Pete Pinnell on sun 14 mar 10
)
(Here's the rest of my post).
I just did a quick Google and came up with a page that shows some =3D20
manufactured devices that would be used in official tests- these will =3D20=
=3D
provide a better illustration for all us visual types:
http://www.testresources.com/applications/bendflex.aspx
There are two ways to figure the diameter of the bar for our =3D20
calculations, and to my mind they each have some validity. The =3D20
official way is to use a micrometer and measure the exact diameter of =3D20=
=3D
each broken bar. As you might expect, glazed bars are thicker than =3D20
unglazed ones, and clays that shrink less in drying and firing are =3D20
thicker than those that shrink more. This takes more time when =3D20
calculating, but provides a scientifically more accurate result.
The process that I actually prefer for the class is to use a nominal =3D20
figure for all unglazed bars and not worry about the small difference =3D20=
=3D
from the actual diameters. For one thing, it=3D92s a lot easier for a big =
=3D20=3D
bunch of art students to do (and besides, I only own one micrometer, =3D20
which greatly slows down the process). While I can hear you wincing =3D20
when you read this, it does make sense from the perspective of the =3D20
potter. If I take a pound of porcelain and throw a cup and then take a =3D2=
0=3D
pound of earthenware and thrown an identical up, we all know that the =3D20=
=3D
porcelain cup will be smaller (and thinner) when it is finally fired. =3D20=
=3D
By using a nominal figure, I=3D92m acknowledging that reality. As for the =
=3D20=3D
difference in glaze thickness, that=3D92s more relevant, and to get the =3D=
20=3D
most accurate results from glazed bars it=3D92s best to measure the fired =
=3D20=3D
result. That said, it=3D92s not necessarily the thickness that makes the =
=3D20=3D
difference- a crazed glaze dramatically lowers strength, usually to =3D20
far below that of the unglazed bar, even though the glaze makes the =3D20
bar larger. Also, a flawless glaze provides no place for a crack to =3D20
develop, so the surface of the glaze in important. Likewise, a glaze =3D20
that=3D92s in a higher state of compression will also tend to raise the =3D=
20=3D
strength.
By the way, the purpose of doing this exercise isn=3D92t to crown any =3D20
particular material, but to find out what=3D92s actually happening in the =
=3D20=3D
real world of the pottery studio. Like Ron, I=3D92ve always read in =3D20
engineering texts that porcelain is strongest, stoneware next in line, =3D2=
0=3D
and earthenware brings up the rear when it comes to MOR. However, =3D20
those quoted tests always involve optimized clay bodies and processes =3D20=
=3D
measured in laboratories, and potters rarely (if ever) live up to =3D20
those standards. The reality shown by our tests is that there is an =3D20
enormous amount of overlap among clay bodies, and a poorly mixed, =3D20
poorly formulated, or underfired clay will be weaker, regardless of =3D20
the kind of clay it is. On the other hand, a well mixed and formulated =3D2=
0=3D
low or medium-fire clay that=3D92s fired to its full maturity will be more =
=3D
=3D20
than strong enough for pottery use, and its MOR can easily exceed =3D20
those of our typical high-fire bodies. Don=3D92t believe me? As you can =3D=
20=3D
see, it=3D92s not that difficult to measure- just run some tests!
I=3D92m afraid that I don=3D92t have the MOR formula here at home- its in m=
y =3D
=3D20
office at school. I can post that separately tomorrow, or perhaps =3D20
someone else has it handy and can post it sooner. The formulas are =3D20
included in =3D93ASTM C674 - 88(2006) Standard Test Methods for Flexural =
=3D20=3D
Properties of Ceramic Whiteware Materials=3D94, but they want me to pay =3D=
20=3D
$33.00 if I access it from home so I think that I=3D92ll just wait till =3D=
20=3D
tomorrow.
BTW, this is not a test for hardness (that=3D92s a different standard =3D20
altogether), and chip resistance is also a completely different animal =3D2=
0=3D
that=3D92s tested in another way.
I hope this helps- sorry about the length
Pete
PS: This seems interesting enough to many potters that I may write =3D20
some more detailed results for Clay Times this fall when I next teach =3D20=
=3D
a clay and glazes class here at UNL. If I do it for the magazine I =3D20
could include pictures as well as the specific results of the tests. =3D20
Would you want me to?
Peter Pinnell
Professor of Art, Department Grad Chair
120 Richards Hall
University of Nebraska
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 Beumee on sun 14 mar 10
)
Hi Pete,
Thanks very much for all the info, and yes, a write up in Clay Times of
your conclusions would be most welcome. Since there is so much interest in
cone 6, perhaps you and your class might want to consider an experiment
comparing relative strength between similar types of bodies at mid range an=
=3D
d
high fire temperatures. Please include your points about crazed versus
uncrazed glazes concerning fired strength, the need for well mixed and
formulated bodies, and firing to the point of full maturity for maximum
strength and glaze fit.
