Melting Metals in the Microwave

Yes, you can melt metals in your microwave, and it's way easier, faster, and more efficient than you think!

For the last few years, I've been experimenting with "MMM" - Microwave Metal Melting. I've gone through many prototypes and experiments. The project was delayed over and over again because I kept trying to find ways to make it simpler so more people could replicate it. I am very pleased with the final result.

It's fast - You can melt aluminum in a matter of minutes (depending on the quantity). You can melt cast iron and stainless steel - Even though it's best to use MMM for lower melting point metals (aluminum, brass, copper), it's possible to melt cast iron and stainless steel in smaller quantities.

The Basics

The basics of MMM are very simple. We use a silicon carbide crucible (that I'll show you how to make in step 17). Silicon carbide "absorbs" microwaves and heats up. If we insulate the crucible, it's possible to reach very high temperatures, like stainless steel melting point high. That's all there is to it!

In this Instructable, I'll demonstrate how to make an insulating chamber, the base, and a silicon carbide crucible. I'll be using a collage of different chamber pictures to make this Instructable more interesting.

Silicon Carbide | https://amzn.to/3MJz4nb

Sodium Silicate | https://amzn.to/3GK32DA

Ceramic Fiber Blanket | https://amzn.to/3ofizVN

Kapton Tape | https://amzn.to/40a8qXy

Kiln Wash | https://amzn.to/4166PmY

Kaolin Clay | https://amzn.to/3KWsjNv

Boric Acid | https://amzn.to/3zZyMRB

Aluminum Oxide | https://amzn.to/3ohJ2lK

Gas Torch | https://amzn.to/40p9dEt

Important: Wear a respirator when working with Ceramic fiber.

We'll create a chamber by wrapping a ceramic fiber sheet around a cylindrical object. The cylinder can be an empty cup, a food can, a 3D printed object, etc. It should be larger and taller than the crucible, but I don't recommend going too big or making it very tight.

While having the chamber cavity much larger can be an option if you want to use it with different-sized crucibles, it's not the best choice. Small crucibles placed into large-cavity chambers will heat up much slower. Personally, I use two different-sized chambers: one for small crucibles and the other for large.

Making the chamber cavity too tight can be hazardous. The crucible might end up sticking to the chamber if there is not enough space left for error.

We'll cut a strip of ceramic fiber sheet, and then we'll chop one end of the strip at an angle.

We'll wrap the cylinder around the ceramic fibre sheet until the desired thickness is reached.

Then we'll make a cut keeping in mind that we'll also need to cut the other end in an angle.

I rarely make the chamber thicker than 3 layers (1 layer = 25mm).

Most of my smaller chambers are only 2 layers thick.

We'll cut the other end of the strip in an angle.

We'll be using a kapton tape to wrap the chamber.

Just make sure the kapton tapes long range temperature resistance is at least 250°C (482°F)

It's important, because not all kapton tapes have the same temperature resistance.

Once you finished wrapping, feel free to roll the chamber backwards/forwards if necessary.

This action can sometimes make the chamber look more round…other times it can be the opposite.

Here is what I do: If the chamber looks round, I will not roll it or I might give it a gentle roll.

If the chamber is noticeably not-round, I might try to fix it by rolling

Depending on what kind of cylinder was used, you can leave it in or take it out by twisting and pulling.

I prefer to leave it inside of the chamber as long as possible....if it's not too tall.

We'll put the chamber on top of 2-3 sheets of ceramic fiber.

Then we'll cut away the extra material around the chamber.

We'll flip it around and use kapton tape to join it with the chamber.

Using a lazy Susan/turntable is helpful

We'll cut out a circle using the cylinder as stencil and put it inside the cavity.

If you have a blowtorch, I recommend heating the inside of the chamber to harden the fibers a little bit.

It's an extra step that can be done to make sure the fibers don't move around too much while we work on the chamber as they will become a bit harder.

That said, if you don't do it it's fine.

Just like before, we put the chamber on top of a few ceramic fiber sheets and remove the excess material.

Then we wrap it with kapton tape.

I like to stick the edge of kapton tape onto the chamber, so it has somewhere to hold to.

Then we choose the ugly/less smooth side and cover it with kapton tape.

We'll make kapton tape and ceramic fiber pillows that we'll use as feet.

Sometimes I add 3 other times 4 feet.

To make the bottom of the chamber and top of the base nice and smooth, we'll use a sanding glass.

All you need is a piece of glass and some sandpaper or you can use silicon carbide (that you'll need to make the crucible anyway)

In this example I used silicon carbide because it's my preferred method.

We'll hot-glue the glass onto the table.

Hot glue can be easily removed with some alcohol.

If hot-gluing is not an option, you can also cover the table with cling-film/plastic wrap to stop the glass from moving.

Just make sure the glass is clean or it won't stick.

Pour some silicon carbide onto the glass and add some water.

Use another piece of glass or a drinking glass with a flat bottom to sand the big glass.

Now we can use the sanding glass to make the bottom of the chamber and top of the base nice and smooth.

Wet the glass and bottom of the chamber/base and start mashing those layers together.

I say sanding, but in reality it's more like mashing layers together.

The bottom will start to close up from mashing/sanding.

More you sand, more it will close.

Use a sharp knife to cut it open.

When sanding base make sure to slide it off the glass when checking on progress.

If you try to lift it, it might stick to the glass a bit too good. That can result in layer separation.

Next step is putting the chamber and the base in the microwave to dry them.

If there is a lot of water present, take out the chamber during the drying process and to wipe the microwave's walls.

