Neon-Cat on sat 22 may 10
Glauconite is a green, alkaline, iron-rich, non-expandable,
dioctahedral, 2:1 layer phyllosilicate of the illite group with a
layer charge that is satisfied by potassium ions (K+). Glauconite is
sometimes called green mica or glauconitic mica and has been widely
used as a green earth pigment by non-clay artists. It formed eons ago
in marine environments all over the world. Glauconite has the ideal
structural formula of K0.8(FeIII1.0,Al0.4,Mg0.6)(Si3.8Al 0.2)O10(OH)2
but in real life it is highly variable depending on where in the world
it is collected or mined.
Texas greensand that contains glauconite has about 19=3D9622% iron and
5-7% potassium. Glauconite is characterized by a low disintegration
temperature of the crystal lattice and a low melting point. Using
greensand adds strength to both greenware and fired ware, helps bind
unfired clay bodies, acts as a flux decreasing sintering temperature,
and helps reduce the porosity and permeability of finished fired
products while increasing the intensity of their (red) color.
When fired, glauconite dehydrates between 80=3D96300 C forming a fully
oxidized ferric glauconite. Dehydroxylation then occurs, with most
taking place as the lattice distorts between 520=3D96825 C. Its maximum
stable temperature is 910=3D96970 (average 945 C) and it starts forming
liquid in this temperature range. In an oxidation atmosphere beginning
about 860 to 970 C glauconite transforms to maghemite (gamma-Fe2O3)
plus an amorphous material (sometimes with some trydimite). Upon
further heating (1000-1200 C), maghemite inverts to hematite
(alpha-Fe2O3) accompanied by a glassy phase. In the clay bodies large,
shiny deep-red, red-brown, to dark-brown hematite forms complete with
a fine metallic luster.
Increasing the firing temperature from, say pit fire or pellet stove
to cone 06 and firing for four hours as opposed to 30-50 minutes,
hematite becomes progressively more iron-rich and the amount of Fe3+
that incorporates into the crystal lattice of other compounds that are
forming diminishes as compared to reactions of red clay bodies that
potters typically use (kaolin and ball clay). In clay bodies using
greensand the red color coordinate increases and the ware or glaze
darkens and becomes deep-red to reddish-brown. The finer the mesh-size
of glauconitic greensand the more the color will intensify.
Discrete bright grains of red hematite are visible up close in all
glazes and vitreous engobes. Both cone 10 firings for clay bodies and
glazes took over 14 hours to reach peak (2230 F with slight holds).
Glazed ware was bisqued to cone 05 prior to glaze application; clay
bodies were, for the most part, all single fired. The glauconite slips
and glazes in the photos were only screened to 80 mesh and mixed by
hand but were very well behaved and their application was smooth;
glazes tended to stay in a uniform suspension that seldom requires
mixing. Illites generally have a CEC (cation exchange capacity in
meg/kg) between 200=3D96500 and one studied glauconite had a CEC of
50=3D96400. Hematite has a CEC of up to 1000 at pH >8.3. By comparison
kaolin has a CEC of only 30-50.
The clay bodies in the photo examples on Flickr have been screened
only to 17-20 mesh. They were mixed by hand. Larger agglomerations of
hematite formed in the fired ware and are readily visible. In the
photos the charm of their in-person look is somewhat lost =3D96 in person
the clay bodies sparkle in a toasty-orange-brown background, some
brown-red, some dark red, some almost black, while others have a
metallic sheen. On unglazed clay bodies a semi-gloss, 1/64th inch
glaze-layer forms on vessels wall surfaces and surrounds the large
dramatic hematite particles.
Like any clay, glauconite may form beautiful and durable sandstones.
At one time in east Texas glauconitic stone was mined for building.
Here=3D92s a good photo of a lovely old church built with green
glauconitic stone around the year 1200 in Germany as posted by Frank
Buin on TrekEarth:
http://www.trekearth.com/gallery/Europe/Germany/West/Nordrhein-Westfalen/So=
=3D
est/photo1030824.htm
Here is an unofficial analysis of the Texas greensand I use:
Silicon - 25.00 % - SiO2 - 53.48
Aluminum -1.90 % - Al2O3 - 3.58
Sodium - 0.27 % - Na2O - 0.36
Potassium - 5.49 % - K2O - 6.62
Magnesium - 2.28 % - MgO - 3.78
Iron-19.62 % - FeO - 3.37
- Fe2O3 - 24.31
Trace: zinc, nickel, copper, phosphorus, and 14-19 additional elements
There is a distinct correlation between greensand color and its chemistry:
Blue-green pellets: K2O 7-10%; contain Mg (>4.8), and have very low Al;
Dark-green: increased Fe2+ and K;
Bright-green pellets: K2O <7% with MgO (<4.8) and Al2O3 (<14.4%);
Olive-brown =3D96 K2O <3.8% and high Al2O3 (14 =3D96 28%);
Light brown: Al2O3 rich with SiO2, Na2O, and CaO, but with less K2O and Fe2=
=3D
+;
Deep-brown: may contain TiO2, Cr2O3, or be Al enriched but depleted in Fe a=
=3D
nd K.
These initial clay body experiments at high temperature utilized
primarily OM-4 ball clay (two pieces were formed from a Gold
Art-greensand mix). I don=3D92t have scales sensitive enough to measure
absorption, but shrinkage was reduced from values for the OM-4 and
Gold Art alone. The bodies did not crack, slump, or warp and stayed
right were I put them while being plastic enough to perform well when
coiled.
OM-4 & greensand (34/34 plus mix):
Shrinkage:
dry =3D96 2%
at cone 06-05 =3D96 3%
at cone 10 oxidation =3D96 10%
Fired color:
at cone 06-05 =3D96 red to orange with small white specks
at cone 10 oxidation =3D96 medium orange-brown to golden-brown with large
dark to metallic hematite filling pores.
