David Finkelnburg on fri 12 mar 10
I wrote this in response to an offlist note...
Strength and hardness of fired clay are most definitely not the same
things.
Strength, typically measured in force per unit area (pounds per square
inch or its metric equivalent) can be thought of like rope. You can take a
piece of old, rotten rope and pull it in two with your hands. On the other
hand, really thick, strong rope can be used to tow ships! Two ropes with
remarkably different strengths! Strength of bone dry greenware versus fire=
d
clay is another example with which you are very familiar.
Hardness, on the other hand, is an indication of resistance to abrasion.
In terms of ceramics in this regard I always see in my mind my
motherinlaw, rest her soul, sharpening her kitchen knife on the unglazed
rim of an antique ceramic bowl. The bowl, clearly harder than the steel
knife, made an excellent, readily available sharpening stone!
Hardness is typically measured with a somewhat subjective scale, like
Moh's hardness, or a more objective measure like Brinnell,
Rockwell, Vickers hardness or others where a specific indenter of a specifi=
c
size, shape, hardness and weight is dropped onto a sample at a specific
speed and the size of the indentation created is measured. This sort of
measure is most useful for glass, ceramic or metal samples but doesn't work
as well for softer materials like rubber or wet clay!
Good potting,
Dave Finkelnburg
http://www.mattanddavesclays.com
Ron Roy on sat 13 mar 10
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 chipability  that would have been
another story for sure.
RR
Quoting David Finkelnburg :
> I wrote this in response to an offlist note...
> Strength and hardness of fired clay are most definitely not the same
> things.
> Strength, typically measured in force per unit area (pounds per square
> inch or its metric equivalent) can be thought of like rope. You can take=
a
> piece of old, rotten rope and pull it in two with your hands. On the oth=
er
> hand, really thick, strong rope can be used to tow ships! Two ropes with
> remarkably different strengths! Strength of bone dry greenware versus fi=
red
> clay is another example with which you are very familiar.
> Hardness, on the other hand, is an indication of resistance to abrasio=
n.
> In terms of ceramics in this regard I always see in my mind my
> motherinlaw, rest her soul, sharpening her kitchen knife on the unglaze=
d
> rim of an antique ceramic bowl. The bowl, clearly harder than the steel
> knife, made an excellent, readily available sharpening stone!
> Hardness is typically measured with a somewhat subjective scale, like
> Moh's hardness, or a more objective measure like Brinnell,
> Rockwell, Vickers hardness or others where a specific indenter of a speci=
fic
> size, shape, hardness and weight is dropped onto a sample at a specific
> speed and the size of the indentation created is measured. This sort of
> measure is most useful for glass, ceramic or metal samples but doesn't wo=
rk
> as well for softer materials like rubber or wet clay!
> Good potting,
> Dave Finkelnburg
> http://www.mattanddavesclays.com
>
Lee Love on sat 13 mar 10
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 chipability  that would have been
> another story for sure.
>
> RR

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
Michael Wendt on thu 18 mar 10
After Pete announced his MOR testing methodology,
I did some calculations about the role of the diameter of
the rods in terms of MOR.
I used the general engineering formula
sigma =3D My/I
where:
sigma is the extreme fiber stress which occurs at the
surface
M is the bending moment (measured)
y is the distance from the centroidal axis to the surface
I is the plane moment of Inertia
which for a rod is:
Ix=3DIy=3D pi * r^4/4
Notice that the radius is a factor to the fourth power.
From this, it is easy to see why the porcelain and stoneware
rods appear to be weaker than the earthenware rods.
They are probably 57% smaller in diameter.
For the rod tests to be valid, the extrusion die must be
scaled to produce finished diameters that are as close to
the same as possible when fired to maturity.
Put simply: even a slightly smaller diameter results in a
huge loss of moment of inertia and it is the governing
factor in this type of test.
I would be interested to learn what the results are once
this error is corrected.
Regards,
Michael Wendt
Pete Pinnell on thu 18 mar 10
I promised last weekend that I would provide the formula for figuring =3D
Modulus of Rupture MOR), but I ran into an interesting issue. Several =3D
formulas are provided on Wikipedia, but the important one (the one that =3D
is used for round bars) is different from the one that I've used for the =
=3D
last 20 years (which came from an engineering textbook published in =3D
1972). I was afraid that the standard might have changed since then, so =3D
I decided to wait until I could get my hands on the ASTM standard, which =
=3D
I wasn't able to do until today. Sorry about the wait.
Here's that Wikipedia entry pertaining to Flexural strength, which =3D
contains the 3point and 4point formula for rectangular bars:
http://en.wikipedia.org/wiki/Modulus_of_rupture
The formula for figuring 3 point testing, both rectangular and round are =
=3D
on this page:
http://en.wikipedia.org/wiki/Three_point_flexural_test
The rectangular formula are the same on both pages, and the same as that =
=3D
which is in the book that I've worked from. On the other hand, the =3D
formula for round bars (or rods, if you will) is different from what =3D
I've used. The ASTM formula is the same as the one I've been using, so =3D
I'm pretty confident that it is correct. Here's the listing for the ASTM =
=3D
standard:
http://www.astm.org/Standards/C674.htm
The formula is M=3D3D 8PL/pi d3 (read out loud, the formula is "M equals =
=3D
8PL, all divided by pi times d cubed". "M" is the modulus of rupture =3D
(the ASTM standard is to figure that in pounds per square inch and if =3D
desired to then convert that to MPa.). "P" is the load (the weight that =3D
it took to break the bar). We just weigh the bucket of sand and list it =3D
in pounds (decimal). So, for instance, 14 lbs 8 ozs would, for =3D
calculation purposes, be 14.5 lbs. "L" is the span between the =3D
supporting points, listed in inches (six inches is our standard). "d" is =
=3D
the diameter of the bar, in decimal fractions of an inch (a quarter inch =
=3D
would be .25", for instance). The rest is pretty straightforward, I =3D
think. I hope that my post won't be one of those that's filled with =3D
extraneous symbols, which would make this formula impossible to read.
One more thing: the span needs to be at least 10 times the diameter. My =3D
experience is that if the bar exceeds 3/8 inch diameter with a 6 inch =3D
span, then a full bucket of sand won't be enough weight to break some of =
=3D
the tougher bars. We usually extrude 5/16 inch bars, which end up being =3D
roughly .3 inches in diameter when fired.=3D20
Let me know if this doesn't make sense.
Pete
=3D20
Peter Pinnell
Professor of Art, Department Grad Chair
120 Richards Hall
University of Nebraska
Lincoln, NE 685880114
(402) 4724429=3D
Lee Love on fri 19 mar 10
On Thu, Mar 18, 2010 at 9:50 PM, Michael Wendt wrote:
> I would be interested to learn what the results are once
> this error is corrected.
Please let us know when you get the results!
=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
ivor & olive lewis on sat 20 mar 10
Dear Michael Wendt,
Centroid... A point defined in relation to a given figure in a manner
analogous to the centre of mass of a corresponding body....
What is a Centroidal Axis ? What is a plane moment of inertia ? (Recalling
from a Physics lesson sixty years ago where I learned that "Every Couple ha=
s
its Moments")
If I take Ix =3D Iy =3D pi * r^4/4 and substitute r =3D 1 then Ix =3D Iy =
=3D 0.25 but
when r =3D 0.999 Ix =3D Iy =3D 0.7823 . More than three times larger for a =
smaller
diameter. What are the dimensions of these values ?
Interesting !
Ivor Lewis,
Redhill
 
