iandol on mon 24 sep 01
Appreciating that there are sound environmental reasons for decreasiing =
the maturing temperatures of clay bodies and glazes, I follow the =
discussion which are developing around this topic with some degree of =
interest. The focus seems to be on energy saving and hence benefits to =
society as well as adding to the bottom line.
The points made about substitution of manufactured frits as alternatives =
to boron bearing minerals show that it may be a case of six, or two =
threes since the price advantage seems to slip away if more =
expensive,energy demanding material sare necessary. But I feel =
insufficient attention is being paid to underlaying chemical principles =
which govern the melting, dissolution and recombination of the materials =
we put in glaze and clay. These are, as I understand it, chemical =
reactions. After all, new substances result from what we do. So with =
regard to that thing called "Heat Work" (is this tautology?) it is =
interesting to note that the higher the temperature the faster the rate =
at which any reaction will proceed. I have in my chemistry book a figure =
of doubling the rate of reaction for every ten degrees Celsius increase =
in temperature.
If this figure is used for comparisons of ^6, ^8 and ^10, things happen =
about thirty times faster at ^8 than at ^6 and possibly more than a =
thousand times as fast at ^10, for identical reactions. Now, it would be =
wrong to assume that this is a fact when a ^ 10 glaze is refomulated to =
melt at ^6, but is it permissible to think that by making things happen =
faster we will save fuel? Are those high cone, fast wood fire buddies =
onto something after all?
Just thoughts.
Best regards,
Ivor Lewis, Redhill, South Australia
John Hesselberth on mon 24 sep 01
on 9/23/01 2:00 PM, iandol at iandol@TELL.NET.AU wrote:
> These are, as I understand it, chemical reactions. After all, new substances
> result from what we do. So with regard to that thing called "Heat Work" (is
> this tautology?) it is interesting to note that the higher the temperature the
> faster the rate at which any reaction will proceed. I have in my chemistry
> book a figure of doubling the rate of reaction for every ten degrees Celsius
> increase in temperature.
> If this figure is used for comparisons of ^6, ^8 and ^10, things happen about
> thirty times faster at ^8 than at ^6 and possibly more than a thousand times
> as fast at ^10, for identical reactions. Now, it would be wrong to assume that
> this is a fact when a ^ 10 glaze is refomulated to melt at ^6, but is it
> permissible to think that by making things happen faster we will save fuel?
> Are those high cone, fast wood fire buddies onto something after all?
Hello Ivor,
While it is true that the "rule of thumb" is that chemical reaction rate
doubles for every 10 degrees Celsius, that is only relevant if chemical
reaction rate is the limiting phenomena. While I have no data on that I
would be extremely surprised if that were the case. I would bet that this
process is physics limited, probably by the rate of diffusion of the
materials into one another and/or the rate of melting those crystals of
individual ingredients. I would bet the actual chemistry takes place very
quickly once the molecules are in proximity to each other. These are
extremely thick fluids we are dealing with and they will mix into each other
fairly slowly.
Regards,
John
Web site: http://www.frogpondpottery.com Email: john@frogpondpottery.com
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
iandol on tue 25 sep 01
Dear John,
Some good points there. I wonder what the other buddies have go to say =
about it. I think your notion of diffusion is a most important one, but =
don't diffusion gradients depend upon concentrations and other factors?
Thanks for the input.
Best regards,
Ivor
Joseph Herbert on tue 25 sep 01
I also doubt that much of what goes on in a glaze is a "chemical reaction"
in the sense that most people understand it. Of course many people do not
understand it at all, but that is another question.
I believe we create a complex solution of metal oxides in a silica/alumina
liquid mixture. The temperature at which this starts to happen depends on
the silica/alumina ratio and the presence of fluxing agents to help create
the liquid. Once everything is melted, and that may not actually happen,
the look of resulting glaze will depend on many processes and events, most
of which are considered physical rather than chemical. The real chemical
reactions take place because of the reducing atmosphere or (possibly) the
introduction of ion vapors into the kiln atmosphere.
First of all, ion migration takes place in the dense viscous liquid. The
diffusion rate is controlled by the viscosity of the fluid which is, in
turn, controlled by the temperature of the fused material. This alone is a
reason for some mixtures to require "heat work" to produce a particular
result.
A second factor in the look of a glaze is the possible existence of
immiscible phases in the glass. I cannot say how much of a factor this is
without research, but the appearance of many glazes gives that impression,
of separate blobs of material in the melt. If such phases exist, they could
disappear at some temperature and reappear later, or the flow structures
caused by the movement of such phases might require "heat work" to develop.
There are also bubbles present in many glazes and they may disappear,
congregate, or get larger with time and mobility.
Crystal growth, to the extent that it occurs, takes time and ion mobility to
happen, again heat work. In matte glazes, this is quite important as can be
seen by shiny spots where the glaze layer is too thin to support crystal
growth, or the reverse. One of the parts of crystal growth that is
sometimes forgotten is the requirement that the crystal grows by removing
material from the liquid. The growth of crystals depletes the material
around the crystals so that it has a different composition than when the
crystal growth started. This may not be observable if the crystals are
small but there is an influence. In crystal glazes, the liquid is extremely
fluid so that ions can migrate relatively large distances to form the large
crystals that are the reason the glazes exist.
These all seem to me to be physical processes, except for the reducing kiln
atmosphere.
An exception is the inclusion of silicon carbide in a glaze. the reduction
that occurs during the decomposition of the carbide is a "chemical"
reaction.
This kind of discussion interests me. The glaze materials are mostly ground
rocks or metal oxides like rust (there are a few exceptions) but are
uniformly referred to as "chemicals". This is in spite of the fact that no
chemical reactions take place during their use, and, as oxides, relatively
little can happen to change them (chemically) at earth surface conditions.
Joseph Herbert
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