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## fired clay-strength and hardness are not the same thing (pt. 1=

### Pete Pinnell on sun 14 mar 10

)

(This was kicked back for being too long- here's the first half).

I see this topic has come up again (though I'm sorry- I haven't read =3D20
Clayart in a few days so I'm not aware of what's already been said, =3D20
aside from the quotes from Lee and Ron below). It is an interesting =3D20
question, as there are different ways to measure strength and each =3D20
reflects a different way that a pot can =3D93fail=3D94. In this case, we =
=3D
bars of clay that were fired and broken, in order to measure modulus =3D20
of rupture (aka MOR or flexural strength), the amount of bending force =3D2=
0=3D

it takes to break each body. The methodology is simple, and any potter =3D2=
0=3D

would have the skill to do it (it takes hardly any equipment at all). =3D20=
=3D

The most important thing is that you are careful and consistent with =3D20
all steps in the process.

First, clay samples are mixed very well, aged a couple of days, then =3D20
wedged well. A ball of wedged clay is placed in a thin-wall plastic =3D20
bag and put in the extruder (just a ordinary hand extruder bolted to =3D20
the wall). The extruder has a plastic hole die- this can be whatever =3D20
size you want (within reason, of course), but I=3D92ve found that 5/16=3D94=
=3D
is =3D20
a good size. The plastic bag is =3D93poked=3D94 with a needle tool through =
=3D
the =3D20
extrusion hole so that the plastic will tear and lay down around the =3D20
edge of the hole. The gives the extrusions a very smooth, glassy =3D20
surface- you want to avoid having any tears in the surface of the clay =3D2=
0=3D

as this will effect the fired strength. The plastic bag is also handy =3D20=
=3D

as the extruder won=3D92t need to be washed between different clays.

The bars are cut 8- 9 inches in length and placed on a smooth board to =3D2=
0=3D

slowly dry (drywall with the edges taped works well). In locations =3D20
that are drafty the bars may need to be gently =3D93rolled=3D94 (turned =3D
over) =3D20
from time-to-time to avoid warping. The bars do need to be relatively =3D20=
=3D

straight for the breaking test to be accurate.

All the bars are bisqued (even those that are to be left unglazed) so =3D20=
=3D

that there won=3D92t be that additional variable between glazed and =3D20
unglazed bars. Bars to be glazed are dipped in glaze, all but about =3D20
1/2=3D94 to 1=3D94 at one end of the bar. Soft bricks are drilled with hole=
s =3D
=3D20
so that the glazed bars can stand on end, with the unglazed bar ends =3D20
stuck into holes in the brick. Then all the bars (glazed and unglazed) =3D2=
0=3D

are loaded in the kiln and fired in the normal way. OBVIOUSLY it is =3D20
important that firings be accurate and consistent- underfiring or =3D20
overfiring samples can greatly effect results.

We break the fired bars on (what I jokingly call) The Neolithic =3D20
Instron (Instron, for those of you who don=3D92t know, makes high-tech =3D2=
0
laboratory testing machines). I cut two pieces of angle iron about =3D20
12=3D94-14=3D94 long (I used 1 1/2=3D94 x 1 1/2=3D94 x 1/8=3D94 because we =
some left =3D20
from kiln building, though any small angle would do- you could even =3D20
cut up an old bed frame). These are clamped or screwed down to the =3D20
edge of a strong table so that they extend out about 6=3D94 from the edge =
=3D20=3D

of the table and the upright edges of the two angles are exactly 6=3D94 =3D=
20=3D

apart. The amount that they extend out from the table isn=3D92t critical, =
=3D
=3D20
but the distance between them IS critical, since this is the =3D93span=3D94=
=3D20=3D

measurement for our calculations.

Take a 5-gallon plastic bucket and remove (break off) the plastic =3D20
handle covering the wire bale. The top (middle) of the wire bale =3D20
should be bent slightly to make something approximating a slightly-=3D20
rounded right angle. Take the fired bar that is to be tested and, =3D20
using an indelible marker, place a line around the center point of the =3D2=
0=3D

bar (equal distance from either end). The bale is placed over a fired =3D20=
=3D

bar (on the mark), then the bar is placed across the angle iron so =3D20
that the bucket is suspended by its (wire bale) handle. If you=3D92re =3D20
following this, the bar is now holding up the bucket. Place a small =3D20
platform of bricks or concrete blocks below the bucket so that when =3D20
the bar breaks the bucket will not have far to fall.

Slowly scoop DRY sand into the bucket until the bar breaks. The bucket =3D2=
0=3D

is then weighed and this information (along with the diameter of the =3D20
bar and the length of the span between the supporting angle irons) are =3D2=
0=3D

plugged into the equation, which will produce the modulus of rupture. =3D20=
=3D

Because our bars are produced by hand (and some may contain mechanical =3D2=
0=3D

flaws), there are several controls that we need to include in the =3D20
process to ensure valid results. First, we never rely on one test bar- =3D2=
0=3D

generally 6-12 are broken for each test, and the more that are broken =3D20=
=3D

the clearer the picture becomes. Second, the break should occur right =3D20=
=3D

on the mark that signifies the location of the wire handle (the point =3D20=
=3D

of force). If the break occurs elsewhere on the bar, that usually =3D20
signifies a bubble in the body, or a flaw on the surface of the bar. =3D20
These anomalous numbers are thrown out- only the numbers from bars =3D20
that break right on the mark are included in the calculations.

