ronroy@CA.INTER.NET on mon 9 jan 12
Earthenware is not as strong as properly formulated and fired =3D20
stoneware and porcelain - there is no question about that and that =3D20
includes chipability as well.
Pete admitted he did not resize all his samples - the earthenware bars =3D2=
0
were bigger than the stoneware and porcelain bars because earthenware =3D20
does not shrink as much.
If you want to do a scientific study on the subject you have to make =3D20
sure the bars are the same size.
I have cut hundreds of bars for my dilatometer - all kinds of clays =3D20
and glazes - the saw goes through earthenware like butter compared to =3D20
stoneware and porcelain.
Pete should be congratulated for doing and sharing any research on =3D20
clay and glazes - it is well needed - and his article has valuable =3D20
information for all potters. It just happens to be inconclusive in =3D20
that one area. All the literature on the subject points to earthenware =3D2=
0
being softer and weaker which is why he said "This will surprise you- =3D20
it certainly did me."
RR
Quoting Lee :
> This is absolutely untrue. Fired to a higher temp, Earthenware
> can be stronger than porcelain. I have found this true with my own
> work. I've always liked Raku much better than porcelain and even
> better than most stoneware.
>
> I include Pete's entire essay on claybody strength below (and
> Kurt Wild's brief thank you to Pete.)
>
> Claybody Strength
> Pete Pinnell on thu 20 dec 01
>
>
> For the final project in my Clay and Glaze class this semester, we mixe=
=3D
d
> 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 wer=
=3D
e
> so weak at cone 04 that we couldn=3D3D92t 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. Uncraze=
=3D
d
> glazes raised the strength of the bars from 50 to 100 %. I had read thi=
=3D
s
> 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=3D3D92s 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=3D3D92s 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=
=3D
.
> As an aside, I define maturity as the point at which a body achieves it=
=3D
s
> 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 =3D3D93high fire porcelain=3D3D94). P=
or=3D
celain=3D3D
> 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=3D3D92t develop delayed crazing).
>
> 4. =3D3D93Smooth=3D3D94 counts for more than =3D3D93glassy=3D3D94, which =
seems =3D
to contrad=3D3D
> ict one
> bit of standard wisdom I=3D3D92ve 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=3D3D92s a bit of a mixe=
=3D
d
> bag, though, because glazes on pyrophyllite bodies tended to craze more=
=3D
.
>
> 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=3D3D92s right. Plain old Redart based, smooth red earthenwares.=
=3D
The=3D3D
> 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=3D3D92t hack it. Surprisingly, taking them to cone 1 did not increas=
=3D
e
> 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=
=3D
,
> and higher compression glazes were strongest of all. Our all-time
> champion (for strength, NOT workability) was the following recipe,
> glazed with Linda Arbuckle=3D3D92s 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 Glaz=
=3D
e
> 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 b=
=3D
ody
> strength".... worth reading!
>
> Kurt
>
>
>
> --
> Lee Love in Minneapolis
> http://mingeisota.blogspot.com/
>
> "Ta tIr na n-=3DC3=3DB3g ar chul an tI=3DE2=3D80=3D94tIr dlainn tri=
na ch=3DC3=3D
=3DA9ile"=3DE2=3D80=3D94that is, "The
> land of eternal youth is behind the house, a beautiful land fluent
> within itself." -- John O'Donohue
>
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