Fred Parker on tue 9 mar 10
Can anyone comment on the relative hardness of fully vitrified fired (^6)=
=3D
stoneware and porcelain? I'm guessing someone somewhere has come up with=
=3D
a
Mohs hardness value (apologies for spelling if needed) but I don't know
where to find it. I am also needing hardness values for glaze, but I exp=
=3D
ect
that will be elusive given so many variations.
Many thanks,
Fred Parker
Lee Love on tue 9 mar 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
--
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
Neon-Cat on tue 9 mar 10
Fred, the ASTM Standards cover just about everything:
http://www.astm.org/
For me to use the standards I've needed to be very specific in
searches and develop research methods that allow me to access
applicable documents for free. The other day when low fire glazes came
up I gathered quite a lot of information on testing both low fire
bodies and low fire glazes that would be applicable to studio potters
making functional ware or architectural tiles, for example. I have
looked at documents regarding stoneware and porcelain before, too.
Application and intended usage are key. What are you making? This is
always a good first question.
There are also institutes and associations that cover applicable
standards for their members and spin-off books and texts and papers on
specific topics within each field. These resources are also quite
informative. For example, if I was into making countertop ceramic
tiles for kitchens the CERAMIC TILE INSTITUTE OF AMERICA, INC. has
some good starter info at http://www.ctioa.org/reports/fr69.html.
I've just gotten past two weeks of hell from a cluster of brown
recluse spider bites, had my Easter miracle, and now, with two good
legs I'm ready to dance again, but it isn't with clay science and
technology on-list. Dusty documents? -- not for me today.
I am confident that you will find just the answers you need. If I can,
anyone can.
Persistence wins the prize.
Marian
Neon-Cat
John Britt on tue 9 mar 10
Hey Fred,
I am sure Ron R. or John P. will step in here but I have gotten this
complaint from several people since I have been doing cone 6 reduction
workshop. They are afraid that cone 6 clay bodies are inferior in strengt=
=3D
h
to cone 10. I assure them that this is not the case.=3D20
I know that the clay manufactures like Laguna or Standard will have the
statistics which is what I think that want.
John Britt
www.johnbrittpottery.com
John Hesselberth on tue 9 mar 10
Hi Fred,
My cone 6 stoneware fired to about 1.5% absorption is between 7 and 8 on =
=3D
the Mohs scale. I suspect there is quite a bit of variability between =3D
various clay bodies.
Regards,
John
On Mar 9, 2010, at 9:29 AM, Fred Parker wrote:
> Can anyone comment on the relative hardness of fully vitrified fired =3D
(^6)
> stoneware and porcelain? I'm guessing someone somewhere has come up =3D
with a
> Mohs hardness value (apologies for spelling if needed) but I don't =3D
know
> where to find it. I am also needing hardness values for glaze, but I =3D
expect
> that will be elusive given so many variations.
>=3D20
> Many thanks,
>=3D20
> Fred Parker
James Freeman on tue 9 mar 10
Hi, Fred...
The Mohs scale brings back memories of my childhood rockhound days. Testin=
g
is quite simple, just a matter of what scratches the sample, and what the
sample can scratch, narrowed down through the process of elimination.
A kit of actual test minerals is the best method, but several common
household objects can suffice. Your fingernail has a Mohs hardness of abou=
t
2.5, so anything that can be scratched by your fingernail has a hardness of
less than 2.5, and anything that can scratch your fingernail has a hardness
greater than 2.5. A copper penny (the real kind, not the new zinc ones) ha=
s
a hardness of about 3. Mild steel, like a nail or a piece of scrap steel
bar, is about 4 or 4 1/2. A knife blade (carbon steel, not stainless) is
about 5.5. Glass is around 6. Hardened tool steel, like a metal file or a
drill bit is about 7, sometimes up to 8. Corundum (aluminum oxide)
sandpaper is 9.
