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