When I do this with a class, we attach at least one breaking station =3D20
to each table in the room, so there are 6 or 8 teams of students =3D20
breaking bars at a time. I like to do things like this in teams so =3D20
that students can check each other through every step of the process, =3D20=
=3D

lessening the variables in quality.

Anyone who has studied the process of measuring MOR will tell you that =3D2=
0=3D

using a machine that applies force on two points (=3D934 point loading=3D94=
- =3D
=3D20
the other two =3D93points=3D94 are the edges of the supporting angle irons)=
=3D20=3D

will produce a more accurate result than a single point (=3D933 point =3D20
loading=3D94) (a slightly different equation is used to figure MOR with =3D=
20=3D

this kind of test). I=3D92ve tried to replicate this with the =3D93Neolithi=
c =3D
=3D20
Instron=3D94 by using a small plywood gizmo, but in the few tests that I =
=3D20=3D

did there was no discernable difference in accuracy. I figure that =3D20
we=3D92re already loosey-goosey enough with our process that it doesn=3D92t=
=3D20=3D

really matter.

(end part 1)

Pete

Peter Pinnell
Professor of Art, Department Grad Chair
120 Richards Hall
Lincoln, NE 68588-0114
(402) 472-4429

> Ron,
>
> Best to go to the source. Please ask Pete. I have Cc'd him in
> this message.
>
> On Sat, Mar 13, 2010 at 2:27 PM, Ron Roy wrote:
>> Hi David,
>>
>> Earthenware clay being stronger than stoneware or porcelain sounds
>> fishy to me - all the literature says otherwise.
>>
>> What test was used?
>>
>> Is there any fired data on the clays tested?
>>
>> Were there any tests done to see if the results were wrong - a usual
>> procedure when you find surprising results.
>>
>> I know they were not tested for chip-ability - that would have been
>> another story for sure.
>>
>> RR

### David Finkelnburg on mon 15 mar 10

)

Pete,
Thanks for including the URL in your post with the pictures of differen=
t
testing machines. Good visual!
Your test procedure is excellent! The NI is a great tool!
Some points to consider with your tests.
Have you tried extruding rods (coils) instead of bars? I know they wan=
t
to roll, but your 3 point bend test, is more accurate for round ceramic rod=
s
than for flat bars. Imperfect orientation causes greater scatter in the
results so unless both ends of the bar are parallel transverse to the long
axis of the bar you get somewhat false readings. When Brian Pinto did
his MS thesis at Alfred, "Effect of Filler Particle Size on *Porcelain
Strength,*" he did all the work with extruded rods because they gave more
accurate results than bars.
Second, a single measure of fired clay dimensions, for glazed or
unglazed bars or rods that are extruded, or better, the average of a sample
of these, should be sufficient for the accuracy you are reporting. I think
you should not apologize for that logical shortcut. Given the scatter in
the breaking strength due to how straight the rods are, etc, I'm not sure
measuring every sample would be productive effort.
Dave Finkelnburg
http://www.mattanddavesclays.com

### Pete Pinnell on mon 15 mar 10

)

Dave,

Thanks for the positive feedback. I didn't make myself very clear by
the text- the tests "bars" are round, extruded rods, for just the
reasons you gave. I don't know why, but I've always called them bars.
I'll change my nomenclature next time- rods makes more sense.

As for the difference between actual measurements and the use of a
"standard", my experience with several classes is that there isn't a
big difference, and it does tend to fall within the normal variation
that occurs with a group of samples. What quickly becomes the
important number when a large group is busy breaking bars (there- I
did it again- "rods") is the amount of sand (in pounds) that it takes
in the bucket before the bar will break. Standing in the middle of the
class, you'll hear someone call out "hey- we got a 12 pounder over
here", to which someone else will reply "That's nothing- we had a 15
pounder a few minutes ago". While we do plug the data into the formula
and convert it, the basic information in pounds (or even the strong
visual of one clay breaking with just a few inches of sand, while
another might require close to a full bucket) is enough to make the
point.

I only teach a glaze class once every three years here, and there is a
LOT of stuff to cover in the course of the semester. I sometimes cut
corners on a project like this in the interest of moving on to the
next topic. That's not supposed to be an excuse, just an explanation.
Since I do (now) plan to publish the data from the class this fall,
I'll do it the "official" way, and also figure some with the shortcut,
so everyone can see the difference. I'm also going to look around here
at UNL and see if I can find someone in Engineering who would be
willing to break a few samples officially so we can compare the results.

I'm off to my Tai Chi class- (where I won't be thinking about ceramics).

