robin clifford on tue 16 jan 01
Hey, just wondering if anyone has some good information for me. It's my
first time teaching glaze calc and I was wondering if anyone knows of any
precice descriptions of what happens in quartz inversion. We've got the
basic alpha to beta crystals, but does there exisist a comprehensible
explanation of the how and why?
Also just wondering why mold is said to add plasticity to a claybody. Does
it just show that its well aged or is there something more to it? Please
respond to robinclifford@hotmail.com
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Snail Scott on wed 17 jan 01
At 04:21 PM 1/16/01 -0700, you wrote:
I was wondering if anyone knows of any
>precice descriptions of what happens in quartz inversion.
>Also just wondering why mold is said to add plasticity to a claybody.
Please
>respond to robinclifford@hotmail.com
No, don't respond directly, post it to the list.
I wanna know, too!
-Snail
Bruce Girrell on wed 17 jan 01
Snail Scott asked:
> I was wondering if anyone knows of any
> >precice descriptions of what happens in quartz inversion.
> >Also just wondering why mold is said to add plasticity to a claybody.
I'll give it a go.
Quartz inversion:
First, remember that quartz is a three-dimensional arrangement of silicon
and oxygen atoms. The silicon and oxygen atoms hold their places in this
lattice due to electrical forces at the atomic level. The atoms are not
rigidly locked in place, though. At any temperature above absolute zero
atoms move about slightly. They bounce around (very, very tiny bounces), but
tend to stay in one general area. If they get too far out of place the
electrical forces push them back. But electrical forces are considered
"weak" forces by physicists and a fairly small amount of energy can change
how the atoms interact with one another.
At room temperature the silicon and oxygen are stable in a form called
alpha-quartz. This form is characterized by a specific position of the
silicon and oxygen atoms relative to one another. As you add energy (heat),
atomic motion increases. At 573 oC enough additional energy has been added
to the crystal lattice that the atoms are vibrating to the extent that they
can overcome the forces that were holding them in the alpha position and
they suddenly rearrange into a new stable position called beta-quartz. No
silicon-oxygen bonds are broken in the process. It is simply a mechanical
rearrangement of the atoms that balances atomic forces within the crystal
lattice. The relative angle between the silicon and oxygen atoms has
changed.
In some ways it is like a child's top which is stable lying on its side. But
when the proper energy is added (as angular momentum) it is more stable in a
new, and somewhat surprising, position - on its tip. At elevated
temperatures the silicon and oxygen atoms are more stable in a new
position - the beta position - and this arrangement takes up more space.[1]
The increase in volume for the beta arrangement is about 3% and the process
is reversible. As the quartz cools below 573 oC the silicon and oxygen atoms
no longer have the energy to stay in the beta position and revert to the
alpha position (the top slows too much and falls over). This change results
in a decrease in volume of about 3%.
If the quartz becomes incorporated in a glass, the regular crystalline
arrangement of the quartz crystal no longer exists and the transition
between alpha and beta quartz does not occur.
Hope that helps. Tom, Ron, Michael - your turn
Now about mold:
Some notes about clay:
1) Plasticity in clay comes about as a result of the water held near the
surfaces of the clay particles.
2) Clay has very poor permeability - the ease with which fluids can move
through the clay.
3) Smaller clay particles have more surface area and can hold more water.
I believe that there are two reasons that mold is a Good Thing with clay,
but I'm guessing here a little. First, when water is mixed into clay, the
water cannot wet all the clay surfaces immediately due to the poor
permeability.[2] Clay particles are very tiny and it takes some time for the
water to get into all the tiny spaces. The presence of mold on clay simply
indicates that the clay has been sitting around long enough for the water to
be more evenly distributed.[3]
Second - and I think this is what most people ascribe the magical powers of
mold to - clay is a weathered material. Weathering takes place through
mechanical (ice, wind, etc.) and chemical means. Mold is a living organism
and, as all living organisms do, produces waste products. These waste
products are often acidic and the acids continue the weathering process,
breaking the clay down into finer particles. Finer particles -> more surface
area -> more water -> more plasticity.