As an example, if your glaze or clay specimen scratches glass but can be
scratched by the file, then your specimen is somewhere in between 6 and 7-8=
,
so you can estimate about 6 1/2 or 7. If you had a mineral sample kit you
could be even more accurate, testing with feldspar at 6, quartz at 7, and
beryl at 8.
I'm guessing your stoneware and porcelain clays will be in the 6-7 range,
and your glazes will be in the 5-6 range, but it will be easy enough for yo=
u
to find out. Not rocket science!
All the best.
...James
James Freeman
"All I say is by way of discourse, and nothing by way of advice. I should
not speak so boldly if it were my due to be believed."
-Michel de Montaigne
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/clayart/
On Tue, Mar 9, 2010 at 9:29 AM, Fred Parker wrote:
> Can anyone comment on the relative hardness of fully vitrified fired (^6)
> stoneware and porcelain? I'm guessing someone somewhere has come up with=
a
> Mohs hardness value (apologies for spelling if needed) but I don't know
> where to find it. I am also needing hardness values for glaze, but I
> expect
> that will be elusive given so many variations.
>
> Many thanks,
>
> Fred Parker
>
steve graber on tue 9 mar 10
i think it's been declared as a Morh's hardness of "6".=3DA0 ?=3DA0 =3D0A=
=3D0Aright=3D
on par with metal of "6" also,=3DA0which indictates why some knives scratc=
h =3D
and some do=3DA0not scratch=3DA0plates (glazes).=3DA0 =3D0A=3D0Ais this har=
dness scal=3D
e really what you're after?=3DA0 the rockwell scales=3DA0provide more detai=
l=3DA0=3D
to split the hardness of metals and other things down further.=3DA0 =3D0A=
=3D0Asom=3D
e useful research sites:=3DA0 =3D0Ahttp://www.engineersedge.com/=3D0Ahttp:/=
/www.e=3D
ng-tips.com/=3D0A=3D0A=3D0A=3D0A=3D0A=3DA0Steve Graber, Graber's Pottery, I=
nc=3D0AClaremo=3D
nt, California USA=3D0AThe Steve Tool - for awesome texture on pots! =3D0Aw=
ww.g=3D
raberspottery.com steve@graberspottery.com =3D0A=3D0A=3D0AOn Laguna Clay's =
websit=3D
e=3D0Ahttp://www.lagunaclay.com/blogs/ =3D0A=3D0A=3D0A=3D0A----- Original M=
essage ---=3D
-=3D0A> From: Fred Parker =3D0A> To: Clayart@LSV.CERAMI=
CS.O=3D
RG=3D0A> Sent: Tue, March 9, 2010 6:29:00 AM=3D0A> Subject: Fired clay hard=
ness=3D
question=3D0A> =3D0A> Can anyone comment on the relative hardness of fully=
vit=3D
rified fired =3D0A> (^6)=3D0Astoneware and porcelain?=3DA0 I'm guessing som=
eone s=3D
omewhere has come =3D0A> up with a=3D0AMohs hardness value (apologies for s=
pell=3D
ing if needed) but I don't =3D0A> know=3D0Awhere to find it.=3DA0 I am also=
needi=3D
ng hardness values for glaze, but =3D0A> I expect=3D0Athat will be elusive =
give=3D
n so many variations.=3D0A=3D0AMany =3D0A> thanks,=3D0A=3D0AFred Parker=3D0=
A=3D0A=3D0A =3D
steve graber on tue 9 mar 10
classic fun discussions always revolve around "strength".=3DA0 and add=3DA0=
term=3D
like=3DA0"toughness" or "brittle" to the discussion and it can go on and o=
n =3D
and on.=3DA0 =3D0A=3D0Acarbon steal is "strong".=3DA0 stainless steal is "t=
ough".=3D
=3DA0 hard candy is=3DA0hard (at least on my teeth) and brittle.=3DA0 saltw=
ater t=3D
affy is "tough".=3DA0 i guess=3DA0gummy bears are tough too, but not so "st=
rong=3D
".=3DA0 =3D0A=3D0Amaterials can be very=3DA0strong to the point of becoming=
brittle=3D
, yet some brittle properties necesary and are used to=3DA0cut metal in mac=
hi=3D
ne shops.=3DA0 a nice lathe cutter will work beautifully on a=3DA0lathe all=
day=3D
