Eric Hansen on tue 22 mar 11
http://www.quartzpage.de/gen_rock.html
incidentally, there are at least 2 types of sediments which form rock
under the ocean, they are the skeletons of ocean based life forms:
silica/crypto-crystaline quartz/flint - and the other is calcium
carbonate/fossilized coral/limestone. Even the purest quartzite such
as the Snowy Peaks of the Medicine Bow range, which is metamorphic
rock, formerly sedimentary sandstones, has some calcium in it. The
product we call "flint" or "silica" when we formulate clay in the U.S.
has considerable calcium in it. "Lime poppers" isn't only a drawback
of using sand, it is also a draw back of using clay, as clay COULD
contain lime fragments. I don't hear anyone saying "don't use clay"
but it is just as illogical as saying "don't use sand"
--
Eric Alan Hansen
Stonehouse Studio Pottery
Alexandria, Virginia
americanpotter.blogspot.com
thesuddenschool.blogspot.com
hansencookbook.blogspot.com
"Simplify, simplify, simplify" - Thoreau
David Finkelnburg on thu 24 mar 11
Eric,
I like your enthusiasm for the information in the nice link. I do have
to point out one important detail, though. Your wrote, "The product we call
"flint" or "silica" when we formulate clay in the U.S. has considerable
calcium in it." That is only true for flint.
Flint, in mineral terms, is a metamorphic silica rock or sea bed origin=
,
and contains, as you mention, a significant amount (3-8%) of mostly calcium
carbonate and also some magnesium carbonate.
In the US most silica sold for ceramic use is ground crystalline
quartz. Quartz is an igneous silicate crystalized from molten rock.
Quartz, by definition a crystal, is quite pure SiO2. Most ground quartz
will be 97% SiO2 or higher.
The Snowy Peaks quartzite you cite is hardly, "the purest
quartzite..." On the contrary, it is less than 90% quartz.
Quartzite here in southeast Idaho mined for electric furnace slag is
more typical...99.7%+ SiO2.
Flint was historically the principal silica source for ceramics in Grea=
t
Britain and Europe and we have inherited the name. It is commonly misapplie=
d
to ground quartz but since ours is not necessarily or entirely a precise
business, we muddle on.
Good potting,
Dave Finkelnburg
-----------------------------
Date: Tue, 22 Mar 2011 19:46:29 -0400
From: Eric Hansen
Subject:
http://www.quartzpage.de/gen_rock.html
incidentally, there are at least 2 types of sediments which form rock
under the ocean, they are the skeletons of ocean based life forms:
silica/crypto-crystaline quartz/flint - and the other is calcium
carbonate/fossilized coral/limestone. Even the purest quartzite such
as the Snowy Peaks of the Medicine Bow range, which is metamorphic
rock, formerly sedimentary sandstones, has some calcium in it. The
product we call "flint" or "silica" when we formulate clay in the U.S.
has considerable calcium in it. "Lime poppers" isn't only a drawback
of using sand, it is also a draw back of using clay, as clay COULD
contain lime fragments. I don't hear anyone saying "don't use clay"
but it is just as illogical as saying "don't use sand"
--
Eric Alan Hansen
Stonehouse Studio Pottery
Alexandria, Virginia
americanpotter.blogspot.com
thesuddenschool.blogspot.com
hansencookbook.blogspot.com
"Simplify, simplify, simplify" - Thoreau
James Freeman on thu 24 mar 11
William...
As always, your posts are incredibly informative. Thanks for providing a
perspective that is very unique. I learned a lot.
All the best.
...James
James Freeman
"...outsider artists, caught in the bog of their own consciousness, too
preciously idiosyncratic to be taken seriously."
"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/resources
On Thu, Mar 24, 2011 at 1:54 PM, William Lucius wrote:
> I found your link to the quartz webpage very informative. It reminded me =
of
> how much of my geologic training has leaked out of my brain. The rabid
> rockhound in me drooled over the various agates!
>
> In terms of your message, I personally avoid sands and clays that contain
> carbonates of calcium. I would refer you to Anna O. Shepard's discussion =
on
> page 22 of her publication Ceramics for the Archaeologist, which is
> available on line:
> http://carnegiescience.edu/publications_online/Ceramics_arch.pdf
>
>
>
James Freeman on thu 24 mar 11
On Tue, Mar 22, 2011 at 7:46 PM, Eric Hansen
wrote:
http://www.quartzpage.de/gen_rock.html
OK, Eric, that quartz site was fascinating! I didn't think I cared about
quartz, but I just squandered a half hour of my life reading it from end to
end. Thanks for sharing the link.
All the best.
...James
James Freeman
"...outsider artists, caught in the bog of their own consciousness, too
preciously idiosyncratic to be taken seriously."
"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/resources
William Lucius on thu 24 mar 11
I found your link to the quartz webpage very informative. It reminded me of=
=3D
how much of my geologic training has leaked out of my brain. The rabid roc=
=3D
khound in me drooled over the various agates!
In terms of your message=3D2C I personally avoid sands and clays that conta=
in=3D
carbonates of calcium. I would refer you to Anna O. Shepard's discussion o=
=3D
n page 22 of her publication Ceramics for the Archaeologist=3D2C which is a=
va=3D
ilable on line:=3D20
http://carnegiescience.edu/publications_online/Ceramics_arch.pdf
=3D20
"Calcium oxide=3D2C the product of decomposition of limestone and other car=
bo=3D
nates
of calcium=3D2C is a fluxing agent. If it is finely disseminated=3D2C it wi=
ll p=3D
romote vitrification=3D2C
but this action will not take place at the lower temperatures of primitive
firing. If calcium oxide does not react with constituents of the clay durin=
=3D
g firing=3D2C
it will take up moisture from the atmosphere after firing=3D2C forming calc=
iu=3D
m hydroxide=3D2C
a reaction accompanied by expansion=3D2C which will cause popping or spalli=
ng=3D
of
the vessel surface."
=3D20
As she notes=3D2C the resulting calcium hydroxide pitting is the hallmark o=
f =3D
historic Acoma pottery. Traditional Acoma potters ground up a hard gray sha=
=3D
le=3D2C which when combined with crushed sherds allowed them to create very=
h=3D
ard=3D2C thin walled and white slipped vessels using pit kiln firing techno=
lo=3D
gy. I bought an Acoma wedding vase years ago and over time the delicate min=
=3D
eral paint design has been nearly obliterated due to pitting. At the base o=
=3D
f each pit is a very soft white mass that indicates that the shale also con=
=3D
tained a fair amount of calcite. I also have on my desk before me a small r=
=3D
eplica of a Pueblo III bowl that was on display at the Mesa Verde National =
=3D
Park Museum. I made and fired it using a native clay from SE Utah. Even tho=
=3D
ugh it was fired to vitrification (approximately Cone 10)=3D2C it too conti=
nu=3D
es to develop pits identical to those on the Acoma vase.=3D20
=3D20
Due in part to the pitting problem many Acoma potters have switched over to=
=3D
commercial clays. I have never used my calcite rich clay again=3D2C despit=
e =3D
the fact that native high fire clays are rare. I actually enjoy observing m=
=3D
y two pots as they have changed throughout my lifetime=3D2C but I doubt tha=
t =3D
most folks who buy your treasures would appreciate the technical process of=
=3D
carbonate decomposition.
=3D20
William A. Lucius=3D2C Ph.D. Board President and Director
Institute for Archaeological Ceramic Research (IACR)
iacr@msn.com=3D20
www.instituteforceramicarchaeology.org
http://www.ourlkcpage.multiply.com=3D20
http://www.leuppkilnconferenceorg.web.officelive.com
=3D
ivor and olive lewis on fri 25 mar 11
Dear Dr. William Lucius,
Thank you for putting forward a counter argument based on your experience
with clay objects that contain Calcium carbonate.
