Ian Macmillan on sun 17 feb 02
A question for the group:
I just tested the absorption of my fired stoneware. A small pot of
64.6 gm weighed only 64.7 gm after boiling and then soaking, making
the absorption of water 0.15%. This seems low. Does this mean I
have overfired this clay? Is there then greater risk of problems if
I refire it for some reason?
Thanks,
Ian
Snail Scott on mon 18 feb 02
At 01:52 PM 2/18/02 -0600, you wrote:
>Snail says that if it doesn't slump and doesn't bloat, it's probably fine.
>My experience is a little different. Overfired tests may look just fine,
>but may have lost a significant portion of their strength...
This is a good point, Wanda. It's not one that I worry
about much, since my sculptural work is quite thickly
built compared with most thrown pottery, and will only
sustain occasional handling in its normal usage. For
pottery in regular use, the issue of strength is more
important, and warrants greater consideration. Thank
you for pointing this out.
-Snail
Snail Scott on mon 18 feb 02
At 02:06 PM 2/17/02 -0500, you wrote:
>I just tested the absorption of my fired stoneware. A small pot of
>64.6 gm weighed only 64.7 gm after boiling and then soaking, making
>the absorption of water 0.15%. This seems low. Does this mean I
>have overfired this clay? Is there then greater risk of problems if
>I refire it for some reason?
Well, if it doesn't slump, and doesn't bloat, I'd say
it's fine, but I'm more likely to suspect a testing
error. That IS a really low number for anything but
porcelain.
You said you boiled it. For how long? You said you
soaked it afterward, so I assume you didn't just
pull the pot out of the hot water and weigh it.
(That's maybe the most common error - a hot test
piece will dry quickly enough to skew the results;
a cool piece will retain its absorbed moisture long
enough to be weighed. You apparently know that,
though.)
Maybe try a cool-water multi-day soak and compare
the result? Or try a different pot, or a different
scale? (Some digital scales can become erratic, and
acquire variable errors of much more than a gram.)
-Snail
>Thanks,
>Ian
>
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Wanda Holmes on mon 18 feb 02
Snail says that if it doesn't slump and doesn't bloat, it's probably fine.
My experience is a little different. Overfired tests may look just fine,
but may have lost a significant portion of their strength. I'd fire pieces
1 and 2 cones below and break all three to test comparative strength. I
fire at cone 6, so I fire individual tests at cones 4, 5, & 6 and test all
three for porosity and strength. At .15% porosity (and I have seen numbers
that low in stoneware), there is a fair chance that the piece is brittle
compared to a cone lower. Wanda
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of Snail Scott
Sent: Monday, February 18, 2002 1:11 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: stoneware water absorption - overfired?
At 02:06 PM 2/17/02 -0500, you wrote:
>I just tested the absorption of my fired stoneware. A small pot of
>64.6 gm weighed only 64.7 gm after boiling and then soaking, making
>the absorption of water 0.15%. This seems low. Does this mean I
>have overfired this clay? Is there then greater risk of problems if
>I refire it for some reason?
Well, if it doesn't slump, and doesn't bloat, I'd say
it's fine, but I'm more likely to suspect a testing
error. That IS a really low number for anything but
porcelain.
You said you boiled it. For how long? You said you
soaked it afterward, so I assume you didn't just
pull the pot out of the hot water and weigh it.
(That's maybe the most common error - a hot test
piece will dry quickly enough to skew the results;
a cool piece will retain its absorbed moisture long
enough to be weighed. You apparently know that,
though.)
Maybe try a cool-water multi-day soak and compare
the result? Or try a different pot, or a different
scale? (Some digital scales can become erratic, and
acquire variable errors of much more than a gram.)
-Snail
>Thanks,
>Ian
>
>___________________________________________________________________________
___
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
>
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Ron Roy on tue 26 feb 02
Hi Bruce,
I keep some high iron cone 6 clays in that range - I don't see it as a
problem - trouble is - if you are using Red Art - or some other material
that varies significantly you may wind up with even less and that could be
a problem with certain forms.
What you need to know is - it's the clay/glaze fit which is the most
important factor in ware strength. Get your glazes far enough below
crazing so thay never craze - not much lower and you will have the best
product for general use.
See Peter Songens article in Studio Potter - vol 28 #1 for the best info on
cassarol making - he comes down on the side of grog to add some flexabilty
- but in the end - the clay/glaze fit is crucial.
RR
>The body I am presently using
>has a very low absorption level.
>definitely under 1% probably 1/2%
>Cone six stoneware...with high iron content.
>When I clang my test bars together
>they sound like coins clinking.
>I thought this was pretty good.
>Have not done strength test as of yet.
>Can you tell me of the "risks"
>I maybe courting...
>This has been a thread that has me wondering
>thanks,
>Bruce
>ps. recieved my copy of your (Johns) book
>last night..thanks again.
>
>----- Original Message -----
>From: Ron Roy
>To:
>Sent: Saturday, February 23, 2002 10:14 AM
>Subject: Re: stoneware water absorption - overfired?
>
>
>> If it is porcelain then the absorption is OK - if it's white stone ware it
>> may still be OK - depending on length of firing and soak. If it's got iron
>> in it is risky - again depending on bisque firing, length of firing and
>> soak.
>>
>>
>> >I just tested the absorption of my fired stoneware. A small pot of
>> >64.6 gm weighed only 64.7 gm after boiling and then soaking, making
>> >the absorption of water 0.15%. This seems low. Does this mean I
>> >have overfired this clay? Is there then greater risk of problems if
>> >
>> >Thanks,
>> >Ian
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
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