Re: [ATM] Microwave heating of glass

[Dominic-Luc Webb <dlwebb@canit.se>]

On Mon, 7 Jan 2008, GARY FUCHS wrote:

> "Since the energy level is readily controlled on any microwave"
>
> I recall hearing that home microwave ovens have only one energy
> level and that the "lower" settings cycle full power for varying
> periods. Is that true? If so, what impact, if any, might that
> have on the annealing?
>
> Gary Fuchs


Hi Gary (and others)!

I am not at all negative on this possibility.

I suspect it may be true for most microwaves that heating control is
accomplished by breaking up the radiation into pulses with varying
duty cycles. This is certainly how my home microwave works. I see
no reason why energy control in a microwave should not by governed
by Joules/s (i.e., Watts) as in other heaters. This variation in
duty cycle is how a glass kiln can be controlled, and I have built
such a control and never heard of anyone complaining about it not
working properly. If it works with a kanthal wire heating
element, why should the same principle not work in a microwave?
I am much more concerned about the non-uniformity of the energy
distribution (e.g., Joules/cm^3 in different zones) than Joules/s.
The latter is easily easily controlled on a home microwave as is.

As for remarks about destroying the microwave, I think this is a
sad case of defeatism. I think if the scientists, physicists and
engineers in here were at their usual jobs and told to make such a
thing, they would more likely just build it than tell their boss
it can't be done. And if told to make a home microwave melt glass,
they would still find a way. Let us think positively, and think on
how this could be made to work.

Why does the microwave burn up? We know from life experience that
you cannot run a microwave when it is empty, and you cannot run
it with metals such as aluminum foil. As shown in the video clip,
there is nothing in the microwave other than the preheated glass.
If the microwave is burning itself up, we can assume this means
the radiation that should have normally struck food has struck
some of the microwave components. The water in food is only one of
numerous food constituents that absorb at this wavelength, about
120 mm, BTW. In fact, water should absorb even better at some other
wavelength. I suspect what we find in a real microwave is a
compromise between what is capable of cooking food, what can
be built efficiently, and what interference the telecommunications
industry will tolerate. Industrial microwave ovens cook food even
better and do not use the same wavelength as a home microwave. In
any event, in the case of the damaged home microwave, microwave
radiation has struck the plastic, the bottle and maybe even
bounced back and hit the magnetron that generated the radiation.
This is what needs to be addressed if this is to work for us.

I would attack this problem by exploring which magnetron (i.e.,
which wavelength) would be most suitable for melting glass.
Specifically, which wavelength produced by which magnetron is
best absorbed by glass? Fortunately, we have a number of
choices here because there are different types of microwaves
that operate at different wavelengths. Risk of damage to the
oven should be reduced as a result of proper wavelength choice.
>From there, I would consider how to further protect the
magnetron, assuming this is the critical component that could be
damaged. The chamber of a microwave oven is a Faraday cage
which I would think can be made of many materials besides
plastic, but maybe this is besides the point. Maybe it would
be possible to add something to the chamber that can absorb
stray radiation to reduce the likelihood of damaging the
magnetron or plastic.

In my lab, I do not melt glass, but I do sterilize it routinely.
In this case, I also include a small beaker of water. Could we
apply a similar approach to melting glass? Maybe someone in
here with some material science background knows a material
(solid or liquid) that absorbs at microwave wavelengths but
would have higher melting or boiling point than the melting
point of glass? Maybe such a thing could be used to transfer
the heat to the glass while protecting the microwave.


Dominic



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