Pete

Peter Pinnell
Professor of Art, Department Grad Chair
120 Richards Hall
Lincoln, NE 68588-0114
(402) 472-4429

On Mar 15, 2010, at 5:26 PM, David Finkelnburg wrote:

> Pete,
> Thanks for including the URL in your post with the pictures of
> different testing machines. Good visual!
> Your test procedure is excellent! The NI is a great tool!
> Some points to consider with your tests.
> Have you tried extruding rods (coils) instead of bars? I know
> they want to roll, but your 3 point bend test, is more accurate for
> round ceramic rods than for flat bars. Imperfect orientation causes
> greater scatter in the results so unless both ends of the bar are
> parallel transverse to the long axis of the bar you get somewhat
> false readings. When Brian Pinto did his MS thesis at Alfred,
> "Effect of Filler Particle Size on Porcelain Strength," he did all
> the work with extruded rods because they gave more accurate results
> than bars.
> Second, a single measure of fired clay dimensions, for glazed or
> unglazed bars or rods that are extruded, or better, the average of a
> sample of these, should be sufficient for the accuracy you are
> reporting. I think you should not apologize for that logical
> shortcut. Given the scatter in the breaking strength due to how
> straight the rods are, etc, I'm not sure measuring every sample
> would be productive effort.
> Dave Finkelnburg
> http://www.mattanddavesclays.com
>

### Ron Roy on wed 17 mar 10

)

Hi Pete,

Thanks for all that - well done!

How did you find the fired absorption for the different clays?

RR

### Pete Pinnell on thu 18 mar 10

)

> How did you find the fired absorption for the different clays?

We do so in the usual way (though I'd love to hear if someone has an =3D
alternative). A fired sample of the body (usually a shrinkage bar) is =3D
weighed on a gram scale, then it's submerged in water and soaked for 24 =3D
hours. It's then removed, all water is dried from the surface and it's =3D
weighed again. The difference is figured in weight percentage. It's been =
=3D
so long since I read the ASTM bulletin on this that I should take =3D
another look at the official process. That document is called ASTM C373 =3D
- 88(2006) Standard Test Method for Water Absorption, Bulk Density, =3D
Apparent Porosity, and Apparent Specific Gravity of Fired Whiteware =3D
Products

You can find it here:

http://www.astm.org/Standards/C373.htm

Suddenly I'm thinking that I need to take another walk over to the =3D
engineering library and look up some of the other industrial standards.

Now, I have a question: what do you consider good numbers for absorption =
=3D
for pottery bodies? There obviously has to be a sliding scale, depending =
=3D
upon the maturing temperature of the body and the kind of clay body it =3D
is. High-fire porcelain would need to be close to zero, but beyond that, =
=3D
what do you use as a standard? (BTW, I'm saying "you" in the general =3D
sense to mean Ron, and anyone else who has worked with this notion). I =3D
have yet to see figures published that are anything more than vague =3D
guidelines, and I've always been able to find valid samples that fall =3D
outside those standards, so I'm just curious.

I've tended, instead, to use glaze fit as an indicator of moisture =3D
expansion, working under the theory that some degree of absorption is ok =
=3D
if it's not causing the clay to expand and create delayed crazing. In =3D
general, it has seemed like some bodies (and temperature ranges) can =3D
have a fairly high absorption and the glazes will not suffer delayed =3D
crazing, while others can tolerate almost no absorption at all without =3D
issues. Have any of you tested along these line?

Thanks,

Pete
=3D20
Peter Pinnell
Professor of Art, Department Grad Chair
120 Richards Hall
Lincoln, NE 68588-0114
(402) 472-4429

On Mar 17, 2010, at 8:02 PM, ronroy@ca.inter.net wrote:

> Hi Pete,
>=3D20
> Thanks for all that - well done!
>=3D20
> How did you find the fired absorption for the different clays?
>=3D20
> RR
>=3D20

### Lee Love on fri 19 mar 10

)

Ron, I don't know what to say. I am drinking Harp beer right now,
out of an unglazed terra sigged mug, made with Continental Clay Smooth
Red Earthenware, and Red Art Terra Sig. fired to a hard cone 03.
I assure you it isn't leaking. I use this ware all the time. I
am guessing many cone 6 clays would not do as well unglazed.

Maybe you should call up Continental and ask them their secret?

--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Ron Roy on fri 19 mar 10

)

Hi Pete,

I have always used the test for absorption outlined in clay and glazes
- boil for 2 hours - cool, and dry and reweigh to get % absorption.

Not sure how that compares with your method but I will do a comparison
after NCECA if you are interested.

So now I have about 30 years of data on fired raw clays and bodies at
cone 04, 6, 8 and 10 ox and reduction.

All the earthenware clays leak at cone 04 - the rest of the bodies
hold water and are continually tested and adjusted - the stoneware
clays need between 1 and 3% absorption to hold water and the
porcelains close to 0%

I did test Colemans porcelain at cone 6 and 6.5 and - even though it
had an absorption of 0.1 to 0.6 it still leaked.

I am interested in the absorption of the clays you tested - do you
have that data at hand?

Do you want any of those clays dilatometered - do you still have the bars?

