Mike Bailey on fri 3 sep 99
Hi all,
Yesterday I had a phone call from a research worker in British Steel
asking for and about pyrometric cones. We chatted around the subject of
cones and into his reasons for wanting them. Now, this is the
interesting bit - they've managed to adapt microwave ovens to give them
the capacity of getting to 1100 deg. C. + and melting iron bars in a few
minutes ! "**** ** ****" I said, (expletives deleted). The reason he
needed cones is that the microwaves were playing havoc with their
thermocouples, which isn't surprising. When I asked how it was done he
became a bit coy and said it was technically 'sensitive' at the moment.
An article has also just appeared in 'Ceramic Review' (No 179) by Mike
Walford about the use of microwave heating in pottery - although his
initial experiments seemed just to result in blowing up the pots and
being banned from using the family's microwave oven. In the conclusion
to the article he does give some information about the developments
within this emerging technology.
Anyway, I thought some Clayarters might be interested in this bit of
info. as the development of this energy efficient form of heating is
clearly entering our domain. It could also be a pointer to the kilns of
the future - clearly, getting to glaze firing temperatures is possible
with the use of microwaves and all it now needs is a bit of control.
Has any else got any thoughts, comments, information on this subject?
Cheers,
Mike.
--
Mike Bailey. Bath. U.K.
Gavin Stairs on tue 7 sep 99
At 04:05 PM 9/3/99 -0400, you wrote:
>----------------------------Original message----------------------------
>Hi all,
>
>Yesterday I had a phone call from a research worker in British Steel
>asking for and about pyrometric cones. We chatted around the subject of
>cones and into his reasons for wanting them. Now, this is the
>interesting bit - they've managed to adapt microwave ovens to give them
>the capacity of getting to 1100 deg. C. + and melting iron bars in a few
>minutes ! "**** ** ****" I said, (expletives deleted). The reason he
>needed cones is that the microwaves were playing havoc with their
>thermocouples, which isn't surprising. When I asked how it was done he
>became a bit coy and said it was technically 'sensitive' at the moment.
The interesting thing about microwave heating is that there's no upper
limit. The microwaves are converted to heat in the object, and the
temperature just keeps on rising. There are lots of tricky bits, including
absorbing enough of the microwaves so that you don't end up overheating the
magnetron, but getting to temp is not one of them.
The upper limit (I know, I lied about there not being an upper limit) is
just the temperature at which the heat radiates away from the object as
fast as the microwaves deposit it. If you were to put the object in a
perfect mirror box, with no air, you could proably turn it into a plasma,
or at least a gas before the gas conduction would quench the process.
As to a practical limit, it is probably well above our region of interest.
But...
1 An ordinary microwave will be damaged by the re-radiated heat. The
walls will get hot, and the paint will decompose. You need a box with
highly conductive, heat tolerant walls. Maybe aluminum, lined with highly
pure alumina.
2 The heat will not be deposited uniformly. It will be absorbed by
certain polar compounds, like ferritic material, and by water. If these
materials are not distributed evenly, the object may shatter from thermal
stress. Not much of a problem with steel, but rather nasty with ceramics.
This really means that you may have to regulate the heating rate.
3 Some clays and glazes will not absorb the microwaves efficiently, so
there may be risk of overheating the magnetron. Not good.
4 The heating will be dependent on the load, and on placement in the
cavity. There may be pronounced cold and hot spots. A stirrer or
turntable is required. Less of a problem heating conductive metals like
steel than with pottery.
5 Most of the time (except with highly absorbent or conductive materials)
heat is lost from the outside, but absorbed in the interior. Thus the
interior will get hotter than the exterior. This means that careful
ramping is necessary.
6 The air temperature is not a good indicatior of the internal ware
temperature. Some other strategy is required for temperature measurement.
Probably the best would be infrared radiometry, but even that won't tell
the interior temperature. With experience, it might be possible to measure
temperature by closely following the microwave absorption by standing wave
reflectometry or some such. Since cones will not necessarily absorb
microwaves the same way the ware does, they won't be much help, except
after tests with a given load, type of ware, etc.
7 Since each piece will absorb differently (different shape, different
glaze, different body ... almost any difference), it will not be possible
to mix up a load of many different things to be fired (cooked?) together,
since they will come out differently. What will work to even this out is
reradiation. If the pots start to glow, they will heat each other up, but
careful: they will also heat up the oven. So, the oven will have to be
lined with refractory insulation, and it will have to be non-absorbent of
microwaves, or be a very good heat sink, to get rid of the heat as soon as
it is reradiated. Probably a combination of those two in a practical oven:
water cooled walls, with a pure refractory liner.
8 The collong rate will be very fast. It will be necessary to fire down
below all the inversions, and see note above about uneven heating.
I don't want to be totally discouraging, but it does seem that resistive
heating is much simpler for the case of firing pots.
It might help to preheat pots, to drive off water and even to reach a low
bisque point. So, for some special purposes I'm sure there is an niche.
But not to use the household microwave to fire pots, I think. As a side
note, microwaves and radio frequency heating are used in very large
industrial kilns to dry wood. They are also used for other purposes, in
many kW units. And inductive heaters, which are somewhat similar in
operation, have been in use for heating metals, particularly steel, for a
long time. In an industrial setting, where loads are controllable and
consistent, the drying and firing of ware might be accelerated with a
vacuum microwave drier as a pre-process in a semicontinuous batch stream.
But in the studio? I don't think it makes sense.
Gavin
Tom Buck on tue 7 sep 99
Hey! Mike, that microwave business is exciting! And perhaps not too
shocking. After all, the magnetron tube has been around a long long time
(50+years). But I would not get your knickers in a bind because the
general principle of microwave energy may not be readily feasible with
clay biscuit or air-dried clay forms.
Yet, something is new: how come steel (an alloy of iron, carbon,
other small amounts of metals) would absorb the MW energy enough to melt a
test piece in such a short time? Usually, the kitchen MW unit rebels at
having a metal nearby and the reflected MW energy burns up the MW
generator or control components.
I bet this is more than a marginal improvement over the
magnetron---likely to be a full-step innovation that will have immense
effect on our future (one could be a quick installation of Solar Power
Satellites).
Again, WOW! be well. Peace. Tom.
Tom Buck )
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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