, but drop it and it shatters.=3DA0 is that a good thing?=3DA0 it is for=3D=
A0use =3D
as a lathe tool but if the product was meant to survive a drop, you need=3D=
A0=3D
something less brittle, more "tough".=3DA0 =3D0A=3D0Alee's testing sounds l=
ike it=3D
was=3DA0dancing around the concept of IZOD impact testing.=3DA0 i don't kn=
ow i=3D
f he took it=3DA0thru complete data compilation of the IZOD impact methods.=
=3D
=3DA0 it's one useful=3DA0method of many to evaluate materials.=3DA0 they u=
se tes=3D
t bars and get wacked by a=3DA0pendulm with known foot-pounds of impact for=
ce=3D
.=3DA0 using a standard foot-pound=3DA0impact many materials can be evaluat=
ed v=3D
ery nicely.=3DA0 glass doesn't score very=3DA0high on impact but it does ha=
ve a=3D
very high tensile strength.=3DA0http://en.wikipedia.org/wiki/Izod_impact_s=
tr=3D
ength_test=3D0A=3D0Alee - your=3DA0testing brings out some useful informati=
on for=3D
anyone wanting to select a=3DA0clay=3DA0material for their IMPACT applcati=
on, =3D
but like all applications, the selection of=3DA0the material needs to be fo=
r =3D
THAT applcation.=3DA0 in mechanical engineering,=3DA0materials are evaluate=
d fo=3D
r yield strength (when they bend and stay bent),=3DA0 ultimate strength (wh=
en=3D
they break), modulus of elasticity (how "springy" they=3DA0are).=3DA0 and =
they=3D
are reduced to terms like pounds per square inch (PSI).=3DA0 along=3DA0wit=
h ot=3D
her detail characteristics these help people select materials that are=3DA0=
"s=3D
trong" or "tough" for their needs.=3DA0 =3D0A=3D0Aas it applies to clay, i =
suspec=3D
t=3DA0the general concern is impact.=3DA0 can the piece take a wack against=
the=3D
side of a=3DA0sink or when you "ooops" it against another piece?=3DA0 but =
then=3D
once you know your=3DA0own specific material, you learn you can get away w=
it=3D
h making it thinner, or you=3DA0need to make it thicker to meet a typical e=
xp=3D
ected exposure of the product.=3DA0=3DA0and you learn that a cool looking s=
culp=3D
ture arm simply can not be used as a neat=3DA0towel holder if you start loa=
di=3D
ng that arm up with a bunch of towels and maybe a=3DA0jacket....=3DA0 (that=
's l=3D
ikely a flexural-tension-bending strength failure mode).=3DA0=3DA0=3DA0=3D0=
A=3D0A=3D0AS=3D
teve Graber, Graber's Pottery, Inc=3D0AClaremont, California=3DA0=3D0AUSA=
=3D0AThe S=3D
teve Tool - for awesome texture on pots! =3D0Awww.graberspottery.com=3D0A=
=3D0A=3D0A=3D
=3D0A
John Britt on tue 9 mar 10
Neon,
Sorry to hear about the brown recluse spider bites! You are lucky to be =
=3D
alive!
I contacted several manufactures about data on the durability of cone 6 v=
=3D
s
cone 10. I wanted to see if they had any ASTM numbers to show us,
John Britt
www.johnbrittpottery.com
John Britt on tue 9 mar 10
Got this note back from Jeff Zamek on clay hardness:
"Pots fired to cone 6 reduction or oxidation are as strong as pots fired =
=3D
to
c/10. The important factor is the amount of flux (feldspar) used in the c=
=3D
lay
body. Secondly, the time from c/06 to c/6. I usually recommend at least a=
=3D
75
F. per/hr heat increase to c/6 which allows for body and glaze vitrificat=
=3D
ion.=3D20=3D20
=3D20
At c/6 the absorption figures should be from 1- 3% absorption indicating=
=3D
a
dense water holding clay body. Shrinkage rate are comparable to c/10
throwing body's. Many ceramics suppliers do sell c/6 porcelain clay body'=
=3D
s
which are translucent when thin indicating glass build up in the clay bod=
=3D
y.