For its time, Anne O. Sheppard's manuscript would have been a welcome
publication. But there have been considerable changes in ideas relating to
the chemical and physical properties of ceramic raw materials since it was
published.
A better view of the ceramic landscape is given in a text book by Kingery,
Bowen and Uhlmann, "Introduction to Ceramics" ISBN 0-471-47860-1. Chapter 1=
0
on Grain Growth, Sintering and Vitrification seems to me to be a better
description of the ceramic process.
I appreciate your contributions to Clayart,
Best regards,
Ivor Lewis,
REDHILL,
South Australia
Neon-Cat on fri 25 mar 11
Dr. Lucius, hey Eric - Anna Shepard's 1956 work is a classic reprint.
However, the mid-1980's saw the beginning of a boom in technical
research in the field of ceramic archeology and elsewhere as new test
instruments became available.
My work with calcium-containing clay began about three years ago and I
turned to chemistry to solve the problem of hydroxide formation and
then began finding articles from all over the world that detail
potters=3D92 use of calcareous clays from all ages and places. I have
saved numerous articles. Many I cannot give a link to as they are
for-pay on-line. If you click this link
http://www.ims.demokritos.gr/archae/download/Maniatis_CV_Web(10).pdf
to one of my favorite researchers and his friends you will find on
pages 11 and 12 articles #11, #12, and #20 that have direct links for
these publications on calcium-bearing clays =3D96 there are even excellent
micrographs of vitrification at different (low) temperatures:
11. Y. Maniatis and M.S. Tite, "Technological Examination of
Neolithic-Bronze Age Pottery from Central and South East Europe and
From Near-East", J. Arch. Science, 8, 59-76, (1981).
12. Y. Maniatis, A. Simopoulos, A. Kostikas, "M=3DF6ssbauer studies of the
Role of Ca in Raw and Fired Clays", J. Amer. Ceram. Society, 64,
263-269 (1981).
20. Y. Maniatis, A. Simopoulos, V. Perdikatsis and A. Kostikas, "The
Effect of Reducing Atmosphere on Minerals and Iron Oxides Developed in
Fired Clays: The Role of Ca". J. of Amer. Ceram. Society, 66, 773-781,
(1983)
Early on the newer generation of ceramic archeology, Bronitsky
provided a comprehensive review of the physical properties of ceramics
and the methods by which they could be measured. Bronitsky and Hamer,
Feathers, Feathers and Scott, West, and Hoard et al. all provided
experimental data for the strength, toughness and, in some cases,
thermal shock resistance of low-fired pottery tempered mainly with
quartz sand, limestone and shell. I=3D92ve got a very long list of temper
materials that potters from around the world have (and are using).
That studio potters today aren=3D92t aware of the many very inventive
methods potters have successfully used in the past is rather a shame.
There is nothing preventing an inquiring potter of today from
learning, down to the atom or ion, everything he or she wishes to
know. To present any less than the best to fellow potters is like
going to visit one=3D92s beloved wearing underwear that=3D92s not been chan=
ged
all week. Here are some relevant articles on temper, especially as
pertains to the successful use of calcareous clay.
=3D93The Use of Materials Science Techniques in the Study of Pottery
Construction and Use=3D94, G. Bronitsky,.Advances in Archaeological Method
and Theory, Volume 9 (ed. M. Schiffer), Academic Press, New York, 1986
G. Bronitsky and R. Hamer, =3D93Experiments in Ceramic Technology: the
Effects of Various Tempering Materials on Impact and Thermal-shock
Resistance=3D94, American Antiquity, 51, 1986
J. F. Feathers, =3D93Effects of Temper on Strength of Ceramics: Response
to Bronitsky and Hamer, American Antiquity, 54, 1989
J. F. Feathers and W. D. Scott, =3D93Prehistoric Ceramic Composite from
the Mississippi Valley=3D94, Ceramic Bulletin, 68(3), 1989
S. M. West, =3D93Temper, Thermal Shock and Cooking Pots: a Study of
Tempering Materials and their Physical Significance in Prehistoric and
Traditional Cooking Pottery=3D94, Unpublished M.Sc. thesis, University of
Arizona, Tucson. 1992
R. J. Hoard, M. J. O'Brien, M. G. Khorasgany, and V. S. Gopalaratnam,
=3D93A materials-science approach to understanding limestone-tempered
pottery from the midwestern United States=3D94, Journal of Archaeological
Science, 22, 1995
EMAC'07 Budapest, 24-27 October 2007 24 L-02
COARSE CERAMICS WITH CALCITE INCLUSIONS: A TECHNOLOGY FOR ALL AGES
Bruno Fabbri -- Sabrina Gualtieri
C.N.R., Institute of Science and Technology for Ceramics, Faenza, Italy
This is just the tip of the iceberg on studies involving calcium
carbonate and may not represent my favorite articles, just some I had
handy. The truth is out there. We should all keep current. Calcareous
clay is a wonderful material =3D96 it=3D92s my most preferred clay.
I have these below saved as files (and a nice summary I did) along
with numerous other publications on just temper materials; I am not
sure if these are available on-line for free, but they are good for
quartz temper in low-fired ware:
"Toughening of Ceramic Earthenware by Quartz Inclusions: An Ancient
Art Revisited",
V. Kilikoglou, G. Vekinis and Y. Maniatis
Acta metall, mater. Vol. 43, No. 8, 1995
"Mechanical Performance of Quartz-Tempered Ceramics: Part I, Strength
and Toughness",
V. Kilikoglou, G. Vekinis, Y. Maniatis and P. M. Day
Archoeornetry 40.2, 1998
"Strength, Toughness and Thermal Shock Resistance of Ancient Ceramics,
and Their Influence on Technological Choice",
M. S. Tite, V. Kilikoglou, and G. Vekinis
Archaeometry 43, 3, 2001
I have to wonder if calcium in clays in the form of calcium sulfate
(CaSO4) materials (gypsum) and resulting CaS from mixed atmosphere or
reduction firing might not be the culprit in many reports of bad
experiences from potters in the past.
Between 212 =3D96 572 F (100 - 300 C), there=3D92s any needed dehydration o=
f
calcium sulfates through a number of steps until the anhydrate forms.
Around 658 F (348 C), the anhydrate converts in an exothermal reaction
to beta-calcium sulfate. It is unlikely to combine with silica to form
calcium silicates as calcium carbonate does. At 1219=3DB0C the
beta-calcium sulfate converts to alpha-calcium sulfate. Above cone 7,
2282 F (1250 C) the sulfate begins decomposition with conversion to
calcium oxide =3D96 this continues well above cone 13. Calcium sulfate in
reduction may form calcium sulfide (CaS). Calcium sulfide decomposes
in moist air to give the problematic hydroxide Ca(OH)2, as well as
Ca(SH)2, and Ca(SH)(OH). .
Marian Gooding
Neon-Cat Ceramics
www.neon-cat.com
http://www.flickr.com/photos/neon-cat/
(Most all the work in the last 2 years is done with calcareous clay =3D96
not a marvel, a fake, or a mystery =3D96 rather the result of working
smart)
Ron Roy on sun 27 mar 11
Just a gentle reminder - using any of the alkaline earths to flux high
fire clays will result in cristobalite formation - not what you want
in functional pottery.
Try fluxing a stoneware body at cone 10 with red art and that is what
you will get - that includes trying to flux a high fire body with
calcium and/or magnesium.