RR

Quoting Pete Pinnell :

>> How did you find the fired absorption for the different clays?
>
>
> We do so in the usual way (though I'd love to hear if someone has an
> alternative). A fired sample of the body (usually a shrinkage bar)
> is weighed on a gram scale, then it's submerged in water and soaked
> for 24 hours. It's then removed, all water is dried from the surface
> and it's weighed again. The difference is figured in weight
> percentage. It's been so long since I read the ASTM bulletin on this
> that I should take another look at the official process. That
> document is called ASTM C373 - 88(2006) Standard Test Method for
> Water Absorption, Bulk Density, Apparent Porosity, and Apparent
> Specific Gravity of Fired Whiteware Products
>
> You can find it here:
>
> http://www.astm.org/Standards/C373.htm
>
> Suddenly I'm thinking that I need to take another walk over to the
> engineering library and look up some of the other industrial
> standards.
>
> Now, I have a question: what do you consider good numbers for
> absorption for pottery bodies? There obviously has to be a sliding
> scale, depending upon the maturing temperature of the body and the
> kind of clay body it is. High-fire porcelain would need to be close
> to zero, but beyond that, what do you use as a standard? (BTW, I'm
> saying "you" in the general sense to mean Ron, and anyone else who
> has worked with this notion). I have yet to see figures published
> that are anything more than vague guidelines, and I've always been
> able to find valid samples that fall outside those standards, so I'm
> just curious.
>
> I've tended, instead, to use glaze fit as an indicator of moisture
> expansion, working under the theory that some degree of absorption
> is ok if it's not causing the clay to expand and create delayed
> crazing. In general, it has seemed like some bodies (and temperature
> ranges) can have a fairly high absorption and the glazes will not
> suffer delayed crazing, while others can tolerate almost no
> absorption at all without issues. Have any of you tested along these
> line?
>
> Thanks,
>
> Pete

### Lee Love on fri 19 mar 10

)

On Fri, Mar 19, 2010 at 4:45 PM, Ron Roy wrote:

> All the earthenware clays leak at cone 04 -

You probably have to fire a little higher. I do. And have no leakage.

--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Lee Love on fri 19 mar 10

)

On Fri, Mar 19, 2010 at 6:01 PM, Lee Love wrote:
> On Fri, Mar 19, 2010 at 4:45 PM, Ron Roy wrote:
>
>> All the earthenware clays leak at cone 04 -
>
> =3DA0You probably have to fire a little higher. =3DA0 =3DA0I do. =3DA0And=
have no=3D
leakage.

Pete explains this in the article. But it probably depends on your clay t=
=3D
oo.

--=3D20
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Ron Roy on fri 19 mar 10

)

Hi Lee,

Most have 7 or 8 % absorbency - have to fire more than a little higher
to stop water.

Firing earthenware to vitrification is tricky. I have years of data on
red art that shows a big variation from shipment to shipment. That
means you will have to test continuously to make sure it's vitrified
enough - you won't have to test if it's starts melting - it will be
obvious.

The big problem is - you have to add so much flux at the lower
temperatures that once the melting starts it goes very quickly - good
luck trying to get water proof clay at low temperatures - many have
tried but the odds are against you.

RR

Quoting Lee Love :

> On Fri, Mar 19, 2010 at 4:45 PM, Ron Roy wrote:
>
>> All the earthenware clays leak at cone 04 -
>
> You probably have to fire a little higher. I do. And have no leakag=
e.

### David Beumee on sat 20 mar 10

)

Lee,
Your cone 03 mug has a terra sig finish on it. Surely that makes a
difference as to whether the piece leaks or not.

David Beumee
Lafayette, CO

On Fri, Mar 19, 2010 at 8:58 PM, Lee Love wrote:

> Ron, I don't know what to say. I am drinking Harp beer right now,
> out of an unglazed terra sigged mug, made with Continental Clay Smooth
> Red Earthenware, and Red Art Terra Sig. fired to a hard cone 03.
> I assure you it isn't leaking. I use this ware all the time. I
> am guessing many cone 6 clays would not do as well unglazed.
>
> Maybe you should call up Continental and ask them their secret?
>
>
> --
> Lee, a Mashiko potter in Minneapolis
> http://mashikopots.blogspot.com/
>
> =3D93Observe the wonders as they occur around you. Don't claim them. Feel
> the artistry moving through and be silent.=3D94 --Rumi
>

### Lee Love on sat 20 mar 10

)

On Sat, Mar 20, 2010 at 6:49 AM, David Beumee wrote=
=3D
:
> Lee,
> =3DA0Your cone 03 mug has a terra sig finish on it. Surely that makes a
> difference as to whether the piece leaks or not.

Yes, especially on a non-glazed piece. It also give some hints
about sigging feet on other bodies.

My cone 03 glazes, majolica and clear, are equally funcitonal.
I use those every day too with no leaking.

Sometimes, we are satisfied by our testing results quickly, so we
can dismiss what we want to dismiss, rather than seeing there is
something in our testing procedure that effects the outcome.

It is simply not so difficult to make functional earthenware.
I'm reading :"Raised in Clay" right now and many of those southen
potters made earthenware pots for cookware, along with their stoneware
pots.