=3D20
Just to take it one step further your can easily develop c/06 clay body
formulas that are dense and strong and have low absorption rates if you u=
=3D
se
frit in the clay body. Unfortunately, frit has a very short maturing rang=
=3D
e
and these bodies have to be fired carefully."
I will try to get more data,
John Britt
www.johnbrittpottery.com
Eric Hansen on tue 9 mar 10
Normally the ceramics industry doesn't concern itself to much with Moh's
scale. There are a typical series of tests done on floor tile which involve
resistance to abrasion - which is determined using several different method=
s
& equipment
h a n s e n
On Tue, Mar 9, 2010 at 3:01 PM, John Britt wrot=
e:
> Neon,
>
> Sorry to hear about the brown recluse spider bites! You are lucky to be
> alive!
>
> I contacted several manufactures about data on the durability of cone 6 v=
s
> cone 10. I wanted to see if they had any ASTM numbers to show us,
>
> John Britt
> www.johnbrittpottery.com
>
ivor & olive lewis on wed 10 mar 10
Dear Fred Parker,
From Memory, Mohs' Scale of Mineral Hardness
1. Talc,
2. Gypsum.
3. Calcite.
4. Fluorite.
5. Apatite.
6. Felspar.
7. Quartz.
8. Topaz.
9. Corundum.
10. Diamond.
You should be able to buy a set of stones from a gemstone merchant
Best regards
Ivor Lewis,
Redhill,
South Australia
John Britt on wed 10 mar 10
Eric,
I did not mention anything about Moh's scale. I know that there are lots =
=3D
of
tests run in industry, including modus of rupture, and all are outlined i=
=3D
n
the ASTM book.
Here is one link:
http://engineers.ihs.com/news/2008/astm-ceramics-test-method.htm
John Britt
www.johnbrittpottery.com
Neon-Cat on wed 10 mar 10
Thanks John, and others, for looking into all this for us (I=3D92m way
behind in basic life and school stuff right now).
I do want to mention that looking fearlessly into a question does not
mean to imply that the work of studio potters or their clays are =3D91less
than=3D92 or not as good as ware or products made industrially. We may
find that we meet or exceed normal expectations and standards and what
a nice marketing point (not to mention point of well-earned pride)
that would be.
If I were to start looking for a set of dishes today and opted to
purchase dinnerware from Mel, I might expect many years of safe
service from great looking plates and bowls. Did you know that in
parts of the world many busy restaurants consider it normal to replace
their tableware at least twice a year? I bet Mel=3D92s plates and those
of others on-list would exceed the quality of some of the top
industrial producers today.
So, as Steve and Jeff have touched upon there are all kinds of
stresses and strains and ways to measure and make things. Knowing more
about processes and terms and standards can only be helpful. I can
take my same wonky native clay body, formulate it in different ways,
mix it by hand, and get a cone 06 to cone 03 product with widely
varying properties depending largely on the fluxes or frits I use. I
can do the same thing at higher temperatures, too =3D96 change the fired
properties. Some of these clay body versions will work better for some
intended purpose than others. If I am making just art ware, well, I=3D92ve
now got a large range of colors and textures handy from just one clay
fired in the same range or over a wider range. If I settle on one line
of work, say tiles for outdoor applications, wouldn=3D92t it be nice for
me to claim that they meet or exceed ASTM standards for such tiles? My
competitors will be making these claims, why not me? It=3D92s part of
smart marketing and one of the ways of the world. I=3D92d rest easier,
too, knowing I met certain standards and had some reasonable
expectation that if I did an installation today for a friend she=3D92d not
be looking at crumbling tiles five years down the road.