If anyone cares to do some experiments - or have a suspect clay
dilatometered to see if there is cristobalite let me know - depending
on the situation I do it for free. That means if I can see the recipe.
RR
Neon-Cat on sun 27 mar 11
Dear List,
I meant to also write that the phases (the things we create during
firing, like natural and synthetic minerals) are basically the same
whether I fire my calcium carbonate containing clay to Cone 05 or Cone
10. At low temps the calcium and silica (and aluminum, etc.) combine
to make mostly gehlenite and some anorthite. At Cone 10 I=3D92ll get some
gehlenite, more anorthite, along with mullite, maybe some
wollastonite, and possibly a few other things. No cristobalite at Cone
10 whether I use my white native clay or my red native clay. The
articles I referenced show this. There are hundreds of such articles.
If we think of what=3D92s made in a fired clay body and compare it to
baking, if I mix eggs, flour, butter, and sugar together and bake this
mix, the ingredients combine during heating to give us a cake or if we
add some chocolate, brownies. At the end of baking we don=3D92t still
have isolated eggs, flour by itself, butter we can let congeal and
collect, or sugar we can reuse in our morning coffee. We have cake. Or
brownies. I am not a cook, and this is but an extremely simplistic
example, but I hope you get the idea.
Scientists and researchers, as in the papers I referenced, detect
these phases using their equipment (not a dilatometer) as readily as
we see to read list posts. It=3D92s the way of the world now and new
information and ways of thinking about things can only benefit and
help us as studio potters and artists. Do you have to be able to read
high tech science and understand it? No. I do because I was trained to
do so, sometimes I find it relaxing, and I=3D92ve kept reasonably current
with advances in understanding since my BS in chemistry some years ago
(I was going for a PhD but had to withdraw due to progressive lyme
disease). You need only keep an open mind to avail yourself of the new
understanding unfolding around you.
If you want to take children or new clay students out on a field trip
to collect clay, make a few things, and then fire them and you know
the clay contains calcium carbonate in some form (limestone, sea
shells, fossil shells), all you have to do is follow the not too
complicated directions I gave in the post entitled =3D93Working with
Calcareous Clays=3D94. It is not hard and adds but a new paragraph to
basic explanations. If you=3D92re more adventuresome, try clay with
whiting as an ingredient, watching your time to bisque and the
temperature attained, allowing for temperature lag (what your kiln
temp reads where measured vs. the actual temp of your ware). Easy.
Some of our list-members work with talc-containing stoneware bodies
and have had good success for years =3D96 they will get a different set of
new products in their fired clay body but no cristobalite at our
normal firing temperatures. Industry makes use of MgO-containing
bodies for a lot of things and cristobalite is not an issue.
Marian Gooding
Neon-Cat Ceramics
www.neon-cat.com
Neon-Cat on sun 27 mar 11
Yes, Ron, around Orton Cone 27 or so -- if we were firing to 3000 F
(1650 C) plus -- this would be a problem (in a CaO-Al2O3-SiO2 system).
Not even Mel or some of our hot wood-firing potters are going to get
us there. I top out at cone 10 myself.
Artist=3D92s fantasies are so very interesting.
In a MgO-Al2O3-SiO2 system, we=3D92d need to get only to about 2912 F
(1600 C) to start producing cristobalite. Still a reach for me.
In a past post I mentioned that in the normal range for porcelain and
whiteware recipes in the K2O-Al2O3-SiO2 system cristobalite forms only
around 2732 =3D96 2912 F (1500-1600 C) with tridymite being common between
2192 =3D96 2552 F (1200 =3D96 1400 C). Na2O-Al2O3-SiO2 systems are similar=
,
with cristobalite generally appearing around 2912 F (1600 C).
Decades ago cristobalite was mentioned here and there in our
literature =3D96 people didn=3D92t know any better and scientists didn=3D92=
t have
the modern equipment and computer capabilities that they do now.
A dilatometer is not a proper tool for a clay body analysis. Any
assumptions about the existence of cristobalite in a fired clay body
taken from a mere dilatometric chart can never be anything but
guesswork regardless of the number of years one might have been
running tests using a dilatometer. Given the pioneering work by Bowen
and N. L. Schairer (Yale, Jan 1, 1947, Geophysical Laboratory Carnegie
Institution of Washington, American Journal of Science), and all the
others who followed with further research and refinement of silicate
systems, for us to worry and fret now over cristobalite is just
ludicrous. We've gone to the moon and sent vehicles out into space,
plumbed the ocean depths, and enjoy the Internet today because of the
hard work these men and women have done and are doing. Can anyone
really believe they are all wrong?
For my geeky science buds, here=3D92s a very interesting mind-spin:
Thermodynamic Assessment of the CaO=3D96Al2O3=3D96SiO2 System
Mao, Huahai and Hillert, Mats and Selleby, Malin and Sundman, Bo
(2006) Thermodynamic Assessment of the CaO=3D96Al2O3=3D96SiO2 System. Journ=
al
of the American Ceramic Society, vol. 89
http://docs.google.com/viewer?a=3D3Dv&q=3D3Dcache:WuWnJp-AOHEJ:oatao.univ-t=
oulo=3D
use.fr/2606/1/Mao_2606+oatao.univ-toulouse.fr/2606/1/Mao_2606&hl=3D3Den&gl=
=3D3D=3D
us&pid=3D3Dbl&srcid=3D3DADGEESif_ORr2fGTvrTH3pqeyW-tWESnP3A9N5M3szvZlXy5b59=
T7lH=3D
NkVvoVzyFSu_gLMJobRvvggGW9zsm5uRvoVnaQpnuLVlUE2nyfQ7KR6hK3laf9AkIo2aau4MFuN=
=3D
TqN_inPdJf&sig=3D3DAHIEtbTy_saj3ntYXOfFbuVPPNaWp0yJFA
If this link fails to open, just type the article title into Google
search then click on =3D93quick view=3D94 for the author=3D92s pdf copy of =
the
full text article. It=3D92s got good review of past research as well as
interesting science discussions of what=3D92s happening in the
CaO-Al2O3-SiO2 system.
For a peek at the crystalline phases in the MgO-Al2O3-SiO2 system,
check out the teaching aid from Higher Technological Institute Calkin=3DED
(Mexico). It=3D92s easy and simple (and correct):
http://www.itescam.edu.mx/principal/sylabus/fpdb/recursos/r68012.PDF
Ah, artists, ya gotta love them.
But when it comes down to making and firing, functional or
non-functional, get serious and please don=3D92t worry about cristobalite
=3D96 I don=3D92t.
Check the archives for the discussions on iron-bearing claybodies and
cristobalite -- cristobalite just isn't a concern with them either at
temperatures we can hope to obtain.
Marian Gooding
Neon-Cat Ceramics
www.neon-cat.com
The cat=3D92s back for a brief visit and is still on a round-the-world
freedom and liberty mission. I=3D92m rocking along to the cat=3D92s tunes f=
rom
Neelix like this one:
http://www.youtube.com/watch?v=3D3Dvn0GiLNngn4
Neelix: =3D93Give Them Free=3D94
On Sun, Mar 27, 2011 at 12:13 PM, Ron Roy wrote:
> Just a gentle reminder - using any of the alkaline earths to flux high
> fire clays will result in cristobalite formation - not what you want
> in functional pottery.
>
> Try fluxing a stoneware body at cone 10 with red art and that is what
> you will get - that includes trying to flux a high fire body with
> calcium and/or magnesium.
>
> If anyone cares to do some experiments - or have a suspect clay
> dilatometered to see if there is cristobalite let me know - depending
> on the situation I do it for free. That means if I can see the recipe.
