Russel Fouts on fri 1 aug 03
Ron
I'm coming into this discussion late but wanted to interject these
findings by Pete Pinnel. Pete posted this to Clayart almost 2 years ago.
When it was posted, I expected it to stimulate some discussion because
his findings contradict the commonly held wisdom regarding the strenght
of earthenware. But nobody bit.
I've excerpted the part that I wanted to focus on and have included the
full text below that. Enjoy!
Russel
----------------------------------------------------
"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's right. Plain old Redart based, smooth red earthenwares.
They 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't 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's Majolica and fired to a full cone 04."
Redart, 60%
KT 1-4 Ball Clay, 30%
Wollastonite, 10%
-----------------------------
Date: Thu, 20 Dec 2001 10:15:23 -0600
From: Pete Pinnell
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.
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't 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's 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's 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 "high fire porcelain"). Porcelains
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't develop delayed crazing).
4. "Smooth" counts for more than "glassy", which seems to contradict one
bit of standard wisdom I've 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's 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's right. Plain old Redart based, smooth red earthenwares. They
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't 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's 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
-----------------------------
--
Russel Fouts
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