Do not write me and tell me we studio potters don=3D92t have the processes
and equipment and techniques of industry and so attempt to excuse what
we do.
Maybe we need no excuses.
So take another look.
If you and your students are meeting or exceeding world expectations =3D96
claim it! Use it. Be proud.
But first let=3D92s see what it is we can claim and how.
Here is an interesting article on architectural terra cotta. I drop it
into our stoneware thread here because I have it handy and it gives an
excellent history spanning a hundred years of how people have looked
at terra cotta and how generations have measured and assessed these
products and their service while in use. Some methods mentioned are
applicable to work done using stoneware and porcelain.
=3D93Glazed and Confused: Exposing the Mysteries of Glazed Architectural
Terra Cotta=3D94 by Xsusha Carlyann Flandro, 2009:
http://www.scribd.com/doc/19095555/Glazed-and-Confused-Exposing-the-Mysteri=
=3D
es-of-Glazed-Architectural-Terra-Cotta
Marian
Neon-Cat
David Finkelnburg on wed 10 mar 10
Fred,
The hardness of fired clay would be approximated by the hardness of the
materials that make up the fired clay at the end of the firing. The
significant phases, for mid and high fire, are mullite (~Moh 6.5), glass an=
d
any unmelted quartz (~Moh 7.0) or other filler. The hardest of those would
control the abrasive nature of the fired body. The glass acts like a binde=
r
in an alumina grinding stone. The binder is an important ingredient but th=
e
alumina controls the hardness of the stone!
The hardness of glass;, ~Moh 5.5, is a good starting point for the
hardness of glazes. As you note, though, there are many variations. High
alkali glazes and high boron glazes will be relatively soft. High calcia
and alumina would promote harder glazes. If significant anorthite were
present (Moh 6), as in a crystallized matte glaze, then the anorthite may
control the hardness.
James Freeman's comments on Moh's scale show how to test specific
samples.
Good potting!
Dave Finkelnburg, astounded at how quickly this thread morphe=
d
from hardness to strength...
-----------------------------
Date: Tue, 9 Mar 2010 09:29:00 -0500
From: Fred Parker
Can anyone comment on the relative hardness of fully vitrified fired (^6)
stoneware and porcelain? I'm guessing someone somewhere has come up with a
Mohs hardness value (apologies for spelling if needed) but I don't know
where to find it. I am also needing hardness values for glaze, but I expec=
t
that will be elusive given so many variations.
John Britt on wed 10 mar 10
Got a note from someone who makes a lot of clay for potters and he said:
"He took 10 random ^5 clays ---1 in. wide x 1/4 thick fulcrum 4 in.
apart----average load 52lbs to snap them in half
=3D20
Then he took 10 random ^10 clays ---average 49 lbs to snap them.
=3D20
He found that the ^5's out performed the ^10's!
=3D20
As a side not he --- threw out one ^5 body because it's a hotel china bod=
=3D
y
and it took 120 lbs to snap. by far the strongest body I tested. The next=
=3D
strongest was a ^10 porcelain that took 78 lbs ."
That is one and I am hoping to get a few more.
John Britt
www.johnbrittpottery.com
Lee Love on wed 10 mar 10
On Wed, Mar 10, 2010 at 10:05 AM, John Britt w=
=3D
rote:
> Got a note from someone who makes a lot of clay for potters and he said:
Pete explained that glaze fit and firing temp were the most important
variables. Some compression is good. Earthenware usually needs to
be fired a little hotter than cone 05, but over 03 doesn't have any
effect. A cone 10 body is weaker at cone 9.
--
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
John Britt on wed 10 mar 10
Lee,
I kinda wanted just clay body durability because glaze adds another whole=
=3D
set of variables.=3D20
I know that the firing cone makes a difference, so a cone 10 body fired t=
=3D
o
cone 9 is different that one to 11 or 10 1/2, etc. And we all know the
variations in pottery kiln.=3D20
But the general superiority of cone 10 clay bodies over the general cone =
=3D
6
body is what is at issue. It is hard to convince people that cone 6
reduction is good and then you throw in the red herring that cone 10 bodi=
=3D
es
are stronger anyway, and now you really have a problem.