>
> RR
David Beumee on mon 28 mar 11
Hi John,
The formation of cristobalite in sufficient quantity can be a problem in
utilitarian ware because, although cristobalite is chemically identical to
silica, its expansion and contraction properties are quite different,
meaning that cristobalite has a very pronounced expansion/contraction withi=
n
the range of heat used in the kitchen, enough to crack or split a piece. In
high fired utilitarian work, say a platter or casserole that gets used for
baking, if the oven heats a pot that contains a certain degree of
cristobalite to around 450 degrees F, you might hear a loud BANG and open
the oven to see dinner frying on the bottom of your oven and your casserole
in shards. It's a problem because not only do you lose that customer, you
lose a dozen of his or her friends who get told how unfortunate it was to
have purchased your work.
It's not necessarily true at all that a minimum of 20% spar is needed to
eliminate the formation of cristobalite in high fire clay bodies. It depend=
s
on the particular ingredients of the mix, but as you say, it's extremely
unlikely that a porcelain body would exhibit any cristobalite formation at
cone 10 because, as you know, porcelain clay bodies by nature need to
contain a higher percentage of flux (spar) to allow the highly refractory
ingredients to melt sufficiently. There's a true stoneware clay available
from Plainsman Clays in Alberta that can be used alone and has enough
natural flux in it to be a complete clay body all by itself. Such clays are
extremely rare in my experience.
In a sculptural piece that isn't used for utilitarian purposes you might
forget about the possible formation of cristobalite for a body that you
formulated and used yourself, but never if you're responsible for
formulating cone 10 bodies of any kind that will be made by a clay
manufacturer for general use by the public. Another benefit of the addition
of flux (feldspar) is that it causes the clay and silica content of the bod=
y
to fuse together to some lesser or greater degree, meaning a far stronger
fired product. Also, the introduction of flux in the form of slow melting
feldspar is indispensable in lowering the absorption of the fired piece to
an acceptable level for cone 10 high fire work, 2% or less for stoneware,
and just barely above 0% absorption for an everyday workhorse porcelain
body. Many translucent porcelain are far beyond 0% absorption. The more flu=
x
in relation to the clay and silica content of the body, the greater the
degree of translucency.
I do go on. Anyway, that's what all the fuss is about.
David Beumee
Porcelain by David Beumee
Lafayette, CO
On Mon, Mar 28, 2011 at 3:16 PM, John Britt wro=
te:
> David,
>
> Thanks for the reply.
>
> I really want to know why cristobalite is a problem ?
>
> I know about free silica and mullite formation and the ejection of free
> silica
> when kaolin is heated as well as the fluxing property of feldspar and tha=
t
> having over 20 feldspar will melt all the cristobalite in a body and make
> it
> cristobalite free, but I want to know why it is a problem unless you rehe=
at
> it to
> the temperature it expands approximately 3% (450 F). So in sculpture or
> dinnerware/ other objects excluding ovenware.
>
> I use a lot of cone 10 porcelain and so don't have cristobalite in my bod=
y
> but
> still want to know what all the fuss is about?
>
> I know that cristobalite is desirable in low-fire bodies because the
> correct
> amount can counteract crazing (a common problem in low fire work). I also
> know that naturally dug clays contain calcium and magnesium oxides and so
> cannot be removed.
>
> So I am just trying to find out why it so bad.
>
> Thanks,
>
> www.johnbrittpottery.com
>
>
>
John Britt on mon 28 mar 11
David,
Thanks for the reply.
I really want to know why cristobalite is a problem ?
I know about free silica and mullite formation and the ejection of free s=
=3D
ilica=3D20
when kaolin is heated as well as the fluxing property of feldspar and tha=
=3D
t=3D20
having over 20 feldspar will melt all the cristobalite in a body and make=
=3D
it=3D20
cristobalite free, but I want to know why it is a problem unless you rehe=
=3D
at it to=3D20
the temperature it expands approximately 3% (450 F). So in sculpture or=3D2=
0=3D
dinnerware/ other objects excluding ovenware.
I use a lot of cone 10 porcelain and so don't have cristobalite in my bod=
=3D
y but=3D20
still want to know what all the fuss is about?
I know that cristobalite is desirable in low-fire bodies because the corr=
=3D
ect=3D20
amount can counteract crazing (a common problem in low fire work). I also=
=3D
=3D20
know that naturally dug clays contain calcium and magnesium oxides and so=
=3D
=3D20
cannot be removed.=3D20
So I am just trying to find out why it so bad.
Thanks,
www.johnbrittpottery.com=3D20
David Beumee on tue 29 mar 11
Hi John,
My recommendation would be to make sure that any high fire clay body that
you use, sculptural or otherwise, would not create the formation of
cristobalite.
David
On Tue, Mar 29, 2011 at 10:39 AM, John Britt wr=
ote:
> David,
>
> Thanks for the lengthy reply.
>
> I can assume from reading your post that there is no problem with
> pottery/sculpture that has cristobalite as long as it is not used as oven
> ware? Is
> that correct.
>
>
>
>
John Britt on tue 29 mar 11
David,
Thanks for the lengthy reply.
I can assume from reading your post that there is no problem with=3D20
pottery/sculpture that has cristobalite as long as it is not used as oven=
=3D
ware? Is=3D20
that correct.
John Britt on wed 30 mar 11
Hey David,
The question is why?=3D20
I have yet to see a reason that it matters if the clay/sculpture is not r=
=3D
eheated=3D20
to 450 F.=3D20=3D20
This is the problem. Everyone harps on cristobalite continually and every=
=3D
one is=3D20
in fear mode. And yet having it in a body is a specific problem not an un=
=3D
iversal=3D20
problem. Potter's are always talking about keeping cristobalite out of b=
=3D
odies=3D20
but they don't even make ovenware.
If you fire consistently and have glazes that fit the body then whether o=
=3D
r not=3D20
your sculpture clay has cristobalite won't matter. (You have a 1 -2%=3D20
exp./cont. of quartz and a 3% exp./cont. of cristobalite.) If you make=3D2=
0=3D
anything that doesn't go into an oven, then you have no worries.=3D20
Since you have to remain above 2100 F for 3 or 4 hours to produce cristob=
=3D
alite=3D20
it is not likely a problem in cone 6 bodies. It is not a problem for porc=
=3D
elain=3D20
potters since any cristobalite is melted to glass. Rarely if at all at 04=
=3D
. So we=3D20
are only talking about cone 10 potters and woodfirers and only ones who m=
=3D
ake=3D20
ovenware. That is a pretty small percentage of potters for as many potter=
=3D
's=3D20
that are afraid of cristobalite.
I could be wrong but I really like reasons and not just fear mongering wh=
=3D
ich=3D20
helps no one. The reason I like to know is because if I am wrong I don't =
=3D
want=3D20
to be teaching this in my classes. I want a reasoned well thought out app=
=3D
roach=3D20
to clay and firing.
www.johbrittpottery.com
Taylor Hendrix on wed 30 mar 11
Hey John B.,
I'm wondering if, in addition to the ovenware issues already
discussed, cristobalite in problematic levels wouldn't create
unnecessary problems in the bisque and glaze firings. These are not
problems that I have much experience with, just book knowledge.
Taylor, in Rockport TX
wirerabbit1 on Skype (-0600 UTC)
http://wirerabbit.blogspot.com
http://wirerabbitpots.blogspot.com
http://www.flickr.com/photos/wirerabbit/
On Tue, Mar 29, 2011 at 11:39 AM, John Britt w=
rote:
> David,
>
> Thanks for the lengthy reply.
