Pete Pinnell on thu 20 dec 01
For the final project in my Clay and Glaze class this semester, we mixed
about 50 clay bodies for testing, including red and white earthenwares,
stoneware, porcelain, and sculpture bodies. Besides other tests, we
extruded numerous bars of each body and broke them to measure MOR
(Modulus Of Rupture, which is a measure of the bending strength). There
are other strength tests that can be done (chipping tests, for
instance), but MOR is a quick and easy way to predict how well a body
will hold up to the bumps of everyday use.
Out of all these tests, there were a number of interesting trends:
1. Any amount of grog weakens clay bodies, especially in sculpture
bodies that are essentially underfired. Some of the sculpture clays were
so weak at cone 04 that we couldn=92t measure them- the bars broke at
initial contact before any stress was applied. Any texture in the clay
tended to have the same result, though the texture from using 50 mesh
fireclay seemed to have only a minimal effect. Really fine grogs- those
less than 80 mesh- also had little effect.
2. Glaze made a huge difference in strength. Crazed glazes lowered
results 50% or more from the strength of the same bar unglazed. Uncrazed
glazes raised the strength of the bars from 50 to 100 %. I had read this
before, and assumed that it was mostly related to the lack of surface
flaws on a smooth glaze (cracks like to start at a flaw- take away the
flaws and it=92s more difficult for a crack to start). What I found
interesting is that the amount of compression also mattered. We glazed
the porcelain bars with three different versions of my Pete=92s Clear
glaze, which ranged from mild compression for the original version to a
very low expansion version that places the clay in a very high
compression. Consistently, the higher compression versions produced
higher MOR results.
3. Clays have to be fired to maturity to get good strength. Even firing
porcelain bodies to cone 9 rather than 10 lowered strengths a good deal.
As an aside, I define maturity as the point at which a body achieves its
best strength and glaze fit, and no longer suffers from marked moisture
expansion. Absorption, in my opinion, is not a good indicator except
within one clay body group (such as =93high fire porcelain=94). Porcelain=
s
may need to have less than 1% absorption to avoid moisture expansion
problems, while mature white earthenwares can have upwards of 20%
absorption (which is why those cheap white tiles on our shower walls
don=92t develop delayed crazing).
4. =93Smooth=94 counts for more than =93glassy=94, which seems to contrad=
ict one
bit of standard wisdom I=92ve heard in the past.
5. Quartz seems to be a problem- at least in a minor way. Porcelain
bodies that used a combination of pyrophyllite and quartz were stronger
than those which used only quartz as a filler. It=92s a bit of a mixed
bag, though, because glazes on pyrophyllite bodies tended to craze more.
What were the strongest clays? This will surprise you- it certainly did
me. The strongest clays, consistently, were (drum roll, please) red
earthenware clays fired to a full cone 04.
Yep, that=92s right. Plain old Redart based, smooth red earthenwares. The=
y
were stronger than smooth, brown or gray stonewares, and even stronger
(over all) than porcelain, which I had assumed would be best.
Yes, it was very important to fire them to a full cone 04: cone 06
didn=92t hack it. Surprisingly, taking them to cone 1 did not increase
MOR, though they certainly were denser and felt more solid and chip
resistant. Within red earthenwares, we got consistently higher strength
from those using wollastonite as a secondary flux (5 to 10%), rather
than talc. It seemed best to use red clay in amounts of 50 to 70%, and
while Redart alone (for the red clay portion of the body) gave the best
strength, we got much better workability (and only a tiny bit less
strength) by using a mixture of red clays, such as Redart mixed with
Ranger Red (from Texas) and Apache Red (from Colorado).
As with porcelain, the clay was made much stronger with glazes that fit,
and higher compression glazes were strongest of all. Our all-time
champion (for strength, NOT workability) was the following recipe,
glazed with Linda Arbuckle=92s Majolica and fired to a full cone 04.
Redart, 60%
KT 1-4 Ball Clay, 30%
Wollastonite, 10%
I thought you might find this interesting. I only teach a Clay and Glaze
class one semester every three years, so while I plan to do some follow
up tests (these tests raised as many questions as they answered), don't
look for those results any time soon!
Pete Pinnell
University of Nebraska at Lincoln
Kurt Wild on thu 20 dec 01
>A thank you to Pete Pinnell on his interesting post of 12/20 on "clay body
>strength".... worth reading!
Kurt
Wanda Holmes at Alistia on thu 20 dec 01
Pete,
Would you please comment on the instruments and or methods you used to
measure MOR? I rigged a torque wrench to an arbor press and it works just
fine for testing relative strength within my own studio, but I don't have a
clue how to turn my results into MOR numbers which would allow me to compare
my results to those of others.
By the way, I spent a good part of the last year testing locally available
commercial claybodies at lowfire and midfire ranges. To my surprise, I too
found the red earthenwares to compare very favorably with stoneware and
porcelain.
Wanda
David Hendley on sat 22 dec 01
Thanks for this most interesting report, Pete.
Did you have access to real MOR testing equipment for
this project, or is there some low-tech method of testing
you can tell us about?
