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Re: [ATM] Microwave heating of glass
> 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
>
Yes, I think all kitchen microwaves control the time-averaged power
output of the magnetron with a switched-mode - turning it on and off for
varying durations. Glass 'forming' from molten glass to a non-annealed
solid form typically takes 5-20s for the amounts of glass we are
considering. This is a similar time period to the switching period of
my home microwave, i.e. my microwave turns off for too long when set to
a low power. So, to control the rate of microwave heating of the glass
using a microwave oven, we might need the switching rate to be
increased. This is only an issue if we want to use the microwave to
anneal the glass and not just form it.
> 3. Getting broken glass melted enough to mix and fuse properly
> without a lot of bubbles and stress is a whole separate issue.
> ...However, when starting with a lot of broken glass, the pieces first
> become kind of high viscosity liquid, much like syrup, but the
> different pieces won't truly mix, although they might fuse as the
> system cools.
This is where the rumours of 'microwave stirring' sound interesting.
When the glass starts to behave in the magnetohydrodynamic regime (a
conducting fluid) microwaves start to induce motion in the fluid, just
like in a magnetohydrodynamic drive.
http://www.evilmadscientist.com/article.php/SimpleMHD
This is something the glass industry has taken advantage of only recently:
> MICROWAVE MELTING TECHNOLOGY
> Hajek, M
> Glass. Vol. 79, no. 9, pp. 317. 2002
>
> Abstract: The results of research into the microwave melting of glass
> are reported. Microwave melting is a simple and economic process with
> the following benefits: reduced energy use, reduced emission of
> pollutants, faster heating rates, any form of glass can be melted, use
> of cullet and homogenisation of glass melt via self-stirring effect.
Regarding the form for the mirror:
> 1. A form would be needed, either for the shapre of the mirror
> or as a crucible. We would need to think on appropriate
> material for a microwave.
Well, cement has a lower dielectric constant than glass. Is it possible
to make cement/concrete moulds for glass that won't crack from the
thermal stress?
Ian Williams
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