Introduction: Microwave Smelter

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Remember how your parents used to say "don't put metal in the microwave"? Prepare to throw that cardinal nugget of advice straight out the window because we're going to do just that. We're going to use a domestic, unmodified microwave to melt metal! 

I happened upon an article in an old Popular Science magazine (c.2003) about microwave smelting and thought it was so awesome I had to try it for myself. So, just what happens when you try to smelt metal in the microwave? Turns out, it works!

Also, if you do it wrong you end up with a fiery microwave disaster:

Want to see how it's not done? Let's go!

Step 1: Tools + Materials


tools:
  • oven mitts
  • safety goggles + face shield
  • metal tongs / grips
  • old microwave
.		materials:
  • stainless steel measuring cup (crucible)
  • silicon-carbide block (details in step 2)
  • ceramic casserole dish / fire bricks
  • tin / lead / silver solder (details in step 2)
  • mold for metal ingots (pancake mold)


Step 2: Science + Disclaimer

How does it work?
In it's most basic form the microwave is being used to generate heat to an element which then melts the metal, while not arcing the magnetron to the metal to be melted.

The microwave I used was an 850W microwave (model: GE 3850W3W081A), I used regular bricks to build the hearth to keep the crucible and a silicon-carbide material as the heating element (I also used building insulation to try and keep the heat directed inwards, a terrible terrible mistake). The insulation was an addition I incorporated after reading another smelting article and attempting to blend methods to achieve more effiecient results. However I didn't anticipate the reactions of all the elements together in my microwave. The outcome was a success, and a failure.

Next, I had to choose metals that I were readily available to anybody and wold have a low enough melting point to be melted in a regular microwave. For this experiment I chose 2 types of common plumbing solder, 50/50 blend of tin/lead and silver solder, having a melting point of 180-190 °C (360-370 °F) and 450 °C (840 °F) respectively. There's other metals that could be smelted this way, like zinc (and plutonium?). Maybe you can find other metals with low melting points, here's a good place to start.

To help focus the energy of the microwave I used silicon-carbide, which is a microwave susceptor: meaning it absorbs microwave energy and turns it into heat energy. Silicon-carbide can be found in block-knife sharpeners, but I found they were too expensive. Instead, I used a silicone-carbide rubbing stone for under $14 found at the hardware store.

The crucible I used was metal with a higher melting point that the melting I was attempting to smelt. My crucible was a regular stainless steel measuring cup (melting point of 1510°C [2750°F])

To recap:
Microwave: 850W
Tin/lead solder: 180-190 °C (360-370 °F)
Silver solder: 450 °C (840 °F)
Microwave susceptor: silicon-carbide rubbing stone

Step 3: Safety (seriously)

Smelting in a microwave isn't a bad idea, but does require some precautions to be safe.

It should go without saying that molten metal, hot bricks, radiation and fire can cause harm you you and everything around you., Be aware of your surroundings and always use appropriate safety measures when dealing with hazardous methods and materials. 

Here's my setup:
  • Face shield
  • Eye protection
  • Oven mitt
  • Metal tongs/grips
  • Fire extinguisher
  • Bucket of water
  • Outside environment
The water bucket shown here was our emergency backup. As you can see from the video we ended up using it. Always unplug your microwave and other electrical equipment before introducing any water. Electrical equipment and water do not mix!

Step 4: Build Hearth

The Popular Science article I read mentioned using a casserole dish for the hearth, but then went on to say that the glass top of the dish cracked due to thermal stress after being heated then removed from the microwave. Not wanting to have shattered glass in my face I opted to skip this step and go right for brick as the hearth.

Knowing that I would be reaching tempertures of  500°C (900°F), I wanted something that could handle the heat stress and fracture if it failed and not splinter. I chose brick. There's a specific dense brick used in fireplaces called fire-brick that would have been ideal, however I used think regular bricks and no fractures occurred in over 40 minutes of heating. I also (erroneously) added an additional layer or insulation to the inside of the brick to try and create more heat in the hearth. Do not do this! Brick on it's own will suffice.

Stack your bricks into a simple house-like configuration. Leaving anough room in the middle to have your silicon-carbide slab to sit on teh base of your brick-house with the stainless steel crucible on top. Lay bricks over roof of brick house.

Step 5: Prepare Metal and Crucible

I used a stainless steel measuring cup for my crucible, which has a meting point of 1510°C (2750°F)
 I knew that I was only going to reach about 400/900, so using this as my crucible was a fine choice. Glass would have also worked, as it melts at around the same point as stainless steel (and higher, depending on the type of glass).

Find your metals and coil or snip into your crucible.

Step 6: Timing

The wattage of your microwave will dictate the length of time your smelter will need to be timed for. It's smart to start with a short time duration to ensure your hearth and insulation are assembled correctly, then work your way up from there. The silicon-carbide heating slab will stay warm for a long time, so minimal loss occurs if you open the microwave to check on your smelter.

Here's the data from my 850W microwave:
  • 50/50 lead/tin
    • 5 minutes - mostly melted
    • 10 minutes - completely molten
  • Silver
    • 15 minutes - completely molten

Step 7: Pour Ingots

If all goes well, after a few minutes you'll notice that your crucible has heated up and smelted the metal. Grab your crucible with tongs and pour it into your mold. I used a cast aluminum pancake mold found at a local houseware store. The melting point of cast aluminum is 660.32 °C (1220.58 °F), well above the molten temperature of the lead/tin and silver solder.

Pour molten metal into mold and allow to cool, then gently tap reverse side of mold to release the cast ingot.

Step 8: How Not To

You've watched the video, you saw the inferno. That was caused by double-whammy of not paying attention and using inappropriate materials being used. Using insulation is sound, provided it's rated to be used in high-heat situations. In my effort to keep things accessible and open I chose to use a rigid-type foam building insulation. Bad idea. The heat from the crucible in combination with the duration I had set caused a corner to the foam to ignite. I shut off the microwave and waited to the flame to die out, but it was only getting worse. Fearing a backdraft if I opened the microwave door, I risked it anyway. Yup, huge fireball.


We were able to get our cameras running just when the flames died down from reentry temperature to just immolation inferno.
The lesson here is to use just bricks to create the hearth and wait the amount of time required to to the job effectively, without trying to accelerate the process with insulation.

Science can be messy and dangerous, so be safe and have fun!