--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Ron Roy on sat 20 mar 10

)

You seem to be missing the point Lee,

You call up continental and ask them to send you their records of =3D20
absorption for their different batches of clay. If they are testing =3D20
that will confirm what I an saying.

Do you think Continental tests each batch of clay they make? If they =3D20
don't - how do they know what to do each time a different batch of red =3D2=
0
art arrives?

Drinking bear out of a pot and not having the beer in you lap is not a =3D2=
0
a test for absorbency.

Leaving your container for 48 hours - filled with water - on paper =3D20
will tell you if it is absorbing water.

All this is easily confirmed by testing your clay over several batches =3D2=
0
- you will then have to adjust your firings to get the same absorbency =3D2=
0
for different batches of clay.

RR

Quoting Lee Love :

> Ron, I don't know what to say. I am drinking Harp beer right now,
> out of an unglazed terra sigged mug, made with Continental Clay Smooth
> Red Earthenware, and Red Art Terra Sig. fired to a hard cone 03.
> I assure you it isn't leaking. I use this ware all the time. I
> am guessing many cone 6 clays would not do as well unglazed.
>
> Maybe you should call up Continental and ask them their secret?
>
>
> --
> Lee, a Mashiko potter in Minneapolis
> http://mashikopots.blogspot.com/
>
> =3DE2=3D80=3D9CObserve the wonders as they occur around you. Don't claim =
them. F=3D
eel
> the artistry moving through and be silent.=3DE2=3D80=3D9D --Rumi
>

### Ron Roy on sat 20 mar 10

)

Hi David,

Terra sig does not water proof clay - the clay has to be vitrified =3D20
enough to not leak or absorb water.

You coming to NCECA?

RR

Quoting David Beumee :

> Lee,
> Your cone 03 mug has a terra sig finish on it. Surely that makes a
> difference as to whether the piece leaks or not.
>
> David Beumee
> Lafayette, CO
>
>
>
>
>
>
>
>
>
>
>
>
> On Fri, Mar 19, 2010 at 8:58 PM, Lee Love wrote:
>
>> Ron, I don't know what to say. I am drinking Harp beer right now,
>> out of an unglazed terra sigged mug, made with Continental Clay Smooth
>> Red Earthenware, and Red Art Terra Sig. fired to a hard cone 03.
>> I assure you it isn't leaking. I use this ware all the time. I
>> am guessing many cone 6 clays would not do as well unglazed.
>>
>> Maybe you should call up Continental and ask them their secret?
>>
>>
>> --
>> Lee, a Mashiko potter in Minneapolis
>> http://mashikopots.blogspot.com/
>>
>> =3DE2=3D80=3D9CObserve the wonders as they occur around you. Don't claim=
them. =3D
Feel
>> the artistry moving through and be silent.=3DE2=3D80=3D9D --Rumi
>>
>

### Lee Love on sat 20 mar 10

)

A friend doing outside comminssioned fountains with unglaze low fire
red from Continental Clay was told to fire it to cone 1 to resist the
elements.

The problem folks have with old earthenware, is that it was
underfired. Most of our non-contemporary ceramics in our kitchen
are earthenware. They are very servicable and wear well. I don't
put them in the dishwasher (I have one but would rather hand clean
pots) and I use contemporary ceramics and glass in the microwave.
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Lee Love on sat 20 mar 10

)

On Sat, Mar 20, 2010 at 2:30 PM, Ron Roy wrote:
> Hi David,
>
> Terra sig does not water proof clay - the clay has to be vitrified enough=
=3D
to
> not leak or absorb water.
>

It makes for swell ovenware. Terra sigged pie plates are
non-stick. The original teflon.

--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Marcia Selsor on sat 20 mar 10

)

The Italian red terra cotta exterior architectural ceramics in Bologna =3D
(cold climate) date back centuries.
Marcia Selsor
http://www.marciaselsor.com

On Mar 20, 2010, at 3:44 PM, Lee Love wrote:

> A friend doing outside comminssioned fountains with unglaze low fire
> red from Continental Clay was told to fire it to cone 1 to resist the
> elements.
>=3D20
> The problem folks have with old earthenware, is that it was
> underfired. Most of our non-contemporary ceramics in our kitchen
> are earthenware. They are very servicable and wear well. I don't
> put them in the dishwasher (I have one but would rather hand clean
> pots) and I use contemporary ceramics and glass in the microwave.
> --
> Lee, a Mashiko potter in Minneapolis
> http://mashikopots.blogspot.com/
>=3D20
> =3D93Observe the wonders as they occur around you. Don't claim them. =3D
Feel
> the artistry moving through and be silent.=3D94 --Rumi
>=3D20

### Lee Love on sun 21 mar 10

)

On Sun, Mar 21, 2010 at 4:48 PM, Pete Pinnell wrote:

> it. When I hear someone dismiss earthenware because of its porosity,
> it brings to mind the guy who said that he didn't like sashimi because
> it tasted too much like fish.

Actually, _fresh_ sashimi doesn't taste like fish. ;^)

Shino Mogusa clay has a lot of alumina in it. It is
porous at cone 13. I'd often fire it to cone 13 two or three times.