>
> I can assume from reading your post that there is no problem with
> pottery/sculpture that has cristobalite as long as it is not used as oven=
ware? Is
> that correct.
>
James Freeman on wed 30 mar 11
On Wed, Mar 30, 2011 at 2:48 PM, John Britt wro=
te:
The question is why?
I have yet to see a reason that it matters if the clay/sculpture is not
reheated
to 450 F.
This is the problem. Everyone harps on cristobalite continually and everyon=
e
is
in fear mode. And yet having it in a body is a specific problem not an
universal
problem. Potter's are always talking about keeping cristobalite out of
bodies
but they don't even make ovenware.
John...
I've personally never harped on cristobalite, and have never been in fear
mode about it, nor about much else related to pottery. I say this only to
frame my comment:
Beyond the ovenware problem, which you seem to be conceding, would
cristobalite not be a problem also to anyone who multi-fires their pieces?
I often high or mid-fire my sculptural pieces with only a liner glaze, then
glaze and refire them, sometimes many times, at all sorts of temperatures.
I have lost more than a few pieces on the way up, at low temperatures, afte=
r
a number of refirings. Not sure what caused it, but heard the distinctive
"pop" very early on. Lots of people re-fire, for lots of reasons, like
salvage, luster, enamel, china paint, decals, special effects...
So, IF cristobalite can form in the fired body, and IF it can cause the
piece to crack during reheatings, oven or otherwise, aren't you limiting
yourself if you make such a "flawed" body? (Not saying cristobalite can or
cannot form, that it is or isn't bad, or that it is a flaw, just saying tha=
t
IF it is...) And, if one happens to be a clay manufacturer, or a school
tech, or a group studio tech, how will one ensure that everyone knows that
Body X cannot be put in the oven or re-fired? What if the bisque sits on
the shelf for a year or two, and by the time someone gets around to glaze
it, no one remembers that it is made from the "don't re-heat" body? Is it
prudent, from a commercial perspective, to even manufacture such a body?
Even if you sell it as sculpture clay, what is to stop anyone from using it
otherwise, possibly to bad effect, which effect will accrue negatively to
you as the manufacturer of that "bad" clay, rather than to the user for an
inappropriate employment of your clay? Making such a clay kind of reminds
me of this:
http://m.thetandd.com/news/article_94322564-5505-11e0-940b-001cc4c002e0.htm=
l
Just my perspective, as someone who has absolutely no axe to grind regardin=
g
this topic.
All the best.
...James
James Freeman
"...outsider artists, caught in the bog of their own consciousness, too
preciously idiosyncratic to be taken seriously."
"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/resources
John Britt on wed 30 mar 11
Taylor,
There are no problems in the bisque that I know of.=3D20=3D20
And the only glaze issue I can think of is if you had glazes that were fi=
=3D
tting on a=3D20
body and then that body changed (due to clay source changes which had a l=
=3D
ot of=3D20
free silica as compared to the original) and then the new higher exp./con=
=3D
t. caused=3D20
glaze problems.=3D20
That is all I can think of,
John Britt
John Britt on thu 31 mar 11
James,
It is no big deal, I am just asking for a reason and no one can provide o=
=3D
ne.=3D20
They just harp on ovenware and get all potters in the fear mode.
First, I am not saying that if you are a clay producer you should not thi=
=3D
nk=3D20
about cristobalite. That would be stupid. The point is that clay producer=
=3D
s do a=3D20
good job and "usually" make great bodies. Most people don't make their ow=
=3D
n=3D20
clay and most people don't have their own body, they just buy what the=3D20=
=3D
company sells. So we can rely on them to design and produce excellent bod=
=3D
ies.
If you are refiring bodies then you are going to run into problems. The e=
=3D
xamples=3D20
you gave are vague and all inclusive and then you jump to the assumption =
=3D
that=3D20
it is cristobalite. That is exactly the point. The fear of cristobalite m=
=3D
akes you=3D20
assume that is the cause. It could as well be thermal variance in heating=
=3D
,=3D20
water in the piece from reglazing, firing too fast, etc. If you go slowly=
=3D
through=3D20
the 350 - 500 range even if you have cristobalite it won't crack. only ra=
=3D
pid=3D20
heating will crack it. Same if you go fast through quartz inversion. If y=
=3D
ou refire=3D20
porcelain to fast (around quartz inversion) it will crack but that is no=
=3D
t a flaw in=3D20
the body formulation.
You are also assuming that this problem will not happen with properly=3D20
formulated bodies. Clay bodies are not designed to be refired multiple ti=
=3D
mes=3D20
with no effect. Holding a body over 2100 for 3 - 4 hours (cumulative time=
=3D
- up=3D20
and down) will produce cristobalite. So multiple firings above 2100 F wou=
=3D
ld=3D20
increase cristobalite. No question.
Probably enough on this subject...peace out!
John Britt
ivor and olive lewis on thu 31 mar 11
John Britt asked David Beumee the following question :
"I can assume from reading your post that there is no problem with
pottery/sculpture that has cristobalite as long as it is not used as oven
ware? Is that correct. "
I would suggest plastic earths and clay bodies used in the construction of
ceramic sculpture should be treated with the same respect that is given to
studio pottery.
If the clay medium is one which will generate Cristobalite from free Silico=
n
dioxide as it matures and that form of Silica remains in place as the
article cools then it will be subject to a Phase Change at approx 225 deg C=
.
The cumulative stress caused by volumetric change as the atoms of silicon
and oxygen rearrange themselves may be sufficient to cause dunting. This is
not the same as thermal stress dunting where uneven heating or sudden
cooling causes stress due to unequal volumetric changes in an otherwise
uniform clay fabric.
Distinguishing between Thermal Expansion or Contraction events and Material
Phase Change events is crucial if the problems we face are to be understood=
.
Best regards,
Ivor Lewis,
REDHILL,
South Australia
Edouard Bastarache on thu 31 mar 11
John,
the "cristobalite fear" reminds me of the "lead fear" when I started doing
Clayart some 15 years ago.
Gis,
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://www.flickr.com/photos/30058682@N00/
http://edouardbastarache.blogspot.com/
http://www.facebook.com/edouard.bastarache
http://blogsalbertbastarache.blogspot.com/
----- Original Message -----
From: "John Britt"
To:
Sent: Thursday, March 31, 2011 8:36 AM
Subject: Re: Quartz link - nice
James,
It is no big deal, I am just asking for a reason and no one can provide one=
.
They just harp on ovenware and get all potters in the fear mode.
First, I am not saying that if you are a clay producer you should not think
about cristobalite. That would be stupid. The point is that clay producers
do a
good job and "usually" make great bodies. Most people don't make their own
clay and most people don't have their own body, they just buy what the
company sells. So we can rely on them to design and produce excellent
bodies.
If you are refiring bodies then you are going to run into problems. The
examples
you gave are vague and all inclusive and then you jump to the assumption
that
it is cristobalite. That is exactly the point. The fear of cristobalite
makes you
assume that is the cause. It could as well be thermal variance in heating,
water in the piece from reglazing, firing too fast, etc. If you go slowly
through
the 350 - 500 range even if you have cristobalite it won't crack. only rapi=
d
heating will crack it. Same if you go fast through quartz inversion. If you
refire
porcelain to fast (around quartz inversion) it will crack but that is not =
a
flaw in
the body formulation.
You are also assuming that this problem will not happen with properly
formulated bodies. Clay bodies are not designed to be refired multiple time=
s
with no effect. Holding a body over 2100 for 3 - 4 hours (cumulative time -
up
and down) will produce cristobalite. So multiple firings above 2100 F would
increase cristobalite. No question.
Probably enough on this subject...peace out!
