Ivor and Olive Lewis on thu 27 may 04
Dear Ron Roy and Earl Kruger,
There will be a temperature at which clay will begin to decompose.
This can be calculated if you have access to the Thermodynamic Date of
the chemical species involved.
To find this temperature you imagine what needs to be taken from clay
to make water appear from a substance which does not contain water but
contains the necessary elements to form water. In the first instance
this is the separation of four Hydroxyl (-OH)Radicals from each
Aluminium ion of the octahedral layer. This can be calculated using
Gibbs Rules.
Commence by writing the equation Al4(Si4O10)(OH)8 = 8(OH) +
Al4(Si4O10). The big stumbling block is finding the numbers for the
Thermodynamic Values of each component to substitute into the
equations. After that it is simple arithmetic and a deft hand with a
calculator. When you have finished, remember the assumption which had
to be made, we have not made water, only created conditions from which
water can be made. So now you calculate the energy relationships for
the equation 8(OH)= 4H2O + 4O2. Now remember these answers are only a
suggestion, not cast in concrete fact. But that is nature of Science.
Then remember what Earl Kurgan told us and rely on your observations.
I do wish we could get it out of our heads, strip from the knowledge
base of Clay, that we are dealing with "Chemically Combined Water".
Clay is not like Blue Vitriol, Crystalline Copper Sulphate. There is
no molecular "Water of Crystallisation" in Clay Minerals which distils
from the solid substance as it is heated. (nor is the story of Cu2SO4
that simple either ! !)
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
Ron Roy wrote:
It seems that no two authors can agree - at what stage the chemically
attached water starts to be released. I know it will be a range of
temperatures - could it be different for different kinds of clay. Any
idea when the chemically combined H2O stopped being released?
mailtoandrew@FSMAIL.NET on fri 28 may 04
Just a quick contribution to the debate ... Thermogravimetric Analysis,
TGA, is an established analystical technique. The change in mass is
measured at temperature, examples of events include mass due to
decomposition or gain by oxidation, and are used to characterise and
identify minerals.
Regards,
Andrew
Ron Roy on mon 31 may 04
Hi Ivor,
I would think the first stage of trying to change anyones mind about the
terminology would be to suggest a more accurate name for it. What would be
your suggestion to describe what we call chemically combined water?
RR
>Then remember what Earl Kurgan told us and rely on your observations.
>I do wish we could get it out of our heads, strip from the knowledge
>base of Clay, that we are dealing with "Chemically Combined Water".
>Clay is not like Blue Vitriol, Crystalline Copper Sulphate. There is
>no molecular "Water of Crystallisation" in Clay Minerals which distils
>from the solid substance as it is heated. (nor is the story of Cu2SO4
>that simple either ! !)
>Best regards,
>Ivor Lewis. Redhill, South Australia
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on tue 1 jun 04
Dear Ron,
Changing the name will in no way chance the process or its effects.
There is an anomaly in the thinking of potters who confuse water in
the molecular sense which contributes to the structure of a crystal,
as in the case of Blue Vitriol, with water that discharges as steam
from a kiln of firing clay, which is due to a decomposition reaction.
Though the genesis of minerals continuing hydroxyl radicles which
discharge water as a reaction product during firing may have had their
birth in an environment pervaded by water molecules under horrendous
pressures at extreme temperatures the chemical reaction which ensued
destroyed "Water" at the time those minerals were born.
Warm Regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
Lee Love on tue 1 jun 04
Ivor and Olive Lewis wrote:
>Though the genesis of minerals continuing hydroxyl radicles which
>discharge water as a reaction product during firing may have had their
>birth in an environment pervaded by water molecules under horrendous
>pressures at extreme temperatures the chemical reaction which ensued
>destroyed "Water" at the time those minerals were born.
>
>
I experimented with this in my last firing after reading about the use
of water in wood fired kilns. If water is introduced at high
temperature, it breaks down into hydrogen and oxygen causing a net
reduction effect. When wood is introduced during cool down, the
water acts as an oxidation agent. Chinese brick makers use the
second effect to make normally buff colored bricks (buff from reduction)
turn red (body oxidation.)
I introduced water after cone 10 in three different ways: as
soaked sawdust, between two flat pieces of wood, by spraying through
ports in the door and also by scooping water into the firemouth with a
ladle. I think spraying into the ware chamber caused some of the
kaolin/alumina coating the side walls to leave white specks on glazed
surfaces. Just tossing ladles of water into the firebox was the
easiest way.
When the water is introduced, I always noticed a heat rise on
the digital pyrometer. I introduced the water in the opposite firebox
to where I last stoked.
--
in Mashiko, Japan http://mashiko.org
http://journals.fotki.com/togeika/Mashiko/ Commentary On Pottery
Earl Krueger on tue 1 jun 04
Ivor, Ron,
I'm sure you are familiar with the engineering
concept of the "black box"; a system of
unknown composition which given a certain
stimulus produces a certain response.
Pottery being fired in a kiln can be thought of
as a "black box". The stimulus is the input of
heat energy. The response is the output of
water vapor.
For certain discussions, such as exploding
pots, wherein it is not necessary, nor perhaps
desirable, to get bogged down with the detailed
chemical reactions taking place inside the
"black box" the term "chemically combined
water" is a reasonable and workable abstraction
to facilitate discussion.
After all, when driving an automobile no one
ever really "steps on the gas".
Earl K...
Bothell, WA, USA
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