Microwave Smelter

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Introduction: Microwave Smelter

About: I'm Mike and I make crazy things at Instructables HQ in San Francisco. Follow me and try a few of my projects for yourself!


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!

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    172 Comments

    Doing a bit of research... it appears that a cheap angle grinder disc made from silicon carbide will work

    Your title may have been actually fulfilled if you had let the microwave burn longer, and smelted the microwave. Well, now we know what happens. Imprudent, but glad you took all the safety precautions. It might have been a huge disaster.

    I think I'll gut the machine for the heavy transformer, rewind it's secondaries, and go for a copper induction coil and crucible sort of smelting arrangement.

    Interesting teaching tool, this one. I won't be trying it myself, but thanks for sharing.

    What do you think about using insulating ceramic bricks http://www.ebay.com/itm/K-26-Insulating-Firebrick-9x4-5x-2-5-IFB-Fire-Brick-Thermal-Ceramics-Bricks-K26-/200945542607 and a crucible made of silicon carbide http://www.ebay.com/itm/A3-Silicon-Carbide-Graphite-Crucible-Furnace-Torch-Gold-Copper-Melting-Smelting-/111414338994?hash=item19f0cfe5b2:g:ne0AAOSwPTlTzN8b? If I simply made a plunge hole in a brick and inserted the crucible and put another brick and then put metal in the crucible, is that all I would need to do?

    sometimes I think the technicians get overly distracted by irrelevant details... all the conversation about lead vs lead free solder as well as the technical points of whether melting metal into a liquid form by whatever means is or is not considered smelting are off the point of whether the instructable was useful, helpful, or exceedingly dangerous... for my part, I watched the video, saw the outcome, saw the fire, the guy was smart enough to set it up outside, so, no harm done,... whats the big deal with all the other crap? I just learned that I can melt metals in a microwave! How effing cool is that?! This information can be useful for making bronze (copper/tin/lead) or for smelting other metals into ingots for preservation. Anyway, thanks for the instructable, theres a good chance Ill make one myself someday. The idea about making an induction furnace, or possibly a stick welder from the transformer is a good one too.

    Kaowool (ceramic wool) would be an option for insulation. It can handle temperatures up to 1200C. The insulation is necessary to fool the temperature sensor of the oven so it won't shut down to prevent fire. The bricks you used did the job. Now that the oven is toast (lol) you can take it apart and use the transformer to build an induction furnace. Just rewind the secondary with really heavy gauge wire.

    Interesting experiment. I agree with Void Schism comments, I was wondering if it was going to get more exciting with the water put on the electronics. There are some low temp casting alloys out there that may be interesting to try out.

    You do realize this contains LEAD right? Lead is a carcinogen whether it is being melted or held in your hand a a charm or around your neck as a necklace.

    You don't want long exposure of any kind to this stuff. Seriously.

    13 replies

    Lead isn't a carcinogen. It's a toxic, heavy metal - a poison. That's different. It isn't going to hurt you unless you ingest it, either by eating it, chronically getting lead paint (which isn't sold anymore) on your skin or by inhaling the fumes after it reaches it's vaporization point which is a whole lot higher than it's relatively low melting point. If one were to smelt lead every day, there would be danger. One time isn't dangerous if you are the least bit careful.

    It can also be absorbed through the skin in solution.

    Do you remember good old time of tetraethil lead in gasoline ? I guess some of that stuff is stil spread all around

    So why is there this huge push away from it for consumer (and most other) electronics?

    Because it if it ends up in landfills it contaminates the water table. Of course, every public waste disposal site that I know of keeps electronic junk out of the landfill and recycles it properly but because it once did end up there; politicians created the legislation to eliminate lead in all forms. Makes for good campaign bragging rights. "Senator Jones drafted legislation to eliminate lead and protect our children."

    Then why do we still use Lead Acid Batts for vehicles?

    Because there is no acceptable substitute. It's pretty much as simple as that. No other type of battery can stand up to the thousands of cycles of charge / discharge, put out as much current and withstand the environmental conditions that vehicles are subject to. At least not in that price range. How would you like to have to pay $350 to $700 for a car battery (installation not included)?

    i've read that iron acid batteries are better for lifespan and resiliency, but they're low capacity. now ni-mh could be used, they have low size, high current and capacity. at least acid iron could be built at home.

    Lead Acid batteries are still being use for vehicles because an all around substitute hasn't been created. The hazards of using them have been mitigated to a large degree on both the manufacture, and consumer end. The salvage price for lead has always been constantly high enough that persons in the know, never sent them to the land fill. The high deposits we see now on new batteries is to keep the ignorant, and lazy from sending them to the landfill.

    Also, due to the large deposits encouraging battery exchanges, something like 98% of car batteries are recycled.

    I think too there is a certain paranoia about lead. This paranoia is, in part, spread by certain legislators who, in an effort to look as though they are actually earning their salaries, play up the supposed dangers of "demon Lead" and pass legislation against it. If the web site I looked at earlier is correct, easy flow silver solder contains cadmium which is definitely something you don't want to breathe the fumes. I have read cases of jewelers killed by melting cadmium and breathing the fumes. I have yet to see anything about anyone dying from melting lead. If melting lead was that dangerous plumbers should have been dropping like flies before legislation banned it in plumbers solder. I believe the effects lead has on the brain and nervous systems of immature humans is the main thing we must be careful of when dealing with lead and that is more likely to incur through ingestion.

    This is a good point. It's easy enough to find "Lead Free" solder these days (Sn-Ag-Cu), people should just use that if they plan on handling their ingots (as a paperweight, etc) created here.