John Britt
Neon-Cat on thu 31 mar 11
Hi again -- too much junk in the last post.
Dunting - a large crack through a piece =3D96 rounded crack edges if it
occurs during heating, sharper crack edges if fracture occurs during
cooling. This fault is often given as a reason to pursue testing for
cristobalite by Ron=3D92s method. But dunting due to cristobalite is not
a major problem. So what are some causes for dunting?
Although dunting can be caused by crystal inversions with volume
change (many crystalline substances do this, not just silica), it can
also be caused by the following, in no special order and certainly not
definitive: 1) high thermal expansion clay bodies, 2) large and/or
oddly-shaped pots and vessels that are subject to uneven heating and
cooling over the individual piece, 3) open flame-firing in some types
of kilns may show less favorable results than ware fired in a more
clean and static atmosphere, 4) over-firing, 5) too fast of firing, 6)
too quick of cooling, 7) opening the kiln too soon, 8) large or flat
ware placed directly on kiln shelves, 9) too much refiring of a vessel
or work, 10) ware damaged during making or sometime before firing, 11)
dull making and trimming tools, 12) poor joins, 13) uneven wall
thickness, 14) too thin or thick walls, 15) design failures =3D96 too many
corners and angular areas, 16) clay body texture =3D96 smooth, grog-free,
dense, or non-porous bodies are more prone to dunting, 17) too much
temper or grog, 18) glaze too thick especially in areas where it might
pool such as between throwing rings, or at the base of vessels, 19)
using a glaze that compresses the body too much, 20) in casting =3D96
areas with poor initial drainage during drying.
So, because of one man=3D92s wrong assumption about the nature of
cristobalite and its identification in a fired clay body by
dilatometry (old expression =3D96 =3D91to assume makes an ass out of you an=
d
me=3D92) and the publication of his position in =3D93Mastering Cone 6 Glaze=
s=3D94
and in an article in Studio Potter, we are now subject to the wrongful
condemnation of clay bodies and the materials of our trade =3D96 wake up
and get real. There is nothing wrong with making an honest mistake,
but to knowingly linger in error does have consequences for our work
and relationships, individually and collectively. Covering this error
of Ron=3D92s and all his statements based on the initial wrong assumption
are tearing the Clayart list apart. Good people have left the list,
others are turning away. I=3D92ve lost interest in participation, too,
beyond defending my own work that has been subject to unwarranted
attacks based on ignorance. I've fired my calcareous clay (calcium
carbonate-containing) bodies to cone 10 and repeatedly thrown them
roughly into a pit fire with no results but improvement. No
cristobalite worries here for me.
To the gal who keeps writing to tell me not to post - who are you? Got
any ideas to share with the list? Got any work to show? Just feeling
bitchy? Practicing to be a trained dog? I'll do what I think is best.
Marian
Neon-Cat Ceramics
Neon-Cat on thu 31 mar 11
Dear List, John - Here=3D92s an interesting article (link below) featuring
one of the alkaline earth materials that Ron Roy states will increase
cristobalite production in a clay body.
To summarize, it=3D92s a recipe with barium and the ingredients were
melted first at 1550 C (2822 F) and held for one hour then annealed at
800 C (1472 F) for 50 to 250 hours to show devitrification -
crystallization of various white, opaque, crystalline phases within a
dark, translucent amorphous glass. These were identified as BaAl2Si2O8
(hexacelsian, aka synthetic barium feldspar), BaSiO3 and BaCa2Si3O9.
No cristobalite.
The chemical composition of the glass was analyzed by x-ray
fluorescence (XRF), and crystalline phase identification was by x-ray
diffractometry (XRD) and scanning electron microscopy (SEM) with
energy dispersive x-ray spectrometry (EDS).
Dilatometry was used to give the linear thermal expansion curve of the
glass. The coefficient of thermal expansion (CTE) of glass between
25-640 C was 10.6x10-6 degrees C-1. The CTE of crystalline phases is
also given. It was shown that crystalline phases within an amorphous
glass will increase or decrease the CTE of the glass. In this study
with the given =3D93recipe=3D94, the CTE of the amorphous glass with the
crystalline phases after long-term devitrification at 800 C for 250
hours was 10.8x10-6 degrees C-1 or 1.61% higher than that of the
amorphous glass before devitrification.
Why am I showing you this article?
1) To once again demonstrate the right use of analytical equipment (in
this case XRF, XRD, SEM-EDS) to ascertain crystalline phases;
2) To show you an example of the right use of dilatometry to obtain
the =3D93chart=3D94 or linear thermal expansion curve for materials;
3) To counter the statements by Ron that alkaline earth elements
increase the production of cristobalite.
The article demonstrates that even after firing to a very, very high
temperature followed by an extremely slow cooling designed to develop
all the possible phases, cristobalite is not present in the
BaO-CaO-Al2O3-B2O3-SiO2 under study nor was cristobalite found in
referenced sections of the article. A table for crystalline phases in
alkaline earth systems (those with magnesium - Mg, calcium - Ca,
strontium - Sr, and barium - Ba) is given on page 2.
4) To demonstrate that the crystalline phases within the amorphous
glass change the coefficient of thermal expansion of the glassy phase
and thus the entire material.
5) When Ron tells you he can identify cristobalite in a fired clay
body based on a dilatometer chart reading this is nothing but a very
unfortunate assumption on his part that began many years ago. A fired
clay body may give a rise in the region where one might expect
cristobalite but the rise may be due to many other factors and unless
the proper equipment is used to determine the actual phases present in
a fired clay body, there is no way to say for sure what is causing an
area with abrupt rise in a dilatometric chart. American-trained
ceramic engineers J. T. Jones and M. F. Berard, writing in =3D93Ceramics:
Industrial Processing and Testing=3D94 in 1972, state that quartz, when
liquid is present, may transform to tridymite above 867 C (1593 F) and
cristobalite above 1470 C (2678 F). This is an excellent generality
for the time in which it was written. So, although I may quote, use,
or throw new articles in front of you, the correct idea behind quartz
transformation has been known, at least in industry, for quite some
time. Let=3D92s catch up and think right.
If we do not fire to temperatures high enough to favor the formation
of cristobalite, what is Ron =3D93seeing=3D94? He suggests eliminating a wh=
ole
host of materials and suggests upping the flux. More flux equals more
glassy melt. Of course the chart reading will different. Is it better?
Depends on what you need. Materials will expand and will do so at
different rates. Ron=3D92s charts show only how the entire fired clay body
responds as it is heated and the final linear expansion to a set
temperature. This might be useful to know if it indicated that one
might want to re-fire (or possibly use) a clay body more carefully
through a certain temperature range, or, as dilatometry has been
traditionally used - to help match glaze and clay body for a better
fit. To take the original wrong assumption that we are subject to
cristobalite formation at every turn and that this alleged
cristobalite can be detected in a fired clay body by dilatometry and
then continue with wrong assumptions of how different oxides (fluxes)
affect cristobalite formation is just irresponsible. The different
crystalline substances embedded in the glassy-phase of our fired clay
body will increase or decrease the CTE of the glassy phase and hence,
the overall CTE of our clay bodies.
6) Iron =3D96 it=3D92s just another oxide that combines readily with silica
and the other ingredients in our clay body to create new products
(fayalite and others) during firing. Each of these new products may
effectively increase or decrease the CTE of the entire clay body or
cause areas of rapid rise in a dilatometry chart. Iron has been shown
to increase stability in glasses, glazes, and human ceramic implants
as far as leaching is concerned. One might be safe to assume these
benefits apply also to fired clay bodies =3D96 but I=3D92m not big on
assumptions and iron chemistry in clay science is an enormous field.