Photos of greensand, clay bodies containing greensand (fired and
unfired), as well as some fired versions of glazes, slips, and engobes
are posted on Flickr:
http://www.flickr.com/photos/neon-cat/
Marian
Neon-Cat
www.neon-cat.com
http://www.flickr.com/photos/neon-cat/
Eric Hansen on sat 22 may 10
Marian: Very very interesting. It causes me to wonder in more technical
terms about some of the materials I have here east of the Appalachians, kin=
=3D
d
of a coarse brownish stoneware clay which fires red at mid to high temp.
Pretty sure its NOT an illite clay, in fact lucky to find samples which are
plastic enough. A lot of samples are very sandy, and some of them are
clearly just a mixture of quartz sand, mica sand, and what I assume is
feldspar sand. When fired, it will have bits of melted white material which
has a surface tension to it, it looks like melted feldspar to me. It has
some similarities to Shigaraki clay, but not quite as refractory, it fires
to a darker color. Other samples are just compressed mica mudstone. Oh yes
and there are round quartzite pebbles and field stones in it, of pure
quartzite. Pottery is just so amazing. People otherwise just take their
"dirt" for granted.
h a n s e n
On Sat, May 22, 2010 at 6:56 AM, Neon-Cat wrote:
> Glauconite is a green, alkaline, iron-rich, non-expandable,
> dioctahedral, 2:1 layer phyllosilicate of the illite group with a
> layer charge that is satisfied by potassium ions (K+). Glauconite is
> sometimes called green mica or glauconitic mica and has been widely
> used as a green earth pigment by non-clay artists. It formed eons ago
> in marine environments all over the world. Glauconite has the ideal
> structural formula of K0.8(FeIII1.0,Al0.4,Mg0.6)(Si3.8Al 0.2)O10(OH)2
> but in real life it is highly variable depending on where in the world
> it is collected or mined.
>
> Texas greensand that contains glauconite has about 19=3D9622% iron and
> 5-7% potassium. Glauconite is characterized by a low disintegration
>
>
Neon-Cat on sat 22 may 10
Eric, here is where much of the glauconite can be found in Virginia:
"Some stages in the disintegration of Glauconite"
Benjamin Gildersleeve, University of Virginia
Journal of the Mineralogical Society of America
http://www.minsocam.org/ammin/AM17/AM17_98.pdf
I was born in D.C. and lived in Arlington and Annandale for many
years. Back then clay and geology and things related were not in
consciousness at all. I'm not sure I even realized potters existed.
Had many great times in Old Town. Other parts of Alexandria were handy
on weekends back when there were Blue Laws...
Thanks for the note.
Have a great weekend!
Marian
On Sat, May 22, 2010 at 12:04 PM, Eric Hansen
wrote:
> Marian: Very very interesting. It causes me to wonder in more technical
> terms about some of the materials I have here east of the Appalachians, k=
ind
> of a coarse brownish stoneware clay which fires red at mid to high temp.
> Pretty sure its NOT an illite clay, in fact lucky to find samples which a=
re
> plastic enough. A lot of samples are very sandy, and some of them are
> clearly just a mixture of quartz sand, mica sand, and what I assume is
> feldspar sand. When fired, it will have bits of melted white material whi=
ch
> has a surface tension to it, it looks like melted feldspar to me. It has
> some similarities to Shigaraki clay, but not quite as refractory, it fire=
s
> to a darker color. Other samples are just compressed mica mudstone. Oh ye=
s
> and there are round quartzite pebbles and field stones in it, of pure
> quartzite. Pottery is just so amazing. People otherwise just take their
> "dirt" for granted.
> h a n s e n
Eric Hansen on sun 23 may 10
Marian: Excellent - that is right near here & makes more sense than trying
to figure out what to do with other Virginia materials -
h a n s e n
On Sat, May 22, 2010 at 5:07 PM, Neon-Cat wrote:
> Eric, here is where much of the glauconite can be found in Virginia:
>
> "Some stages in the disintegration of Glauconite"
> Benjamin Gildersleeve, University of Virginia
> Journal of the Mineralogical Society of America
> http://www.minsocam.org/ammin/AM17/AM17_98.pdf
>
> I was born in D.C. and lived in Arlington and Annandale for many
> years. Back then clay and geology and things related were not in
> consciousness at all. I'm not sure I even realized potters existed.
> Had many great times in Old Town. Other parts of Alexandria were handy
> on weekends back when there were Blue Laws...
>
> Thanks for the note.
> Have a great weekend!
>
> Marian
>
> On Sat, May 22, 2010 at 12:04 PM, Eric Hansen
> wrote:
> > Marian: Very very interesting. It causes me to wonder in more technical
> > terms about some of the materials I have here east of the Appalachians,
> kind
> > of a coarse brownish stoneware clay which fires red at mid to high temp=
.
> > Pretty sure its NOT an illite clay, in fact lucky to find samples which
> are
> > plastic enough. A lot of samples are very sandy, and some of them are
> > clearly just a mixture of quartz sand, mica sand, and what I assume is
> > feldspar sand. When fired, it will have bits of melted white material
> which
> > has a surface tension to it, it looks like melted feldspar to me. It ha=
s
> > some similarities to Shigaraki clay, but not quite as refractory, it
> fires
> > to a darker color. Other samples are just compressed mica mudstone. Oh
> yes
> > and there are round quartzite pebbles and field stones in it, of pure
> > quartzite. Pottery is just so amazing. People otherwise just take their
> > "dirt" for granted.
> > h a n s e n
>
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