I would be most interested in hearing the results of chipping
tests (in three years?). I can't help but think that if this
were included in the evaluations stoneware clays might
fare better compared to red earthenware.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
----- Original Message -----
From: "Pete Pinnell"
To:
Sent: Thursday, December 20, 2001 8:15 AM
Subject: clay body strength
For the final project in my Clay and Glaze class this semester, we mixed
about 50 clay bodies for testing, including red and white earthenwares,
stoneware, porcelain, and sculpture bodies. Besides other tests, we
extruded numerous bars of each body and broke them to measure MOR
(Modulus Of Rupture, which is a measure of the bending strength). There
are other strength tests that can be done (chipping tests, for
instance), but MOR is a quick and easy way to predict how well a body
will hold up to the bumps of everyday use.
Stephani Stephenson on fri 1 aug 03
Thank you! thank you, Pete Pinnell for sharing this information on
your class's MOR tests.
I too am somewhat surprised at some of the results you report.
.
Your post makes me realize that it is quite easy to make
'assumptions' of clay strength
based on visual , tactile, input and a bit of 'hearsay'.
For example, I too would guess that a 'glassy' looking glaze would
increases the strength
of the clay bar... it looks 'hard', it feels 'melted in' , whereas
'smooth' would not be something I would first take into consideration
as a contributor to strength,
In addition to sensory input, preconceived notions such as ,
'if it is fired higher , it must be stronger', play into our thinking
Thank you for reminding us that testing opens windows to really knowing
what is happening..
On the bodies you tested with grog content... did each body have only
one mesh/size of grog? or did any contain a mixture/combination of
sizes? I hear from time to time (from the storehouse of tilemaker
adages) that 'the best clay has '3 different sizes of grog'.
Of course that is very general, statement but I wonder if any of your
tests uses bodies with a variety of grog sizes v. a single type/size.
Also, have your students ever done tests using compressed clay bars
(clay that has been pressed... or extruded first then compressed a bit)
versus simply extruded clay bars? I am talking about physical
compression of the moist clay, not glaze 'compression' in firing.
What you say makes sense to me. I have seen some samples of red
earthenware tile clay fired to 04, from Mexico, that was absolutely
smooth, deep red , unusually dense (i.e. very heavy feeling for the
size) and extremely, and to me, surprisingly strong. and sensory
input, and the feel of it and sound of it suggested to me that it was
fired to optimum 'maturity'. This was also working against another
stereotype, which is that of the 'cheap' , crumbly ,Mexican tile clay.
I work with a tile maker who has been making tile for many years, cone
04 red clay, though with grog... and it has worked quite well for floor
tile as well as wall and decorative tile, and architectural components..
Some potters have a hard time accepting this , especially with regard to
the floor tile. I think potter's instincts tell us that 'superior' tile
must be made from high fire clay and fired to cone 10.
I think what happens, and I can understand how it happens, is that when
you become accustomed to firing at higher temperatures, gradually you
become convinced that 'weight bearing' requires 'stronger', and that
'hotter' is the equivalent of 'stronger'.
well thank you for inciting a small riot of thoughts here, I found your
post to be very interesting.
o' yes there's so much to learn!
sincerely
Stephani Stephenson
steph@alchemiestudio.com
http://www.alchemiestudio.com
Stephani Stephenson on sat 2 aug 03
I guess I should say thank you Russell Fouts for re-posting Pete
Pinnell's post!
I never saw the original and while scrolling through the CLAYART
Digest form,
but I missed the connection! Since Pete's message was from 2001
I guess my reply/questions to Pete would be considered
rhetorical!
It would be interesting to find out how his classroom tests
were conducted.
Seems like a good in- studio test to do.
I know the Tile Council of America, states that average 'bond
strength' of unglazed and glazed mosaic, quarry and paver tile
should be 50 psi, and 'breaking strength' of 250 pounds... to
past tests as first quality tile.... exception is glazed wall
tile which is 'breaking strength of 90 pounds'.
I don't have the publication in front of me which defines the
tests or exact definition of terms.
Incidently , the test for crazing involves subjecting tile to
150 psi steam pressure 'for one cycle'
As with studio pottery, studio art tile is not subject to the
same industry testing as commercial, mass produced tile. I think
a lot of small studio tilemakers, as with studio potters, rely
on information from our suppliers on specs/ characteristics for
our clay and even our glazes.
Unlike larger manufacturers, who have full time testing and
quality control and research people.
A studio tilemaker, and potter fits it in with the making, the
designing, the marketing, the firing, the telephone calls, etc.
Too often this type of testing gets put on the end of the list
and understandably so. But I think many studio tilemakers, as
well as sculptors and potter's are very interested in improving
the quality of their ware, so we take to heart information which
can spur us on to do this.
So, looks like some more homework is in order....
and... Thanks Russell!
Stephani Stephenson
steph@alchemiestudio.com
skiasonaranthropos@FSMAIL.NET on tue 25 oct 05
Hello John,
Thank you for the reply, and the explanation with the archive reference.
As you comment note its noting that there are various types of strength.
Pete s original post is certainly a nice summary of his work and most
accords with recognised phenomena, however I was surprised to read =93...
while mature white earthenwares can have upwards of 20% absorption (which
is why those cheap white tiles on our shower walls don=92t develop delayed
crazing).=94 That seems incredibly high; in my experience 10% is high for
earthenware let alone double. And Id argue that wall tiles are not made
from earthenware as well as his description of the mechanism as to why
crazing does not develop
Im still confused with his explanation of high strength results as
suitably fired vitreous bodies will, Id stick my neck out here and suggest
almost without exception, be stronger than those with open porosity.
However my confusion with the post could just be my reading of his brief
synopsis without having access to his full report
Thanks again,
Antony
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