7) Dunting =3D96 covered in a separate post
A Barium-Calcium Silicate Glass for Use as Seals in Planar SOFCs
http://www.science.cmu.ac.th/journal-science/372_07PrachayaAbarium.pdf
Marian Gooding
Neon-Cat Ceramics
www.neon-cat.com
Paul Herman on thu 31 mar 11
Hello Edouard, John and All,
I remember quite a few things that have gotten people all worked up.
Lead, barium, silica, and lately the threads on flame ware and now
(again) cristobalite. I remember being admonished not to make
casseroles at all, because of the fearsome possibility of litigation.
The tactic seems to be intended to stop all conversation on the
subject, and stop the free exchange of ideas. If that is not the
intent, it surely is the result.
In fact, people do this kind of thing on blogs and internet discussion
groups with such regularity that there is now a phrase expressly for
this activity. Now please tell me if this definition doesn't fit some
clayartors to a tee. I found it on wikipedia.
A person who posts on a blog thread, in the guise of "concern," to
disrupt dialogue or undermine morale by pointing out that posters and/
or the site may be getting themselves in trouble, usually with an
authority or power. They point out problems that don't really exist.
The intent is to derail, stifle, control, the dialogue. It is viewed
as insincere and condescending. >
I'm all for free speech, and continued discussion of any subject, for
as long as we darn well please.
best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Mar 31, 2011, at 8:37 AM, Edouard Bastarache wrote:
> John,
> the "cristobalite fear" reminds me of the "lead fear" when I started
> doing
> Clayart some 15 years ago.
>
> Gis,
>
> Edouard Bastarache
> Spertesperantisto
Taylor Hendrix on thu 31 mar 11
Yes, the disingenuous labeling of "crystobalite fear" does remind me
of the "lead fear" label. Sorry, it does not help an argument to call
one's hombre a "fear monger" nor does it invite sincere discussion to
be suspiciously naive.
Ax grinding workshop is NEXT week.
Taylor, in Rockport TX
wirerabbit1 on Skype (-0600 UTC)
http://wirerabbit.blogspot.com
http://wirerabbitpots.blogspot.com
http://www.flickr.com/photos/wirerabbit/
On Thu, Mar 31, 2011 at 10:37 AM, Edouard Bastarache w=
rote:
> John,
> the "cristobalite fear" reminds me of the "lead fear" when I started doin=
g
> Clayart some 15 years ago.
>
> Gis,
>
> Edouard...
Edouard Bastarache on thu 31 mar 11
Taylor,
I am not very good at writing in English but you goofed spelling
"cristobalite",,,
I have already sent maybe 3-4 messages to Clayart, in recent years,
concerning my experiments making high-fired reduction clays without spar.
I did not encountered a single case of cracks among 70 trials; that it is
why
I mentionned the "cristobalite fear".
Later friend,
Gis,
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://www.flickr.com/photos/30058682@N00/
http://edouardbastarache.blogspot.com/
http://www.facebook.com/edouard.bastarache
http://blogsalbertbastarache.blogspot.com/
----- Original Message -----
From: "Taylor Hendrix"
To:
Sent: Thursday, March 31, 2011 1:38 PM
Subject: Re: Quartz link - nice
> Yes, the disingenuous labeling of "crystobalite fear" does remind me
> of the "lead fear" label. Sorry, it does not help an argument to call
> one's hombre a "fear monger" nor does it invite sincere discussion to
> be suspiciously naive.
>
> Ax grinding workshop is NEXT week.
>
>
> Taylor, in Rockport TX
> wirerabbit1 on Skype (-0600 UTC)
> http://wirerabbit.blogspot.com
> http://wirerabbitpots.blogspot.com
> http://www.flickr.com/photos/wirerabbit/
>
>
>
> On Thu, Mar 31, 2011 at 10:37 AM, Edouard Bastarache
> wrote:
>> John,
>> the "cristobalite fear" reminds me of the "lead fear" when I started
>> doing
>> Clayart some 15 years ago.
>>
>> Gis,
>>
>> Edouard...
>
ivor and olive lewis on fri 1 apr 11
The American Ceramic Society, Inc. publish an extensive collection of Phase
Equilibria Diagrams. The complete collection on CD Rom is rather expensive.
But even their 1964 volume covers many of the mixtures we choose to use as
glazes and clay bodies. They also have Wall Charts of Aluminosilicate
systems related to four common fluxing oxides. (CaO, MgO, K2O and Na2O)
One of the advantages of having access to this kind of information is that
it enables experimenters and producers to make informed predictions about
what might happen when mixtures of ceramic raw materials are heated, melted
then cooled .
I feel that using and exploiting this resource should be part and parcel of
any post secondary courses leading to an award in the Ceramic Arts.
Best regards,
Ivor Lewis,
REDHILL,
South Australia
Eric Hansen on sat 2 apr 11
http://ceramics.org/publications-and-resources/
- that is the useful link for those of you who find your wallet open -
On Fri, Apr 1, 2011 at 2:46 AM, ivor and olive lewis
wrote:
> The American Ceramic Society, Inc. publish an extensive collection of Pha=
se
> Equilibria Diagrams. The complete collection on CD Rom is rather expensiv=
e.
> But even their 1964 volume covers many of the mixtures we choose to use a=
s
> glazes and clay bodies. They also have Wall Charts of Aluminosilicate
> systems related to four common fluxing oxides. (CaO, MgO, K2O and Na2O)
> One of the advantages of having access to this kind of information is tha=
t
> it enables experimenters and producers to make informed predictions about
> what might happen when mixtures of ceramic raw materials are heated, melt=
ed
> then cooled .
> I feel that using and exploiting this resource should be part and parcel =
of
> any post secondary courses leading to an award in the Ceramic Arts.
> Best regards,
> Ivor Lewis,
> REDHILL,
> South Australia
>
--
Eric Alan Hansen
Stonehouse Studio Pottery
Alexandria, Virginia
americanpotter.blogspot.com
thesuddenschool.blogspot.com
hansencookbook.blogspot.com
"Simplify, simplify, simplify" - Thoreau
Neon-Cat on thu 14 apr 11
Phase diagrams are available all over the net these days. They are
being updated as new information comes in and is confirmed. One need
not have tons of money to locate a phase diagram. Texts also include
phase diagrams.
A new book I got today is quite nice; I'm just looking it over. The
authors are well-connected and well-respected:
Ceramic Materials, Science and Engineering
C. Barry Carter and M. Grant Norton
Springer, 2007
It deals with a lot out of our area and much in our area. The tech
talk sections usually include a pottery mention as far as relevance
and application go. I am going to enjoy the book very much. No
cristobalite scare anywhere. (Cristobalite is not even mentioned in
some of my other modern ceramic books.)
As I mentioned months ago in some post, the cristobalite polymorph is
formed from tridymite in what is called a reconstructive phase
transformation -- serious stuff requiring the breaking of bonds and a
lot of movement for the tridymite structure to transform to
cristobalite. It's a difficult process at best. Materials go for easy
solutions, just like we do.