The tea masters liked this body because when the bamboo whisk
is worked in it, the sound that is made is very similar to a Raku
bowl. I've been experimenting with a raw Ohio fire clay and
Helmers. They are promising.

--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Pete Pinnell on sun 21 mar 10

)

> All the earthenware clays leak at cone 04 - the rest of the bodies
> hold water and are continually tested and adjusted - the stoneware
> clays need between 1 and 3% absorption to hold water and the
> porcelains close to 0%

Hi Ron,

than you intended, but this sounds as though you're dismissing
earthenware due to its porosity. As someone who (probably unwisely)
works in earthenware, stoneware and porcelain, I've found that I love
each of them for different reasons. In the case of earthenware, I love
it because of it's porosity, rather than (as some assume) in spite of
it. When I hear someone dismiss earthenware because of its porosity,
it brings to mind the guy who said that he didn't like sashimi because
it tasted too much like fish.

At the risk of repeating something we all know, the beauty of of
utilitarian pottery isn't limited to its visual aspects alone- it also
includes both its tactile sense as well as its experiential qualities.
As such, a soft, light, porous earthenware vessel can be really
wonderful to use. On a cold day, a thick earthenware mug with a slick,
wet interior glaze can seem so perfect for a hot drink that everything
else seems second rate. The porosity keeps the drink warm forever. A
terra sigillata exterior on such a vessel can have the warm, soft
texture of skin, and the bright orange color of the clay is as
psychologically warming as the drink is.

Conversely, an earthenware tumbler can be the perfect vessel for a
glass of iced tea on a hot, muggy summer day. The porosity keeps the
tea cold, shelters your hand (some) from the cold surface, and absorbs
the condensation from the air. The bright colors of earthenware glazes
(especially majolica) provide the perfect counterpoint.

Any serious cook will tell you than no material is better for making
baked beans, pot roast and vegetables, or a baked stew than an
earthenware covered casserole. Does it stain? Yeah, sure- but so what?
My cotton jeans fade with age and use, but that doesn't make them
second-rate compared with their polyester equivalent. Like many
people, I like the way that natural materials change with age and use,
and what looks like a stain to some people appears to be a patina to
others.

So, does porosity render a ceramic object non-utilitarian? Of course
not. Does it demand that the user treat the object in a slightly
different manner? Yes, perhaps. It's better to not place earthenware
dishes in the dishwasher, but to hand-wash them. Along the same line,
I own any number of sweaters, and find them to be very utilitarian.
Still, I don't throw them in the washing machine or dryer, and I don't
subject them to Clorox bleach or Tide detergent.

The practical answer to earthenware is to design its use to take
advantage of its positive qualities while not losing sight of its
shortcomings- the same way a sensitive potter would approach any
ceramic material. Yes, a vessel designed to hold liquids should have a
durable, impervious glaze on its interior, but in an age when we have
dozens of commercial frits to choose from, this isn't that difficult a

I hope you don't take this as a rant- I'm saying all of this with a
smile and a wink.

Pete

Peter Pinnell
Professor of Art, Department Grad Chair
120 Richards Hall
Lincoln, NE 68588-0114
(402) 472-4429

> Date: Fri, 19 Mar 2010 18:45:59 -0400
> From: Ron Roy
> Subject: Re: Fired Clay-strength and hardness are not the same thing
> (pt. 1)
>
> Hi Pete,
>
> I have always used the test for absorption outlined in clay and glazes
> - boil for 2 hours - cool, and dry and reweigh to get % absorption.
>
> Not sure how that compares with your method but I will do a comparison
> after NCECA if you are interested.
>
> So now I have about 30 years of data on fired raw clays and bodies at
> cone 04, 6, 8 and 10 ox and reduction.
>
> All the earthenware clays leak at cone 04 - the rest of the bodies
> hold water and are continually tested and adjusted - the stoneware
> clays need between 1 and 3% absorption to hold water and the
> porcelains close to 0%
>
> I did test Colemans porcelain at cone 6 and 6.5 and - even though it
> had an absorption of 0.1 to 0.6 it still leaked.
>
> I am interested in the absorption of the clays you tested - do you
> have that data at hand?
>
> Do you want any of those clays dilatometered - do you still have the
> bars?
>
> RR

### Ron Roy on sun 21 mar 10

)

Hi Lee,

There is no secret Lee -

I have just been to the Continental Clay web site - they have no =3D20
absorbency figures listed for any clays. I did find a shrinkage chart =3D20
- they say the low fire red shrinks 11% at cone 04 and 15% at cone 4 - =3D2=
0
they say that clay is good from 04 to cone 4 - still think it's =3D20
vitrified enough at cone 03 to exclude water?

Here are some red art numbers - lab tested over twenty years. The =3D20
absorbency numbers are from clay from a top bag on a skid - 4 numbers =3D20
mean 4 skids were tested. I only used numbers from clay that was fired =3D2=
0
to cone 04 half down.