There may be some effect of the mold itself. Organic compounds are sometimes
added to increase plasticity and mold may act in a manner similar to a
lubricant when mixed in with the clay. Just speculation. At any rate, the
cells of the mold do hold water and an increase in water content does tend
to improve the perception of plasticity.
Bruce "my take on it anyway" Girrell
in northern Michigan where winter has been on vacation for about a week but
is now returning. Lynne and I tried to do some horsehair pots last weekend,
but the cold shock was too much and we lost every pot to cracking. Thermal
shock resistant body experiments continue.
[1] I'm tempted to make an analogy with ice here too, since everyone is
familiar with the expansion of ice when it freezes. However, ice is ordered
and water is disordered. It is not two different arrangements of one ordered
substance. Also the effect happens with decreasing temperature, not
increasing. So water/ice is not that great of an analogy.
[2] This is why blunging and extracting water from the slip is a produces a
usable plastic clay sooner than adding water to dry material and mixing with
a clay mixer. In blunging the water molecules have a lot more opportunity to
get at the clay platelets.
[3] I would think that some may disagree with me on this, but I think that,
given the short amount of time that we give clay to age in most cases, there
is not really sufficient time for much chemical attack by the mold. I did an
experiment recently where I made a clay "body" out of 100% EPK. When first
mixed I could not wrap a coil of it around my finger without it breaking
into several pieces. After four weeks I could wrap a coil around my finger
without it breaking apart. It still cracked a lot, but it did not break. I
doubt that mold would have chemically attacked the clay to any important
degree in that short of a time span.
Ron Roy on thu 18 jan 01
The is a very good explanation on this in the Hamer book. What I mean by
good is - it's simple enough for me to understand. There are many pages on
silica - all of them are fundamental to our craft.
It seems that the oxygen atom in the SiO2 molecule which is surrounded by
the silicon atoms is such a tight fit that when the oxygen atom gets to
573C it gets too big (through expansion) and the silicon atoms have to
rearrange them selves. When this happens the molecule takes up more space.
Acids add plasticity (flocculate) clay - add some vinegar to a slip and see
it happen - so I am assume mound add acid. Aged clay - through bacterial
action, gets more acid (some bacterial produce vinegar) and become more
plastic through flocculation. If you ever had to try and throw with
defolcced clay you would really understand this.
RR
>Hey, just wondering if anyone has some good information for me. It's my
>first time teaching glaze calc and I was wondering if anyone knows of any
>precice descriptions of what happens in quartz inversion. We've got the
>basic alpha to beta crystals, but does there exisist a comprehensible
>explanation of the how and why?
>Also just wondering why mold is said to add plasticity to a claybody. Does
>it just show that its well aged or is there something more to it? Please
>respond to robinclifford@hotmail.com
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Michael Tannock on thu 18 jan 01
>Snail Scott asked:
> > I was wondering if anyone knows of any
> > >precice descriptions of what happens in quartz inversion.
> > >Also just wondering why mold is said to add plasticity to a claybody.
Bruce Girrell wrote:
>Second - and I think this is what most people ascribe the magical powers of
>mold to - clay is a weathered material. Weathering takes place through
>mechanical (ice, wind, etc.) and chemical means. Mold is a living organism
>and, as all living organisms do, produces waste products. These waste
>products are often acidic and the acids continue the weathering process,
>breaking the clay down into finer particles. Finer particles -> more
>surface
>area -> more water -> more plasticity.
>
>There may be some effect of the mold itself. Organic compounds are
>sometimes
>added to increase plasticity and mold may act in a manner similar to a
>lubricant when mixed in with the clay.
I think that the acid produced by anaerobic decomposition is Humic Acid, I
can't remember where I read about it but I think that someone has done work
on it as an addition to help with pre fired strength of clay. It is a long
chained organic molecule, so it acts like a glue...?
Also the fact that it is an acid helps to flocculate the clay, preventing it
from becoming thixotrophic. I think that clay manufacturers may add calcium
chloride to the clay while a slip if filter pressing it to flocculate it and
allow the water to pass through the clay easier.
Mike Tannock
Dunedin. NZ.
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