Smelting in a microwave

Our scientist zaps tin and silver, shatters glass, and arcs his oven to prove a point.
A firebrick-and-silicon-carbine furnace enclosure inside a microwave with red-hot silver inside it.
After building a furnace enclosure out of firebricks and silicon carbide (shown), the author made silver glow red at about 800 degrees Celsius, slightly below its melting point. Rory Earnshaw

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 website 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.

Achtung! Theodore Gray is a scientist trained in lab safety procedures. Do not attempt this experiment at home. For more information on Gray’s scientific pursuits, visit his website.

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 degrees Celsius). 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.)

A person wearing thick leather gloves and pouring molten tin solder into a fish shaped mold outside of a microwave.
Molten tin solder is poured into a fish mold. Rory Earnshaw

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-carbine furnace enclosure. Same principle as the casserole dish, just better, safer insulation. Although I fell short of melting the silver at 962 Celsius, I did make it glow red-hot at around 800 Celsius.

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.

This story was originally featured in the September 2003 issue of Popular Science magazine.