Israel Amirav on thu 29 oct 98
Hello Clayarters,
Please help ! I had two of my costumers telling me that the new mugs I
sold them , cracked when they
filled them with hot water right after it boiled.
This happened on two different occasions,months apart,so the clay obviously
came from two different bags.
Also,only one of a few new mugs cracked while they were ALL getting the
same "hot water treatment".
So what is wrong ??? I sure checked the mugs before selling them ,and they
were perfectly o.k..
Any advise will be very much appreciated !!!
(I fire to cone 9 oxidation)
Thanks,
Naama in Israel
Ron Roy on fri 30 oct 98
Hello Naama,
Looks like you have a fit problem between the clay and glaze. The glaze -
having a lower expansion than the clay - does not contract enough on
cooling and sets up too much stress in your ware. The glaze winds up too
big for the clay - hot water is poured in - expands the inside glaze a
little more and the ware cracks.
There is a cure for this kind of thing - usually it means raising the
expansion/contraction of the glaze so it will contract more on cooling and
be a better match for the clay. If all of your glazes were cracking pots it
would perhaps be appropiate to change clay bodies.
I can usually change a glaze to cure this problem but I need the recipe,
the type of clay you are using and the cone you are firing to.
>Please help ! I had two of my costumers telling me that the new mugs I
>sold them , cracked when they
>
>filled them with hot water right after it boiled.
>
>This happened on two different occasions,months apart,so the clay obviously
>came from two different bags.
>
>Also,only one of a few new mugs cracked while they were ALL getting the
>same "hot water treatment".
>
>So what is wrong ??? I sure checked the mugs before selling them ,and they
>were perfectly o.k..
>
> Any advise will be very much appreciated !!!
>
>(I fire to cone 9 oxidation)
>
>Thanks,
>
>Naama in Israel
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Joy Holdread on fri 30 oct 98
In a message dated 10/29/98 6:57:46 AM US Mountain Standard Time,
amirav@canaan.co.il writes:
>
> Please help ! I had two of my costumers telling me that the new mugs I
> sold them , cracked when they
>
> filled them with hot water right after it boiled.
>
The key word here is boiled. Tell people to avoid extreme temp changes. If
they must pour boiling water into a mug recommend swishing warm to hot water
in the mugs first. Winter weather & cold houses make this problem even more
prevalent.
Joy in Tucson where you wouldn't really expect that to happen but it does.
Ray Carlton on fri 30 oct 98
MMMMMMMMMM!!
you need to check your packing procedures far more likely though is that
your customer is spinning you a line..cups crack when dropped they dont
always break....i have had mugs that have cracked after a good physical
shock and used them for months even years...i would suggest that you
replace the mugs as a gesture of goodwill and your customer will remain
ever thus and probably buy a lot more more work because they feel guilty
for lying to you about the cups they broke....
good luck in your life
At 08:43 29/10/98 EST, you wrote:
>----------------------------Original message----------------------------
> Hello Clayarters,
>
>Please help ! I had two of my costumers telling me that the new mugs I
>sold them , cracked when they
>
>filled them with hot water right after it boiled.
>
>This happened on two different occasions,months apart,so the clay obviously
>came from two different bags.
>
>Also,only one of a few new mugs cracked while they were ALL getting the
>same "hot water treatment".
>
>So what is wrong ??? I sure checked the mugs before selling them ,and they
>were perfectly o.k..
>
> Any advise will be very much appreciated !!!
>
>(I fire to cone 9 oxidation)
>
>Thanks,
>
>Naama in Israel
>
cheers Ray Carlton
McMahons Creek Victoria Australia
Kathi LeSueur on sat 31 oct 98
In a message dated 10/30/98 8:48:24 AM, you wrote:
>you need to check your packing procedures far more likely though is that
>your customer is spinning you a line..cups crack when dropped they dont
>always break...
I have to strongly disagree with this statement. In my experience customers
usually assume they did something wrong to cause the cup to crack. About a
year ago I traded mugs with a potter whose work I had admired. I brought it
home, made a pot of tea and poured some in the cup. The bottom cracked out of
it. The clay evidenced a very heavy body reduction with some black coring. It
was this I attributed to the problem.
However, this potter does not reduce. So black coring is unlikely.
But........... her post did indicate that she buys her clay premixed. I
suspect a change in the formula made the clay less able to absorb heat shock.
Perhaps it was not well mixed.
My suggestion would be to test mugs from each batch of new clay with boiling
water to make sure that they are not flawed. I would talk with my supplier and
other potters to see if anyone else has this problem.
I believe it is a clay problem. Not a customer problem.
Kathi LeSueur
Craig Martell on sat 31 oct 98
Hello Naama:
The problem could be cristobalite forming in the clay. This usually happens
when there isn't enough feldspar in the claybody to neutralize the effect of
cristobalite by incorporating the cristobalite into the body glass and
changing it to a fused silica which has a lower coefficient of expansion
than crystalline silica. When a claybody is short of feldspar, there is
usually enough cristobalite to cause cracking problems. This will be more
evident in oven use pots than mugs and teapots but there will still be some
problems. Cristobalite goes through a radical and fast volume change at
about 435 degrees F. At lower temperatures, such as around the boiling
point of water, there is still enough expansion from the crystalline form of
silica to cause cracking in some pieces.
If you mix your own clay, you may need to add more feldspar to stop this
from happening. If this is a clay that you purchase premixed, tell the
maker about the problem and they will hopefully give you some help. Clays
that are high in silica will contribute to this problem too.
