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Author: Subject: Utterly mad, but totally practical: microwave furnace!!
Nick F
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[*] posted on 6-7-2004 at 05:45
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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[*] posted on 6-3-2005 at 04:49


The popular science article is no longer available. How about uploading it to the FTP?
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[*] posted on 6-3-2005 at 06:45


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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[*] posted on 6-3-2005 at 07:13


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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[*] posted on 8-3-2005 at 21:36


i migth be wrong hear, but doesn't masers, not effect metal.
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Quince
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[*] posted on 12-3-2005 at 13:01


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.



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searat
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[*] posted on 12-3-2005 at 19:48


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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[*] posted on 13-3-2005 at 08:26
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 :o

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]




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[*] posted on 13-3-2005 at 10:13


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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[*] posted on 13-3-2005 at 10:41


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]




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Chris The Great
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[*] posted on 13-3-2005 at 17:10


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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[*] posted on 23-3-2005 at 14:53
FYI


Silicon carbide is sold cheaply as rock polishing grit.
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SAM4CH
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[*] posted on 28-3-2005 at 02:07
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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[*] posted on 28-3-2005 at 09:49


trick is making the sc crucible. can be done.
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[*] posted on 28-3-2005 at 11:45


SC-graphite crucible can be bought at casting suppiers.
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[*] posted on 29-3-2005 at 00:54


I need more about SC-graphite crucible and its idea in this!
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[*] posted on 29-3-2005 at 01:48


SC, or more accurately S2C > CS2, will have to be cooled quite a bit to make a crucible out of. :P 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. :mad:

Tim
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[*] posted on 29-3-2005 at 08:35


http://budgetcastingsupply.com/Crucibles.html
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