iandol on sun 11 mar 01
Dear John Jensen,
This is a very good point that you make=20
much steam is still escaping at 1400 degrees F.... I had always thought =
all molecular water was gone by around>
I think this may reflect a common misconception about what is happening =
when we fire clay. If by "molecular water" you mean the free water which =
aids plasticity or deflocculation in slips you are most probably =
correct.
But the idea that there is free water which comes from the breakdown of =
clay as temperature rises towards bisque temperatures, noted with the =
elemental oxide designation of Al2O3-2SiO2-2H2O for Kaolinite, then =
there may be a misunderstanding.
In theory, at red heat and above, there is a chemical process at work =
called Dehydroxylation which separates sufficient -OH radicles and -H =
ions from the aluminium octahedral structure where they reside in the =
clay molecule. These liberated entities combine to achieve a loss of =
those two water molecules.
This process may take much longer than expected. Several factors will =
influence the rate of dissipation which may in part depend on the =
diffusivity of each particular clay, the proportion of Kaolinite in the =
clay being used and the partial pressure of water vapour in the kiln =
atmosphere. High local humidity and the burning of hydrocarbon fuels =
could be considered as things which lead to your observations.
Best regards,
Ivor Lewis
John Jensen on mon 12 mar 01
So reading your note again, Just what is this aluminum octahedral structure?
I see Aluminum, I see oxygen...I don't see any Hydrogen in any form in that
....The only Hydrogen I see is in the water component. Are you saying that
afte the water of platicity is gone..at whatever temperature...then the only
water released is from these, Ions. Can you suggest a reference where I
might read further about this process.
John Jensen....
iandol on thu 15 mar 01
John Jensen asked
oxygen...I don't see any Hydrogen in any form in that ...The only =
Hydrogen I see is in the water component.>
In the clay molecular structure Aluminium ions coordinate with Oxygen =
ions and Hydroxyl ions in an Octahedral formation. Silicon ions =
coordinate with oxygen in a Tetrahedral formation. Some Oxygen ions are =
shared between the these geometric formations. To understand this =
structure you need to build your own three dimensional models. The =
structure is almost impossible to understand as a two dimensional =
diagram unless you can imaginatively visualise three dimensional spaces =
from a two dimensional diagram.
There is no "Water Component" in crystalline clay. There is free water =
in plastic clay, as free lubrication water and as ice structured water =
attached to the crystals.=20
It is unfortunate that the convention of representing ceramic compounds =
as collections of elemental oxides prevents people from contemplating =
the structures of the materials we use in pottery. Without the concept =
of a structured compound it is difficult to understand the chemical and =
physical processes which occur when these things are heated.
In the interest of better understanding Ceramics
Ivor Lewis. Redhill, South Australia
John Jensen on thu 15 mar 01
Well, thanks for taking the time to open my eyes. I'm not sure where I
obtained my present misunderstanding, but I can only hope it was from some
reputable source. I've been passing along my vision of what is going on for
some time now, and at least I can stop doing that. In the meantime, I
suppose, a basic chemistry book and some gumdrops and toothpicks may serve
me well.
Does the amount of water released into the atmosphere during a firing if a
porcelain to cone ten (for example) equal the water of plasticity plus the
weight of all those hydrogen and oxygen ions represented by the H2O
component of the molecular formula?
John Jensen
Joseph Herbert on fri 16 mar 01
Ivor Lewis wrote about imagining the alternating layers of silica and
alumina in clay structures and placing the accompanying hydroxyl groups in
the middle of the layers. The imagining part, or building crystal models is
not strictly necessary as crystal lattice structures are posted many places
on the web. The url below has several spaces entered into it. This allows
the transmission without being an evil attachment and so rejected. Remove
the spaces, after p, after // and after alogy/ to reconstitute this as an
address.
http :// mineral.gly.bris.ac.uk/Mineralogy/ clayminerals/main.html
^ ^ ^
In any event, you can treat this site as an example and search for similar
things using "crystal lattice" as one of the search terms.
I think I found a site related to the University of Michigan that had
rotating crystals structures of several rock forming minerals - sorry, glaze
chemicals - but I have misplaced the URL.
At the site above, it may be sufficient to look at the pictures. Look at
several. The diagrams assume that you know the code, like mentally placing
atoms at the points of the tetrahedra (there is one in the center, too) to
make a crystal unit. You may have more trouble, rather than less if you
read much of the text since it is loaded with crystal jargon that could
confuse more than elucidate.
Joseph Herbert
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