Stephen Grimmer on sun 4 mar 01
I'll venture a guess at this one. The systems interesting to us potters,
Al2O3-SiO2, Cao-Al2O3-SiO2, Na2O-Al2O3-SiO2 and K2O-Al2O3-SiO2, can all
include kaolin as a component throughout much of the diagram. But, to get to
the edges along the alkali/alkaline earth-silica and alkali/alkaline
earth-alumina lines requires the use of either silicon or aluminum oxides,
but not kaolin, which introduces both oxides. My guess is that for
consistency's sake, the phase diagrams are done with straight oxides in
mind, even though kaolin and other multiple-oxide suppliers can and may well
be used at many points in the diagrams.
Just a guess.
Southern Illinois University at Carbondale
> From: iandol
> Subject: Kaolin and Eutectics
> After correspondence with David Hewett about his article on Eutectics and
> Phase Diagrams which made me rethink some of my ideas I started to have a look
> through the pictures that most concern potters, which include the Al2O3-SiO2,
> Cao-Al2O3-SiO2, Na2O-Al2O3-SiO2 and K2O-Al2O3-SiO2 systems.
> Now it seems commonly accepted in the literature written for potters that a
> temperature called the Eutectic point becomes opperative during heating of
> clays and glazes, assisting the process and ensuring maturity is achieved. So
> I would expect that Kaolin would feature some where in those phase diagrams
> since it seems to feature in most clays and many glaze recipes.
> It does not. The diagrams relating to Kaolin are at the back of the book,
> round about No 2011, System Al2O3-SiO2-H2O and they relate to the geological
> formation under high pressure of several minerals.
> So if neither Kaolinite nor metakaolin are not featured in the diagrams which
> most concern us, can anyone explain why not?
> Or is this an other piece of inferential evidence that Eutectic points play no
> part in the maturation of a glaze.
> Ivor Lewis. Redhill, South Australia
iandol on sun 4 mar 01
After correspondence with David Hewett about his article on Eutectics =
and Phase Diagrams which made me rethink some of my ideas I started to =
have a look through the pictures that most concern potters, which =
include the Al2O3-SiO2, Cao-Al2O3-SiO2, Na2O-Al2O3-SiO2 and =
Now it seems commonly accepted in the literature written for potters =
that a temperature called the Eutectic point becomes opperative during =
heating of clays and glazes, assisting the process and ensuring maturity =
is achieved. So I would expect that Kaolin would feature some where in =
those phase diagrams since it seems to feature in most clays and many =
It does not. The diagrams relating to Kaolin are at the back of the =
book, round about No 2011, System Al2O3-SiO2-H2O and they relate to the =
geological formation under high pressure of several minerals.
So if neither Kaolinite nor metakaolin are not featured in the diagrams =
which most concern us, can anyone explain why not?
Or is this an other piece of inferential evidence that Eutectic points =
play no part in the maturation of a glaze.
Ivor Lewis. Redhill, South Australia
Louis Katz on sun 4 mar 01
The melting starts at the eutectic or near it as we don't have perfect mixing or equilibrium.
While you are looking into this test 1 Kaolin, 1 Whiting, 1 silica to cone ten. It sits right close to the Cao-Al2O3-SiO2 eutectic.
Ron Roy says it should be not a very stable glaze, I have no reason to doubt it.
Tom Buck on sun 4 mar 01
One likely reason you do not see Al2O3 eutectics in pottery
literature may be because it is almost the standard of comparison. Most
midfire and highfire glazes are CaO.Al2O3.SiO2 plus other components.
WG Lawrence, in "Ceramic Science for the Potter", pages 220-223,
lists various oxide combinations and their eutectic composition and
melting temperature. His brief discussion of the phase diagram lists
components in (weight, unstated) percent, as shown on one Triaxial Chart.
Here are some he cites:
Al2O3-SiO2 ---96%w SiO2, 4%w Al2O3 melts 1595 deg C.
