Re: [ATM] re: heating glass

["Bill Kelley" <flex@commspeed.net>]

Also, for the record, the following information does not apply to high index 
leaded glass.
There seems to be some difference in the rate of microwave heating of most 
glasses, probably due to traces of metals, notably iron.
Bill


> Let's set the record straight.  As far as standard food microwave ovens 
> are
> concerned, glass is a poor absorber of microwaves.  If you want to try an
> experiment, take two identical glasses.  Fill one with water and put them
> both in the microwave at full power for one minute.  Feel both glasses. 
> The
> hotter one is the one with the water!!  This is because food microwaves 
> are
> tuned to the hydrogen bond frequency.  Glass does not have hydrogen bonds.
> The empty glass will heat up a little, but no where near as much as the
> glass with water.  Do not put empty glass in a food microwave by itself.
> This is considered running the microwave empty, which manufacturers warn
> against.
>
> But why can the glass with water be hotter that the water inside?  This is
> because the microwaves only penetrate the water (or food) a short 
> distance.
> So the hottest part of the water is at its surface where it is most 
> exposed
> to the microwaves.  This refers to both the surface in contact with air 
> and
> the surface in contact with the glass.  This hot water zone heats the 
> glass
> by conduction.  The glass heats up, but can only loose heat by conduction
> with the air (poor) and radiation (also poor).  However, the water in the
> hot zone can lose its heat to the cool (central) water zone by the much 
> more
> efficient method of convection.  So the thin (small quantity of mass) 
> glass
> is heated quickly by the hot water at the water/glass surface and has
> difficulty cooling off.  The water (larger quantity of mass) heats more
> slowly and uniformly from the more efficient convection.  The water feels
> cooler until it is completely heated.
>
> There may be microwave magnetrons tuned to some absorption frequencies of
> the silicon or oxygen bonds in glass.  But I doubt there are many of these
> manufactured compared to food microwave magnetrons.  That would make them
> much more expensive.
>
>> -----Original Message-----
>> From: atm-bounces@atmlist.net
>> [mailto:atm-bounces@atmlist.net] On Behalf Of Ellen Mackenzie
>> Sent: Wednesday, January 26, 2005 1:24 PM
>> To: ATM
>> Subject: [ATM] re: heating glass
>>
>> The heating of glass is the result of its' dielectric
>> properties at certain frequencies - - - fill a porcelain cup
>> with water and run the microwave for one minute at full power
>>  - - - you will find the cup is warm or hot and the water
>> inside just starting to warm up.
>> This is an important factor when choosing the right
>> type of insulating material at high frequencies.
>>
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>
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>
> 


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