Marcia Selsor on thu 11 may 00
Dear Gavin,
My rocket scientist husband enjoyed you explanation. He always wanted to
bring his physics classes
over to witness the kilns thermal radiation for his thermal dynamics chapter.
Marcia in Montana
Gavin Stairs wrote:
>
> ----------------------------Original message----------------------------
> 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
--
Marcia Selsor
selsor@imt.net
http://www.imt.net/~mjbmls
http://www.imt.net/~mjbmls/spain99.html
http://www.silverhawk.com/ex99/selsor/welcome.html
| |
|