CaO-Al2O3 --- 52%w Al2O3, 48%w CaO melts 1380 C
K2O-CaO-SiO2 --- 60 SiO2, 30 K2O, 10 CaO melts 950 C
Lithium alumino silicate, Li2O.Al2O3.4SiO2 (spodumene) and
Lithium silicate, Li2O.SiO2 --- this combo has a eutectic
composition of 55%w spodumene, 45%w Li2O.SiO2, and melts
at 1026 C.
Two sodium minerals, nephelite & albite show a eutectic of
67%w albite (Na2O.Al2O3.6SiO2) and 24%w nephelite
(Na2O.Al2O3.2SiO2), and melts at 1068 C.
and finally, CaO-Al2O3-SiO2 --- 62%w SiO2, 14%w Al2O3, 24%w CaO,
which melts at 1160 C.
If you take the last one, and assuming Lawrence cites wt%, then the Seger
Formula for this mix would be 1.03 SiO2, 0.14 Al2O3, 0.43 CaO, or bringing
it to standard unity, 2.40 SiO2, 0.33, 1.0 CaO. Which is not far off from
a Cone 6 glaze recipe.
I conclude that Alumina is cited enough in the Phase Diagram
literature to be of some use to glaze designers, should they wish to make
use of Phase Diagram data.
be well. Peace. To, B.
Tom Buck ) tel: 905-389-2339 (westend Lake Ontario,
province of Ontario, Canada). mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
Tom Wirt/Betsy Price on mon 5 mar 01
Michael and Ivor....
I think we've seen something re: kaolin, but I'm not sure...just a
theory. We had several glazes that called for a material named
"domestic kaolin". Our dear supplier was giving us a material labeled
Hydrite Flat D from Imery's. The tech sheet notes that it is a water
floated (read that as large particle) kaolin for the fiberglas and
In the recent cratering episode, we were trying everything...and
substituted some EPK (air floated) for the Hydrite flat D. Theory,
smaller particle size would give better particle distribution and
maybe lower the eutectic. It seemed to have some effect in making the
glaze smoother. Is this possible?
Also, there was an interesting article in American Ceramic Society
Bulletin in August 1999 that presented a new theory of the surface
electric properties of kaolins that sounds like it might have
applicability to the eutectics discussion. Would be glad to fax it to
you if you sent number off line.
Michael Banks on tue 6 mar 01
Kaolinite does not appear in phase diagrams of utility to potters because it
is a hydrous phase, and ceramic mixtures are essentially anhydrous above red
Metakaolin doesn't appear either, because it is an amorphous mixture of
silica plus alumina (no longer a distinct mineral), which is covered by the
SiO2 and Al2O3 corners of the relevant diagrams.
Eutectic points are defined by composition, not temperature (though they
have an empirically defined temperature). The assigned temperature will
vary depending on physical parameters, such as the grainsize of starting
materials, crystallinity etc, but the eutectic point is invariant.
----- Original Message -----
Ivor Lewis wrote: (in part) ...Now it seems commonly accepted in the
literature written for potters that a temperature called the Eutectic point
becomes opperative during heating of clays and glazes, assisting the process
and ensuring maturity is achieved. So I would expect that Kaolin would
feature some where in those phase diagrams since it seems to feature in most
clays and many glaze recipes.
It does not. ...So if neither Kaolinite nor metakaolin are not featured in
the diagrams which most concern us, can anyone explain why not?
iandol on fri 16 mar 01
Why is the mineral Kaolinite not found in the important Ceramic Phase =
I was surprise that a reasonable answer was not put forward. Some people =
came close to solving the conundrum.
At this point, I suggest reading one of the better texts which describes =
the behaviour of clay and glaze materials may provide information to =
solve the puzzle. Pioneer Pottery, p 67 by Michael Cardew is a good =
place to start. Notice that he says there is liquid present when the =
temperature has reached 950 degrees, without mentioning the intervention =
of a "Eutectic". Then look at the definition of Eutectic. It is a term =
with several meanings.
Best regards and thank you to those people who did have a go a giving me =
a reasonable answer.
Ivor Lewis. Redhill, South Australia.