Oct 91.....9.3,9.3,9.0,9.5,9.3

Jan 94.....10.5,9.8,10.5,10.5,10.6

Jan 96 ......6.6,6.5,6.5,7.0,7.4

June 99 .....8.5,8.5,8.3,7.8,8.5

Jan 02 ...... 9.3,9.3,9.0,9.5,9.3

Aug 04 ......11.0,10.3,10.8,10.5,10.5

Oct 06 .....8.5,8.3,8.3,8.4

Oct 08 .....9.3,9.5,9.3,10.0,10.0

So - as you can see the biggest difference could be between 6.5 and =3D20
10.8 - over 4%. That is why you can't rely on enough vitrification =3D20
without reformulating when using red art.

I'm still interested in any absorbency figures from Continental if you =3D2=
0
can find them - care to send me 5 LB so I can test to see just how =3D20
much water is getting in at cone 03?

RR
Quoting Lee Love :

> Ron, I don't know what to say. I am drinking Harp beer right now,
> out of an unglazed terra sigged mug, made with Continental Clay Smooth
> Red Earthenware, and Red Art Terra Sig. fired to a hard cone 03.
> I assure you it isn't leaking. I use this ware all the time. I
> am guessing many cone 6 clays would not do as well unglazed.
>
> Maybe you should call up Continental and ask them their secret?
>
>
> --
> Lee, a Mashiko potter in Minneapolis
> http://mashikopots.blogspot.com/
>
> =3DE2=3D80=3D9CObserve the wonders as they occur around you. Don't claim =
them. F=3D
eel
> the artistry moving through and be silent.=3DE2=3D80=3D9D --Rumi
>

### Lee Love on sun 21 mar 10

)

These were my first terra sigged redware pieces, after the pie plates:

http://redware.blogspot.com/

--=3D20
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Lee Love on mon 22 mar 10

)

On Mon, Mar 22, 2010 at 9:35 PM, Pete Pinnell wrote:
> Lee, that's a beautiful bowl, and I really like that foot. I confess to
> being lost by the term "Swat bowl". The only thing I can come up with is
> that it's from the Swat Valley, perhaps?

Yes, it is the part of Pakistan where Al Queda is hanging out. We
bought our first ones about 20 years ago. These are all at least
50 years old.

You can see the first ones here (the green on the bottom of the one
bowl is not as electric green as it appears):

039bc9=3D
b
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Pete Pinnell on mon 22 mar 10

)

Lee, that's a beautiful bowl, and I really like that foot. I confess =3D20
to being lost by the term "Swat bowl". The only thing I can come up =3D20
with is that it's from the Swat Valley, perhaps?

Just wondering.

Peter Pinnell
Professor of Art, Department Grad Chair
120 Richards Hall
Lincoln, NE 68588-0114
(402) 472-4429

On Mar 22, 2010, at 9:05 PM, Lee Love wrote:

> Ron,
> You inspired our first trip to Indigo since we got back to
> the States. Found a larger Swat bowl. An old one, maybe better
> than our original two. You can see it function with food, empty and
> the foot here:
>
4d3b8=3D
59c3
>
> These are pots that got me interested in earthenware/slipware.
>
> --
> Lee, a Mashiko potter in Minneapolis
> http://mashikopots.blogspot.com/
>
> =3D93Observe the wonders as they occur around you. Don't claim them. =3D
Feel
> the artistry moving through and be silent.=3D94 --Rumi
>

### Lee Love on mon 22 mar 10

)

Ron,
You inspired our first trip to Indigo since we got back to
the States. Found a larger Swat bowl. An old one, maybe better
than our original two. You can see it function with food, empty and
the foot here:

3b859c=3D
3

These are pots that got me interested in earthenware/slipware.

--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/

=3D93Observe the wonders as they occur around you. Don't claim them. Feel
the artistry moving through and be silent.=3D94 --Rumi

### Ron Roy on mon 22 mar 10

)

Hi Pete,

Never said it was not useful - I understand how useful it is - but -
those who buy it should be aware there can be problems. There have
been several reports of people burning their hands after reaching into
a microwave oven to get their heated up food - if a clay - any clay is
not vitrified to exclude water pots can get very hot.

There have been some reports of stuff oozing out of pots under the
same conditions.

I'm just asking that if your clay is not vitrified enough - you tell
those who might buy and use any pot that sucks up water that they may
get hot in a microwave oven.

So like you I see there are some short comings to earthenware clay -
all I really would like is for those who sell it for cooking and
eating off be honest and include some sort of warning - like not
microwave safe.

People can get turned off handmade pots if the pots they buy don't
work the way they think they should. This is a tough enough business
as it is without discouraging some people from buying pots.

You know that non vitrified clay will eventually craze any glaze you
put over it - that is part of the problem - No?