You can also quick cool to below 2000 F to retard the development of
cristobalite. It develops faster at cone 5 and above. The longer one stays
at high temperature, more cristobalite will be formed.
best of luck, Craig Martell in Oregon USA
RedIron Studios on sat 31 oct 98
For the most part this just isn't going to work! Who is
going to be willing to swish thier mug. Shall we include
"swishing" instructions with our mugs? Does asking
customers to "swish" have a possible negative effect
on some people who may have bought a mug from me.
Lets call these people the "refuse to swishers" I'm willing
to bet the majority fall into the "I refuse to swish" category.
So I guess we must make glazes that fit our bodies well
so they can handle this thermal shock.
The glaze gurus on the list probably will have something to
offer. I believe the solution to the problem if one wants to get
really serious about it. Is to develop a glaze for your mug that
has the characteristics you wish (within reason) whilst
maintaining a proper fit. I believe the glaze that you design
will be ideally under a slight compression at room temperature.
Once you have realized a glaze of this type and have fitted it
to your clay body. I believe at that point when you pour hot water
into these mugs they are relieved.
Another factor to look into is if they are fired for a long period in
an excessive amount of cristobolite can form therby making the
mug much more susceptable to thermal shock.
Anyways the Guru can help you (us ;) more I just wanted to
help get some dialouge going on this subject.
Cheers
>----------------------------Original message----------------------------
>In a message dated 10/29/98 6:57:46 AM US Mountain Standard Time,
>amirav@canaan.co.il writes:
>
>>
>> Please help ! I had two of my costumers telling me that the new mugs I
>> sold them , cracked when they
>>
>> filled them with hot water right after it boiled.
>>
>The key word here is boiled. Tell people to avoid extreme temp changes. If
>they must pour boiling water into a mug recommend swishing warm to hot water
>in the mugs first. Winter weather & cold houses make this problem even more
>prevalent.
>Joy in Tucson where you wouldn't really expect that to happen but it does.
>
Bonnie Staffel on sat 31 oct 98
----------------------------Original message----------------------------
> Hello Clayarters,
>
> Please help ! I had two of my costumers telling me that the new mugs I
> sold them , cracked when they
>
> filled them with hot water right after it boiled
> This happened on two different occasions,months apart,so the clay obviously
> came from two different bags.
>
> Also,only one of a few new mugs cracked while they were ALL getting the
> same "hot water treatment".
>
> So what is wrong ??? I sure checked the mugs before selling them ,and they
> were perfectly o.k..
>
> Any advise will be very much appreciated !!!
>
> (I fire to cone 9 oxidation)
>
> Thanks,
>
> Naama in Israel
>
>Greetings from the Legend of Charlevoix Hollow:
):o))
This happened to me a number of years ago. My
analysis of this problem is the result of throwing
each mug individually. If they were cast, this
problem may not occur. As you may or may
not know, clay expands upon heating, contracts if
cold. So if you happen to make a mug that is
a little heavier at the bottom than the top, or
vice versa the stress of expanding may result in a verticle crack occurring when
comes in contact with the mug. I use an ovenware
clay for all my pots which is made to withstand
shock so have not had this problem since changing.
Check into the thermal expansion qualities of the
clay you are using.
Bonnie Staffel
bstaffel@freeway.net
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Grimmer on sun 1 nov 98
Hi,
Sounds like at least a glaze fit problem, and quite likely a clay body
problem to boot. I would guess that the expansion of the glaze is too low
for the clay body being used. The glaze can actually pull the pot apart! If
the clay body is insufficiently fluxed, it will not be as strong as a
balanced body, and will typically have too high an expansion for most
glazes to fit well. It's a real one-two blow to the strength of your ware.
If you post the claybody, glaze, and firing conditions, there are lots of
people on the list who could offer good advice.
steve grimmer
marion illinois
On a soap box, here. A well fitted clay/glaze combination should be able to
withstand boiling water being poured into the pot straight from the freezer.
It takes a fair amount of fine tuning, but it's considerably easier than
replacing customer's pots. It can be done, and is a reasonable goal for
anyone who is a functional potter.
ok. I'll stop now.
>----------------------------Original message----------------------------
>In a message dated 10/29/98 6:57:46 AM US Mountain Standard Time,
>amirav@canaan.co.il writes:
>
>>
>> Please help ! I had two of my costumers telling me that the new mugs I
>> sold them , cracked when they
>>
>> filled them with hot water right after it boiled.
>>
>The key word here is boiled. Tell people to avoid extreme temp changes.
If
>they must pour boiling water into a mug recommend swishing warm to hot
water
>in the mugs first. Winter weather & cold houses make this problem even
more
>prevalent.
>Joy in Tucson where you wouldn't really expect that to happen but it does.
amy parker on thu 5 nov 98
My grandma taught me to ALWAYS put a silver spoon into ANY ceramic container
before I poured boiling water into it, be it the finest bone china or some
funky clunky pitcher. Of course I personally am using stainless steel (WG)
instead of sterling, since Grandma did not leave the sterling to me, but
I have never ever cracked anything.
The thermal shock of pouring down one side, or splashing up one side, can
certainly be enough to caused sudden expansion in one area, shattering the
object.
Unfortunately, not every one had a clever Grandma, but all consumers should
learn this trick - never mind the liability issue, it is the ultimate
responsibility of the consumer to use things wisely. Perhaps we should
educate our buyers? I am thinking of including this tip in each of my
iced tea pitchers. No one needs to have the hot foot from an avoidable
accident!
amy parker Lithonia, GA
amyp@sd-software.com
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