Gavin Stairs on wed 10 may 00
At 02:25 PM 5/9/00, Joyce wrote:
>----------------------------Original message----------------------------
>Eureka!!! I think I've got it!.....or at least some part of it. Now,
>tell me in VERY SIMPLE TERMS if I'm right or not. ...
Hi Joyce,
This is my few cents on heat work.
To really understand, it helps to think of what is happening at an atomic
scale when you heat a ceramic. The materials are made up of atoms more or
less bound to each other by forces that make them act as though they were
connected by springs, or elastic bands. As the materials are heated,
energy is added to the material. Thermal energy. This makes the atoms
vibrate at the ends of the springs. The springs are storing the heat
energy. Temperature is just a measure of how violently the springs are
vibrating. Hotter means more violent action.
Now, well below the melting temperature, nothing much happens besides this
vibration. And there is always some vibration, even down at what we call
absolute zero, the lowest temperature possible, ~300C below room
temperature. But when we get up near the melting temperature, something
strange and un-spring-like begins to happen: sometimes when a spring gets
extended to its farthest reach, it finds that rather than go back to its
original spot, it prefers to jump over to a neighboring spot instead. Each
little atom or ion has a finite chance of doing this at each vibration, and
the higher the temperature, the faster the vibrations, and the more often
an atom finds itself in this condition. Eventually, virtually all the
atoms are doing this all the time, and we have a liquid. But before we get
the liquid, atoms can mix with neighbors. This is called diffusion, and it
tends to mix things up into uniform compositions. In other words, it will
mature a clay body.
This diffusion and related transport mechanisms are what change a clay body
into a uniform vitreous mass of fired clay. The key word is transport:
atoms have to move about to make this change. Moving about takes
time. The speed at which the mixing can take place is dependent on the
temperature, as discussed above. So, the hotter the kiln is, the faster
the transformation will occur.
Now, we don't want to get to the fully liquid state. So there is a
practical limit to how hot we can go. It is a state in which particles of
solid material (material in which the heat dance is still constrained by
the springiness) are still in contact with each other throughout the mass
of clay body, but they are surrounded by liquid material in the gaps. At
the outset of firing, there is a lot of solid stuff, and a little very
runny stuff: the flux. As time goes by, and the temperature rises a bit
more, the runny stuff dissolves some of the solid, and in doing so, changes
from very runny to rather sticky and lethargic, like molasses
(treacle). The solid stuff also changes, from a material with lots of
silica to a material with less silica, and more refractory. Refractory
simply means a material that has a higher melting temperature.
This transformation takes time. It happens faster at higher temperature,
up to the liquid limit at which the piece is no longer dominated by solid
to solid contact. At this point the piece may sag or bloat. Porcelains
are prone to sagging, because they are sintered (the technical name for the
process just described) very close to the liquid limit. Stonewares are
sintered farther below the limit, and earthenware still lower.
All of these thermal processes act with a speed described broadly as an
exponential function of absolute temperature. What this means is that at
low temperatures, almost nothing happens. As temperature rises, nothing
seems to change for some time, until at a certain point, change begins to
occur quite rapidly. The characteristic description of this is that for
each degree interval of temperature rise, the activity rate is multiplied
by a constant value, no matter at what temperature you look. As the
activity at low temperature is very small, multiplying by a constant
doesn't make much difference. But eventually, the difference becomes
noticeable, and after that, each degree rise in temperature makes a
steadily increasing rise in significant activity. We see that below a
certain temperature, it seems as though nothing will happen for as long as
we care to watch: for example at room temperature. At some critical
temperature, things will begin to happen, but we will have to wait for a
very long time: an interminable soak. Above that temperature, perhaps only
a few degrees, or a few tens of degrees, things will move along quite fast,
and may even seem to be happening instantaneously. Then a few degrees
above that, and the piece or cone sags rapidly to the shelf, over fired.
So, heat work is a catch phrase to include all of this. It is not
temperature: it is the change that temperature makes possible. Temperature
is usable as an indication of firing point only because of the way in which
the rates of the transport reactions increase rapidly over a narrow range
of temperature.
Cones and other similar firing indicators work not by sensing the
temperature directly, but by acting as models of the very changes we want
to bring about in the work being fired. The witness cone, or the sitter
cone, is calibrated to begin to slump just as the slightly more refractory
ware is coming to maturity. Because of the similarity of the materials in
the two cases, the heat work required for the two is similar, and the
system makes a reliable indicator of firing maturity, or heat work.
This description is by no means complete. Because the chemical makeup of
the material changes during the process, and because different materials
(substances) have different heat properties, just getting to maturity may
not be the end of the game. This is certainly true in the case of glazes,
especially variegated ones, like crystalline glazes. In these cases, after
the glaze becomes liquid, it may begin to form refractory crystals out of
the glass melt. This may cause the glass to become more runny, or less. A
hare's fur or similar glaze may separate into two somewhat immiscible
fractions of liquids, which may run and tear. And in the case of bodies, I
have already alluded to the change that clay undergoes, changing from one
sort of solid substance to another, more refractory solid. The heat
dynamics of these very complex mixtures is by no means simple, and many
changes may alter the simplified picture I have drawn above.
So, the cone does not give the whole picture. It may simply indicate when
one process gives over to another one. But what it does do is measure
an exponential process similar in nature to the process under control,
whereas the simple temperature measurement only tells you at what rate the
reactions should be proceeding.
Hope this is of some help, Gavin
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