Here (below) is the order of how the silica polymorphs convert to one
another. This is long-accepted science and is beyond refute now. But
do remember that our silica, be it from sand or released from clays,
wants to combine with other clay body substances to create new
substances -- in this day and age with the material processing
utilized, there just is not going to be lots of uncombined silica to
transform into other polymorphs (polymorphs are materials that have
the same chemical composition but different crystal structures). Both
tridymite and cristobalite can be utilized as the alpha and beta
quartz is used, for example, when making wollasonite or fayalite or
the glassy phase or whatever. Pressure, temperature, or a combination
of both are needed to affect silica transformations. In our kiln
firing we only deal with temperature changes. Displacive phase
transformations as mentioned below require atoms to move only slightly
relative to one another -- low temperature forms (the alpha phase) are
a distortion of the high temperature forms (the beta phase). Alpha to
beta structural changes are displacive phase transformations and are
relatively easy and rapid.
low quartz <---> high quartz
displacive at 573 C
high quartz <---> high tridymite
reconstructive at 867 C
Should we get high tridymite, two things can happen. The path one way
gives us middle tridymite via a displacive phase transformation at 160
C with another displacive transformation following on its heels to
give low tridymite at 105 C. The reverse is also true.
Or, we can go from high tridymite to high cristobalite at 1470 C
through the more difficult reconstructive phase transformation
process. After that high cristobalite transforms to low cristobalite
through a displacive phase transformation at 200 +/- 35 C. The reverse
is also true here, too.
Once in awhile cristobalite will already be present in some of the
materials we use (some native clays from the SW, sometimes as a small
percent in mullite, etc.). Potters have been known to add cristobalite
on purpose (for clay body-glaze fit issues). The non-equilibrium
conditions of our kiln firings also may have cristobalite form for a
time, say at 1300 C, only to disappear as work is fired slightly
higher. But all this worry just seems so needlessly awful to me. So
does the condemnation of perfectly wonderful alternative clay bodies
based on unfounded information. As a newcomer trying to defend correct
clay science I find the out-of-hand condemnation of my own work and
clay bodies (and that of my friends) quite trying. I know how things
work and am perfectly happy knowing this on my own, but I do like to
share some things, some times -- it often seems that there is no
"nice" way of doing so. So I encourage everyone to seek good, modern
information should they have a desire to learn a bit about the clay
science behind what they do.
Handy schematic diagrams of these silica transformation processes are
in many texts these days (the book mentioned above included). It's
real; it's what's happening. Learning better craft techniques is a
small price to pay for freeing the studio potters' world of needless
cristobalite concerns.
Anyway, I'll be around but not posting as often. I really have got to
get a handle on marketing this year so I'll be busy trying to figure
that out - a gal's got to eat. It's not easy for me. I wish it were
all as easy as making things with clay or understanding clay
science...
Marian
Neon-Cat Ceramics
www.neon-cat.com
On Sat, Apr 2, 2011 at 10:16 AM, Eric Hansen
wrote:
> http://ceramics.org/publications-and-resources/
>
> - that is the useful link for those of you who find your wallet open -
>
ivor and olive lewis on fri 15 apr 11
Yes,
Ceramic Materials, Science and Engineering
C. Barry Carter and M. Grant Norton
is a comprehensive book. Interesting price range.
The utility of ACerS Wall charts of the main ceramic systems to studio
potters and ceramic artists resides in their scale. Yes, there is an
abundance of Phase Equilibrium Diagrams to be found trolling the Web. But
from how many of those can a person estimate compositions to one tenth of
one percent if they are exploring a specific temperature range for firing o=
r
estimating the potential firing range of a particular clay body or glaze?
Favoured reading this month is Ernest G. Ehlers, "The Interpretation of
Geological Phase Diagrams". 1972. ISBN 0-7167-0254-1.
Best regards,
Ivor Lewis,
REDHILL,
South Australia
Neon-Cat on sat 16 apr 11
Gee, Ivor, why calculate at such a small scale to "estimate" compositions?
Phase diagrams are nice up to a point. They provide a general guideline.
We are always working at non-equilibrium conditions. And we're all
working under different conditions.
Despite my seeming exactness at times, if it is not easy and
convenient (a clay body or glaze) I don't want to mess with it. I am a
"pinch of this, pinch of that" kind of gal but do keep records and do
measure the pinches and pats for reference or in case someone asks.
I like what I do to be fun.
For example, I might make alpha or a higher brass ceramic coating with
zinc and copper but would never attempt to reduce copper with zinc. A
phase diagram would tell me relative temperature ranges to work in.
Ellingham diagrams might be fun to look at -- briefly.
Just this morning I spent a pleasant 45 minutes reading about graphite
so I know at just what temperature (in my electric kiln, oxidation) I
want to fire the pieces I made with graphite in porcelain yesterday
(while listening to a rockin' CD and imagining the happy hub-bub of
Seattle NCECA and the clayart runway -- good lord, doesn't that man
know that cats love little danglie things??...). I know now what to
expect should I fire all the work in the pit enclosure, too.
There is no profit (time-wise, result-wise) in over-stepping what one
can understand and use. Keeping it simple can be a wonderful way of
working. For those who have gotten their feet wet or for the
experienced potter, working intuitively quite often produces fabulous
results.
Science should serve our needs not make one a slavish servant who
thinks one must have this chart, this book, that set of expensive
diagrams, etc. before one can make anything, enjoy making it, and then
go on to share and talk intelligently about our mutual passion.
What will you make after your "favoured reading"?
Just curious.
Science should be fun -- and useful.
Marian
Neon-Cat Ceramics
On Fri, Apr 15, 2011 at 1:20 AM, ivor and olive lewis
wrote:
> Yes,
> Ceramic Materials, Science and Engineering
>
> C. Barry Carter and M. Grant Norton
>
> is a comprehensive book. Interesting price range.
>
>
> The utility of ACerS Wall charts of the main ceramic systems to studio
> potters and ceramic artists resides in their scale. Yes, there is an
> abundance of Phase Equilibrium Diagrams to be found trolling the Web. But
> from how many of those can a person estimate compositions to one tenth of
> one percent if they are exploring a specific temperature range for firing=
=3D
or
> estimating the potential firing range of a particular clay body or glaze?
> Favoured reading this month is Ernest G. Ehlers, "The Interpretation of
> Geological Phase Diagrams". 1972. ISBN =3DA00-7167-0254-1.
Edouard Bastarache on sat 16 apr 11
Hello Neon,
I agree, Denis Caraty, aka Smart.Conseil does not use phase diagrams at the
Gien Factory in France; so why us dumb potters, shoulsd use them...
http://www.gien.com/cms/accueil.php
Gis,
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://www.flickr.com/photos/30058682@N00/
http://edouardbastarache.blogspot.com/
http://www.facebook.com/edouard.bastarache
http://blogsalbertbastarache.blogspot.com/
ivor and olive lewis on sun 17 apr 11
Dear Marian,
You ask: "What will you make after your "favoured reading"?"
Why should I do anything? To gain insight is reward enough in the short ter=
m
Enjoy your work.
Regards,
Ivor
Neon-Cat on mon 18 apr 11
Why NOT do something, Ivor?
But I understand. Really. I have a new thermodynamics of metallurgical
processes text I sometimes read to still my heart after reading
clayart. I am wondering about Boa color now -- for NCECA Seattle. And
I'm still pondering donkeys. And why some seem not to have outgrown
guilt (the need for priests and all that). But the reading does give
me ideas and spring boards for actual work. One day apprentices would
be nice -- so many ideas, so little time...
I truly do hope you will gain insight!
(although I don't quite get the concept of short-term. Insight IS, so
how can it then be short-term? But whatever...back to hot tubs and
thangs...my work is done and it was fun, although I wish it was
instantly dried and fired so I can see how it turns out...)
Happy reading, Ivor.
Marian
On Sun, Apr 17, 2011 at 1:16 AM, ivor and olive lewis
wrote:
> Dear Marian,
>
> You ask: "What will you make after your "favoured reading"?"
>
> Why should I do anything? To gain insight is reward enough in the short t=
erm
>
> Enjoy your work.
>
> Regards,
>
> Ivor
>
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