Potters like to believe that cone 10 gas firings are hierarchically the
best, both in terms of history and the long struggle in the West to get
there and also in terms of strength and the look of glazes. (Week long wo=
=3D
od
firing and shino are probably the cause a lot of the later.)
But this is just not the case.=3D20
So I am back to looking for more data. I would love to find a manufacture=
=3D
r
who has some data on the chip test (ASTM) where they take a metal pointy
weight and drop it from various heights until the piece breaks.
John Britt
www.johnbrittpottery.com
Lee Love on wed 10 mar 10
On Wed, Mar 10, 2010 at 1:20 PM, John Britt wr=
=3D
ote:
> Lee,
>
> I kinda wanted just clay body durability because glaze adds another whole
> set of variables.
The data is only valid for unglazed work then.
> But the general superiority of cone 10 clay bodies over the general cone =
=3D
6
> body is what is at issue. It is hard to convince people that cone 6
> reduction is good and then you throw in the red herring that cone 10 bodi=
=3D
es
> are stronger anyway, and now you really have a problem.
Cone 6 is kinda a "kiss or sister" temperature. As Pete points out
from his test, earthenware is the strongest. I see no reason to
mess around with cone 6 when earthenware is so attractive. I am not
sure why cone 6 folks look down on earthenware. You have the broadest
ranges of color there and can go even farther with enamel and luster.
> there and also in terms of strength and the look of glazes. (Week long wo=
=3D
od
> firing and shino are probably the cause a lot of the later.)
I don't think so. Folks like myself doing these kinds of
firing are primarily interested in the effects you get on the claybody
at these high temps. And the nuances you see in cyclical
atmospheres. And, firing with renewable fuel sources like
replenishable fire wood.
> So I am back to looking for more data. I would love to find a manufacture=
=3D
r
> who has some data on the chip test (ASTM) where they take a metal pointy
> weight and drop it from various heights until the piece breaks.
There should be plenty of data for oxidation firing. Would they be
valid for reduction firing?
--
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
Neon-Cat on wed 10 mar 10
Oops, about my last post =3D96 I was quirking-off about a private post I
received that asked about the strength of industrial made and fired
products vs. studio made and fired products. I did not mean to imply
that John or anyone who had previously posted on-line in the thread
had anything to do with my comments about mid-post.
Strength is relative and depends on how it is defined.
The private post came with a photo titled =3D93Jomon=3D94. I love Jomon
pottery and the photo seemed an interesting juxtaposition to the email
text. Those Jomon potters lived in caves or pit-dwellings from 10,000
=3D96 400 BC and pit or bon fired their slab or coil constructed pots =3D96
pots strong enough to meet their needs and strong enough that we get
to have some with us now after many centuries.
Sorry =3D96 too much dancing and not enough mental clarity...
Carry on.
Marian
John Britt on thu 11 mar 10
Lee,
You would think that there would be plenty of data since the majority of
people who work in clay fire at cone 6! Got any?
John
David Woof on thu 11 mar 10
Marian wrote: ..=3D
.
Carry on.
Marian>
____________________________
There are at least four different conditions or qualities of the clay being=
=3D
discussed in this thread using words that are being used interchangebly an=
=3D
d without qualification.
=3D20
So while I have strength=3D2C durability=3D2C hardness=3D2C and plenty of s=
tamina=3D
=3D2C where this party is moving needs clarification before we get to the "=
ch=3D
ip test" or I'm going to put on my glaze and go home!!!
=3D20
David Woof
=3D20
=3D20
=3D20
=3D20
_________________________________________________________________
Hotmail: Free=3D2C trusted and rich email service.
http://clk.atdmt.com/GBL/go/201469228/direct/01/=3D
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