David Beumee
Lafayette, CO
On Sun, Mar 14, 2010 at 2:31 PM, Pete Pinnell wrote:
> (Here's the rest of my post).
>
> I just did a quick Google and came up with a page that shows some
> manufactured devices that would be used in official tests- these will
> provide a better illustration for all us visual types:
>
> http://www.testresources.com/applications/bendflex.aspx
>
> There are two ways to figure the diameter of the bar for our calculations=
=3D
,
> and to my mind they each have some validity. The official way is to use a
> micrometer and measure the exact diameter of each broken bar. As you migh=
=3D
t
> expect, glazed bars are thicker than unglazed ones, and clays that shrink
> less in drying and firing are thicker than those that shrink more. This
> takes more time when calculating, but provides a scientifically more
> accurate result.
>
> The process that I actually prefer for the class is to use a nominal figu=
=3D
re
> for all unglazed bars and not worry about the small difference from the
> actual diameters. For one thing, it=3D92s a lot easier for a big bunch of=
a=3D
rt
> students to do (and besides, I only own one micrometer, which greatly slo=
=3D
ws
> down the process). While I can hear you wincing when you read this, it do=
=3D
es
> make sense from the perspective of the potter. If I take a pound of
> porcelain and throw a cup and then take a pound of earthenware and thrown=
=3D
an
> identical up, we all know that the porcelain cup will be smaller (and
> thinner) when it is finally fired. By using a nominal figure, I=3D92m
> acknowledging that reality. As for the difference in glaze thickness, tha=
=3D
t=3D92s
> more relevant, and to get the most accurate results from glazed bars it=
=3D
=3D92s
> best to measure the fired result. That said, it=3D92s not necessarily the
> thickness that makes the difference- a crazed glaze dramatically lowers
> strength, usually to far below that of the unglazed bar, even though the
> glaze makes the bar larger. Also, a flawless glaze provides no place for =
=3D
a
> crack to develop, so the surface of the glaze in important. Likewise, a
> glaze that=3D92s in a higher state of compression will also tend to raise=
t=3D
he
> strength.
>
> By the way, the purpose of doing this exercise isn=3D92t to crown any
> particular material, but to find out what=3D92s actually happening in the=
r=3D
eal
> world of the pottery studio. Like Ron, I=3D92ve always read in engineerin=
g =3D
texts
> that porcelain is strongest, stoneware next in line, and earthenware brin=
=3D
gs
> up the rear when it comes to MOR. However, those quoted tests always invo=
=3D
lve
> optimized clay bodies and processes measured in laboratories, and potters
> rarely (if ever) live up to those standards. The reality shown by our tes=
=3D
ts
> is that there is an enormous amount of overlap among clay bodies, and a
> poorly mixed, poorly formulated, or underfired clay will be weaker,
> regardless of the kind of clay it is. On the other hand, a well mixed and
> formulated low or medium-fire clay that=3D92s fired to its full maturity =
wi=3D
ll be
> more than strong enough for pottery use, and its MOR can easily exceed th=
=3D
ose
> of our typical high-fire bodies. Don=3D92t believe me? As you can see, it=
=3D
=3D92s not
> that difficult to measure- just run some tests!
>
> I=3D92m afraid that I don=3D92t have the MOR formula here at home- its in=
my =3D
office
> at school. I can post that separately tomorrow, or perhaps someone else h=
=3D
as
> it handy and can post it sooner. The formulas are included in =3D93ASTM C=
67=3D
4 -
> 88(2006) Standard Test Methods for Flexural Properties of Ceramic Whitewa=
=3D
re
> Materials=3D94, but they want me to pay $33.00 if I access it from home s=
o =3D
I
> think that I=3D92ll just wait till tomorrow.
>
> BTW, this is not a test for hardness (that=3D92s a different standard
> altogether), and chip resistance is also a completely different animal
> that=3D92s tested in another way.
>
> I hope this helps- sorry about the length
>
> Pete
>
> PS: This seems interesting enough to many potters that I may write some
> more detailed results for Clay Times this fall when I next teach a clay a=
=3D
nd
> glazes class here at UNL. If I do it for the magazine I could include
> pictures as well as the specific results of the tests. Would you want me =
=3D
to?
>
>
> Peter Pinnell
> Professor of Art, Department Grad Chair
> 120 Richards Hall
> University of Nebraska
> 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
>>>
>>
Bryan Johnson on sun 14 mar 10
)
Hi
After reading all the post about attaching handles with or with out
slip or water or magic water or vinegar, I decided to try testing the
strength of the joins by making bars, cutting them and connecting
them. I haven't gotten as far as breaking them yet, so it was good to
read your (Pete's) description of his process.
I'll try to have them at NCECA.
The most bazaar one was one where I dipped the ends in WD 40 before
scoring. They wouldn't stick at all. I added water to the join surfaces
and it got puffy.
I did this in response to a question about what happens when the barrel
of an extruder is coated with WD40. Does it interfere with the join
ability of extrusions?
Bryan Johnson
>
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