Bone is not a good idea as a liner for MSS clay with
Rosies then what would you suggest? I liked the soft colour of it
rather than the dead white of the Shiny White in use at the studio.
Perhaps I would be better off changing to a clay that is medium in
expansion? I wanted to use the same as the studio as I work there
sometimes too.
I see from this experience that there is much for me to learn! I will
reread your book (especially on expansion rates) as soon as I get it
back. I very much appreciate your help in sorting this out. I would
be very happy to send you the mug with the bottom out if this would
help.
Many thanks, Lowell
PS: The mugs that were footed have not shown a problem YET. Does this
make sense? Less area for stress?
By the way, I am loving my new ConeArt kiln.
On 19-Jan-06, at 12:53 AM, Ron Roy wrote:
> Hi Lowell,
>
> I just went through all the dilatometer charts of the Tuckers clay
> bodies
> and find that mid smooth stone is in a group of bodies with higher
> expansion rates than some of the others.
>
> For instace the Mid Cal 5 is in the lower expansion group - but
> the MCS is
> in the higher group - and we used both those clays in our book.
>
> The Bone Glaze has one of the lowest expansion rates of all our
> glazes.
>
> I agree with Michael Wendt - in that if you are combining a low
> expansion
> glaze with a clay body that is on the higher side you need to
> spread the
> stress out - not have it consentrated at any sharp corners.
>
> The type of cracking you are having is almost always due to clay/
> glaze fit
> problems - with the glazes having the lower expansion.
>
> I would select a glaze for the inside (liner glaze) that has a higher
> expansion rate. This means - in the case of our glazes - any other
> than the
> two high calcium glazes.
>
> If you had used the bone inside and out the stress would have been
> probably
> been equalized by the way - I suspect the Rosie's Red may be crazed
> and
> that will help weaken the pot.
>
> I'll be happy to help you work through this by the way - altering
> the form
> and keeping the inside glaze thinner may be all that is needed.
>
> RR
>
>
>> For Christmas I made 10 mugs for my family members. Unfortunately two
>> of the mugs had the bottom crack all around at the level of the floor
>> and immediately start to leak tea. This happened the first time
>> boiling water was poured into them. They were sitting on a cold fake-
>> granite countertop.
>>
>> All were made in Tuckers Mid Smooth Stone. I glazed them with MC6
>> Bone on inside and Rosie's Red on outside with a little bone brushed
>> on top inch of outside which then ran down the outside. Fired to a
>> soft 6 and slow cooled according to Coneart schedule. Gorgeous look
>> and a combo I have done on a lot of pots the last while.
>>
>> Only difference I can see with these two is that they were flat on
>> the bottom whereas I usually put a foot on everything I make. I
>> pushed the bottom of one right out and the thicknesses seem very
>> even....not thick/thin.
>>
>> Is this likely to be a case of thermal shock or is it something else.
>> It is embarrassing to have the bottom crack and fall out!
>>
>> Thanks for any help you can give me.
>>
>> Lowell Ann
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
>
| |
|