Ron

Quoting Pete Pinnell :

>> All the earthenware clays leak at cone 04 - the rest of the bodies
>> hold water and are continually tested and adjusted - the stoneware
>> clays need between 1 and 3% absorption to hold water and the
>> porcelains close to 0%
>
>
> Hi Ron,
>
> than you intended, but this sounds as though you're dismissing
> earthenware due to its porosity. As someone who (probably unwisely)
> works in earthenware, stoneware and porcelain, I've found that I love
> each of them for different reasons. In the case of earthenware, I love
> it because of it's porosity, rather than (as some assume) in spite of
> it. When I hear someone dismiss earthenware because of its porosity,
> it brings to mind the guy who said that he didn't like sashimi because
> it tasted too much like fish.
>
> At the risk of repeating something we all know, the beauty of of
> utilitarian pottery isn't limited to its visual aspects alone- it also
> includes both its tactile sense as well as its experiential qualities.
> As such, a soft, light, porous earthenware vessel can be really
> wonderful to use. On a cold day, a thick earthenware mug with a slick,
> wet interior glaze can seem so perfect for a hot drink that everything
> else seems second rate. The porosity keeps the drink warm forever. A
> terra sigillata exterior on such a vessel can have the warm, soft
> texture of skin, and the bright orange color of the clay is as
> psychologically warming as the drink is.
>
> Conversely, an earthenware tumbler can be the perfect vessel for a
> glass of iced tea on a hot, muggy summer day. The porosity keeps the
> tea cold, shelters your hand (some) from the cold surface, and absorbs
> the condensation from the air. The bright colors of earthenware glazes
> (especially majolica) provide the perfect counterpoint.
>
> Any serious cook will tell you than no material is better for making
> baked beans, pot roast and vegetables, or a baked stew than an
> earthenware covered casserole. Does it stain? Yeah, sure- but so what?
> My cotton jeans fade with age and use, but that doesn't make them
> second-rate compared with their polyester equivalent. Like many
> people, I like the way that natural materials change with age and use,
> and what looks like a stain to some people appears to be a patina to
> others.
>
> So, does porosity render a ceramic object non-utilitarian? Of course
> not. Does it demand that the user treat the object in a slightly
> different manner? Yes, perhaps. It's better to not place earthenware
> dishes in the dishwasher, but to hand-wash them. Along the same line,
> I own any number of sweaters, and find them to be very utilitarian.
> Still, I don't throw them in the washing machine or dryer, and I don't
> subject them to Clorox bleach or Tide detergent.
>
> The practical answer to earthenware is to design its use to take
> advantage of its positive qualities while not losing sight of its
> shortcomings- the same way a sensitive potter would approach any
> ceramic material. Yes, a vessel designed to hold liquids should have a
> durable, impervious glaze on its interior, but in an age when we have
> dozens of commercial frits to choose from, this isn't that difficult a
>
> I hope you don't take this as a rant- I'm saying all of this with a
> smile and a wink.
>
> Pete
>
> Peter Pinnell
> Professor of Art, Department Grad Chair
> 120 Richards Hall
> Lincoln, NE 68588-0114
> (402) 472-4429
>
>> Date: Fri, 19 Mar 2010 18:45:59 -0400
>> From: Ron Roy
>> Subject: Re: Fired Clay-strength and hardness are not the same thing
>> (pt. 1)
>>
>> Hi Pete,
>>
>> I have always used the test for absorption outlined in clay and glazes
>> - boil for 2 hours - cool, and dry and reweigh to get % absorption.
>>
>> Not sure how that compares with your method but I will do a comparison
>> after NCECA if you are interested.
>>
>> So now I have about 30 years of data on fired raw clays and bodies at
>> cone 04, 6, 8 and 10 ox and reduction.
>>
>> All the earthenware clays leak at cone 04 - the rest of the bodies
>> hold water and are continually tested and adjusted - the stoneware
>> clays need between 1 and 3% absorption to hold water and the
>> porcelains close to 0%
>>
>> I did test Colemans porcelain at cone 6 and 6.5 and - even though it
>> had an absorption of 0.1 to 0.6 it still leaked.
>>
>> I am interested in the absorption of the clays you tested - do you
>> have that data at hand?
>>
>> Do you want any of those clays dilatometered - do you still have the
>> bars?
>>
>> RR
>

### James Freeman on mon 22 mar 10

)

On Mon, Mar 22, 2010 at 5:10 PM, Ron Roy wrote:
>
> There have been some reports of stuff oozing out of pots under the
> same conditions.
>
> I'm just asking that if your clay is not vitrified enough - you tell
> those who might buy and use any pot that sucks up water that they may
> get hot in a microwave oven.
>
> So like you I see there are some short comings to earthenware clay -
> all I really would like is for those who sell it for cooking and
> eating off be honest and include some sort of warning - like not
> microwave safe.
>
> People can get turned off handmade pots if the pots they buy don't
> work the way they think they should.

Went to visit my brother and his family a few years ago. They showed
us the beautiful new vase they had purchased the day before at a
higher end art fair in Tempe. It sat atop their new oak entertainment
center, filled with a bouquet of flowers. When they picked it up to
give us a better look, they saw that the pot had leaked overnight,
destroying the top of their cabinet. It never occurred to them that a
ceramic pot could leak, and the potter who sold it to them never
mentioned a thing, and may not have even known herself. A very
expensive lesson.

...James

James Freeman

"All I say is by way of discourse, and nothing by way of advice. I
should not speak so boldly if it were my due to be believed."
-Michel de Montaigne

http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources