Nick F
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Utterly mad, but totally practical: microwave furnace!!
Two links I've found so far:
http://www.popsci.com/popsci/science/article/0,12543,473581,...
http://home.c2i.net/metaphor/mvpage.html
I need to get me a cheap microwave and start melting things!
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CherrieBaby
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The popular science article is no longer available. How about uploading it to the FTP?
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unregistered_
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How about posting the correct URL?
Reposting the article as well.
| Quote: |
Smelting in a Microwave
Theodore Gray
There is an entire subculture of people who derive pleasure from putting strange things in microwave ovens, things that microwave oven manufacturers
would most strenuously suggest should not be put there. In the hands of these people, table grapes produce glowing plasmas, soap bars mutate into
abominable soap monsters, and compact discs incandesce. As a scientist, I'm enthralled by such phenomena (particularly the grapes), but somehow
I've always found the subject a bit unsatisfying: Cool, but what is it really good for? It wasn't until I stumbled upon David Reid's
Web site that I discovered a much more intriguing possibility for a microwave: melting metal.
I know, I know: You've been warned never to put metal in a microwave. There is some merit to this notion, particularly when it comes to
food?metal reflects microwaves and prevents them from reaching the thing you're trying to heat. Microwaves also can ruin metal accents on fine
china and can initiate electric arcs across some metals, which oven-makers consider a fire danger.
Nonetheless, I recently found myself inspired to attempt microwave metal melting?and, for reasons too complicated to explain, the inspiration struck
on a visit to San Francisco. I was 1,500 miles from my Illinois workshop and without any of the required materials: a challenge! From a hardware store
on Fourth Street, I bought a silicon carbide sharpening stone and 3 pounds of tin/silver plumber's solder; from Macy's, I purchased a
microwave-safe casserole dish and a stainless-steel measuring cup; and at Williams-Sonoma, I found a cute cast-iron cornbread fish mold and a pair of
long-cuff leather grilling gloves.
Silicon carbide is a microwave susceptor: It absorbs microwaves and turns them into heat (as does food, but silicon carbide can withstand much higher
temperatures than your average turkey sandwich). The measuring cup sits on the stone, which heats it, and the solder it contains, from below. The
casserole dish traps the heat, allowing it to build up to the tin's melting point (220�C). After microwaving the assembly on high for 15
minutes?some sparks flew among the solder coils at first?I came out with a cup of the molten metal, which I poured into the fish mold. Easy as pie,
except that the casserole lid shattered from the shock of cooling. (Fortunately, I was wearing leather gloves, an apron and sensible shoes as a
precaution against the molten metal.)
Next, I turned to silver?a more difficult metal to melt. The helpful staff at the Exploratorium, San Francisco's world-famous science museum, let
me use their machine shop to fabricate a firebrick-and-silicon-carbide furnace enclosure: Same principle as the casserole dish, just better, safer
insulation. Although I fell short of melting the silver at 962�C, I did make it glow red-hot at around 800�C.
Proof of concept: A microwave is a powerful induction furnace capable not only of creating high-voltage plasmas from table grapes but of melting the
very substance I wasn't supposed to put in there.
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Eclectic
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David Reid's Microwave Furnace:
http://home.c2i.net/metaphor/mvpage.html
[Edited on 6-3-2005 by Eclectic]
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searat
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i migth be wrong hear, but doesn't masers, not effect metal.
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Quince
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If you'd bothered to read the actual posts, you'd have read that another material is used to convert the microwaves to heat, passing it on
to the metal to be melted.
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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searat
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Sorry convecting heat like that, well maybe takeway maicrowvaes.
Even if a microwvae runs allday, it will only produce so much heat.Take a ciq liter and try to metal steel.
DOES IT WORK? No you was't a fuck load of energy in changeing.The boul will get to about 200-300C at max, so maybe solder will(Add a picture
please,).
Edit by Chemoleo: SEARAT, get your spelling sorted. It's so bad that it's hard to decipher the meaning of your
posts. It adds neither to your credibility nor to the forum quality. Next time your message will be just deleted/moved, and you can try again until
you get it right.
[Edited on 13-3-2005 by chemoleo]
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chemoleo
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Re Microwave furnaces
According to what I read, the microwaves are absorbed by the bonds in fats, sugars, and water, and of course related derivatives.
Metal is supposed to reflect microwaves (see this).
Oh wow, just found this:
| Quote: | | in the microwave oven an electronic tube, the so-called magnetron, generates an alternating power field. The molecules within the food - especially
the polar water molecules, but also amino acids, lipids and proteins - are forced to align themselves with the rapidly changing alternating electrical
field. They oscillate around their axis in response to reversal of the electric field that occurs up to 5 billion times per second. This oscillation
creates considerable intermolecular friction that results in the generation of heat. Thus, the food is heated from the inside outwards leaving the
dishes and the oven itself cold, because they are not directly heated by microwaves. |
I am getting more confused with this. Hmm. looks like the microwaves are not absorbed by the bonds, but rather, the polarity of the molecule requires
realignment with the alternating electromagnic fields. This then produces molecular friction.
Anyway, some observations seem to contradict the latter point of metal reflecting the microwaves. For one thing, why does one get sparks if Al foil is
placed into the oven? Obviously a local current is produced, which leads to internal short circuits. So much for reflection of the waves 
The other one is, heat your cup of milk/coffee/whatever with a spoon in the cup. The spoon becomes incredibly hot, way too hot to touch, while the
liquid below is drinkable. Again, to me, it seems the metal spoon happily absorbs the energy.
Surely some physics wiz's have an answer to this apparent contradiction?
Anyway, that's why I am not so sure a microwave-sensitive mold is required to melt metals.
[Edited on 13-3-2005 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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12AX7
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Lossy conductors will dissipate some heat, that's why waveguides are copper, or silver plated. Except in intermittent-use microwave ovens, where
it isn't cost-effective I guess. Stainless (coincidentially about the worst metal you can use!) is easier to clean food mess off of...
It's a badly matched load and hence sends much energy back to the magnetron, causing it to overheat.
Aluminum acts as an antenna, and as anyone experienced in antenna theory knows, resonant circuits can produce very strong voltages or currents. In a
random crumple of foil, there are bound to be a few locations that are series or parallel resonant at 2.45GHz.
Tim
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Quince
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When you are playing with the ball lightning thing, a quarter wave antenna seems to get the best results, but you need some source of carbon
particles, so usually for simplicity you just combine the two by using a graphite rod on the foil base.
Now to figure out how to sustain it outside the microwave. 
[Edited on 13-3-2005 by Quince]
\"One of the surest signs of Conrad\'s genius is that women dislike his books.\" --George Orwell
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Chris The Great
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I find tossing in a little bit of 40 gauge wire works wonders for ball lightning production.
But that's kinda getting off topic.
Anyway, I've found that metals that are fairly thick do not have the resistance required to heat up alot. If you want the furnace to work
efficiently, you need to use something to turn the microwaves into heat, which then goes into your metal.
I have explaination for the spoon however, but I have had spoons/forks etc in my microwave when making food before and they are always less hot than
the food.
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FrankRizzo
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FYI
Silicon carbide is sold cheaply as rock polishing grit.
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SAM4CH
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I need more length arc!
I have some of Microwave Transformer, I tried to run one but the arc was very short (around 1mm), then I tried to run two connected series togother
but no way it is still very short.
My transformer out put= 2070volts, 0.62A
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jimwig
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trick is making the sc crucible. can be done.
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Eclectic
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SC-graphite crucible can be bought at casting suppiers.
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SAM4CH
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I need more about SC-graphite crucible and its idea in this!
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12AX7
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SC, or more accurately S2C > CS2, will have to be cooled quite a bit to make a crucible out of.  If you want to know how an SiC crucible is made, I think they take SiC grit, bond it with pitch and fire to some
ungodly temperature where the pitch first decomposes (leaving amorphous carbon) then recrystallizes as graphite flakes. There may also be
recrystallization or diffusion bonding of the SiC grains, I don't know. Then it's all wrapped in a low-melting black glaze that drips all
over your furnace and makes the plinth stick to the furnace for 20 melts afterwards. 
Tim
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Eclectic
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http://budgetcastingsupply.com/Crucibles.html
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Texium
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