Gavin Stairs on sat 17 mar 01
At 12:00 AM 3/16/01, you wrote:
>Why is the mineral Kaolinite not found in the important Ceramic Phase
>I was surprise that a reasonable answer was not put forward. Some people
>came close to solving the conundrum.
>At this point, I suggest reading one of the better texts which describes
>the behaviour of clay and glaze materials may provide information to solve
>the puzzle. Pioneer Pottery, p 67 by Michael Cardew is a good place to
>start. Notice that he says there is liquid present when the temperature
>has reached 950 degrees, without mentioning the intervention of a
>"Eutectic". Then look at the definition of Eutectic. It is a term with
Ivor, you had your answer a while back, but you may have missed it. I
can't remember the author, but here's the gist of the argument.
What the phase diagram represents is the equilibrium states of mixtures of
igneous minerals, for the most part, or simple oxides. The original ones
were done using simple minerals which could represent whole classes of
minerals, like silica, calcia and alumina. Allied to this work was other
work that used mineral families or archetypes such as talcs, feldspars,
etc. There are very complex mineral families that have also been
investigated, as well as oddballs, like the zircons, borates, etc. All
Kaolin is a metamorphic feldspar derivative. It has lots of water in its
crystal structure, none of which survives in the igneous systems which are
generally plotted in phase diagrams, since they deal with equilibrium
states at temperatures well above the fugitive temperatures for the water
of crystallization in the kaolin. Even in deep plutonic, hydro-thermal
phases of some minerals, there is very little water in these minerals. So
instead of kaolin you can expect to see silica and alumina, or some
combination thereof. If you look closely at the silica-alumina-calcia
diagram, or many other similar ones, you will see marked a few terminal
states of pure kaolin and silica, such as cordierite, and mullite.
The fundamental transformation in clay ceramics is the dehydration of the
kaolinite lattice, resulting in a water free, insoluble structure comprised
of the many phases you may find in numerous phase diagrams. Because we are
dealing with impure, inhomogenous systems, for the most part, we do not get
homogenous end products. Rather we get mixtures of somewhat refractory
phases cemented by somewhat melted phases. This is what our clay bodies
turn into. If we shoot for a pure phase body, like a very finely ground
talc-kaolinite body of mullite proportions, we will end up with a body very
much more difficult to fire, as it will tend to liquify all at once. This
is your worst nightmare of slumping or bloating. So, successful bodies
tend not to reside in the vicinity of eutectics, nor are they necessarily
very finely ground. You can probably make out that a very fine porcelain
is more like the eutectic body than is a standard type stoneware.
In the case of a glaze, I think we've gone over this ground about
enough. However, this is a more highly fluxed system and ends up more
homogenous than a body, and matures at a lower temperature than the body
with which it is matched. Nevertheless, the phase diagram will still
contain no water, and therefore will not list kaolin as a corner composition.
Having noted all the above, it is quite possible that a phase diagram might
be constructed on kaolinite without the water: Al2O3.2(SiO2).2(H2O)
->Al2O3.2(SiO2) + 2(H2O)^. In that case you can read it as being derived
from kaolinite as a raw material. Similar arguments apply to other clay
Hope this satisfies you.
Stairs Small Systems
1A - 921 College Street
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419 firstname.lastname@example.org
Michael Banks on tue 20 mar 01
There is no conundrum. Kaolinite relations are commonly plotted on
phase/stability diagrams by geochemists working, for example on hydrothermal
But, there is simply no call to include a hydrous phase such as kaolinite in
any phase diagram of relevance to ceramists or potters. Kaolinite does not
exist over 650 degrees C. Once it has lost it's hydroxyl groups at ~580
degrees, it consists of a disordered mass of anhydrous molecular fragments
known as "metakaolin". The reaction is not reversible.
There is simple no point in plotting the phase relations of disordered
non-minerals. Metakaolin is not a single phase.
----- Original Message -----
Ivor Lewis wrote:
> Why is the mineral Kaolinite not found in the important
> Ceramic Phase Transformation diagrams.
> I was surprise that a reasonable answer was not
> put forward. Some people came close to solving the conundrum.
iandol on wed 21 mar 01
Thank you for that diffinative comment.