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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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. 