We do the same thing with base.....sometimes the base can grow a toad skin or bumps.

That's normal.......just dry sand them on glass.

Don't worry about the base not looking perfect!

If the base has not come out as smooth as you want it to be...that's fine.

It will shift and shape once used with a crucible. After a few times of use, you can re-sand it on glass to achieve a flatter look.

We'll heat up sanded/mashed surfaces with a blowtorch to make them a little bit harder.

If you burn the kapton tape (the edges), make sure to cut off the burnt part.

Burnt kapton tape can produce arcing and the kapton tape will slowly smoulder.

You can see a detailed demonstration in the video.

So avoid burning kapton tape or make sure to cut off the burnt edge.

For the record: I've never experienced arcing with a kapton tape that was not burnt...I am talking about thousands of hours of use between microwave kilns and MMM chambers. It has only happened with a kapton tapes that was charcoal burnt with a blowtorch.

At this point you could use the chamber and the base, but.......if the crucible is heated too much, it will stick to the base.

That can be a hazard when the crucible is full with molten metal.

One way to stop the crucible from sticking is by using kiln wash.

Mix it to a consistency of a pancake mix or a bit thicker and brush onto the base.

Then microwave-dry it!

I also like to brush some kiln wash onto the bottom of the chamber, but usually I make it a bit thinner.

No we can heat up the crucible for the first time.

Firs time you use the chamber, it will get pretty hot because of water trapped inside of ceramic fiber blanket.

It's normal for the kiln wash to crack. It will crack less once the base stops shifting and shaping.

As I mentioned before, don't worry about making a perfectly flat base. It will not stay that way!

Ceramic fiber will sink in like a Grand Canyon...especially if you melt brass, copper etc.

If you only melt aluminium, then the base can stay pretty flat-ish as the heat is not that high.

I usually re-sand the base and the chamber after using it for x-times. That way we can make it flatter because most of the shifting/moving has already happened.

When re-sanding the base, just spray some water onto the glass as sand as usual. It's ok that the base/chamber has been covered with kiln wash.

Most of the times, I don't re-apply kiln wash to the chamber, It's usually not necessary. There is enough kiln wash soaked in the fibers.

Then microwave dry it and that's it!

If you struggle to make the base smooth, try dry sanding.

Sometimes you have to try both (wet or dry sanding) and see which gives better results.

If you can't find kiln wash, here is one way to make your own.

Just mix 50% of Kaolin clay with 50% Aluminium oxide by weight.

If you want, you can add some pigment, so it's easier to see the kiln wash once you brush it onto the base.

Feel free to play around with ratios. You can add more aluminium oxide if you want.

Because of the pigment, the kiln wash has a grey tint instead of being white.

Feel free to re-sand the surfaces after using the chamber a few times. As I said before, ceramic fiber will shift and shape for the first few times of use. It also depends on how hot the crucible is.

After using the chamber a few times a lot of that shifting/shaping will stop.....not completely, but it will slow down.

That's the time to re-sand the surfaces to make them smoother.....if you want to.

Just sand it like before, on the glass with or without the water.

Most of the times I don't bother to reapply the kiln wash to the chamber after re-sanding. Usually it comes out nice and smooth and there is enough kiln wash soaked in the fibers.

If the base has developed crater from the hot crucible, that's fine. Don't bother making it flat.

By re-sanding the base, we're trying to make the surface around the crater flat-ish, so the chamber and the base "marry" nice and tight without letting too much heat escape.

That said...having some gap between chamber and the base is fine.

Another way to stop the crucible from sticking is by using only aluminium oxide (alumina).

Alumina is great to stop the crucible from sticking, but it does not brush-on very well.

Once mixed with water it's like wet sand. There are methods to apply it, but let's not complicate thing.

Instead, I like to apply aluminium oxide only under the crucible. It will stop the crucible from sticking and it will stay stuck to the surface enough to NOT fall off.

If you don't use kiln wash to cover the bottom of the chamber and top of the base, those surfaces can start to deteriorate.

Kiln wash (even though it cracks) does protect the ceramic fiber.

Deterioration is like holding a chalk in your hand for too long. Ceramic fiber can start to crumble little by little.

It's not always an issue, but it can be.

If I only use aluminium oxide and don't use kiln wash, I like to harden the bottom of the chamber and top of the base.

That way we can stop or slow down deterioration.

This step is optional and can be done later in your MMM journey, if you see there is an issue that needs to be addressed.

Before applying aluminium oxide, we'll harden the surfaces with boric acid solution.

To make the boric acid solution, weight some water and add between 5% - 10% of boric acid.

For example: 400g of water = Add between 20 - 40g of boric acid

Then heat up until boric acid dissolves (wearing a respirator is recommended)

Wait until the solution cools down a little bit before transferring to a spray bottle (to avoid burning yourself or melting the bottle)

I recommend not using more than 10% of boric acid as it will start to recrystalize rapidly once the liquid begins to cool down.

That can clog up the spray bottle.

Wet the sanded and dried surface with boric acid solution, then microwave-dry it!

Depending on the concentration of the solution and how much you spray, you might need to repeat the procedure a few times.

Then heat up the surface with a blowtorch. The flame should start turning green from boric acid.

Now the surface should be noticeably harder.

See the video of a sound/spoon hitting demo.

Now we can apply alumina in the centre of the base.

Make alumina and water slurry and spread in the spot where the crucible will go.

After microwave-drying it, alumina will stay stuck....but not exactly glued to the base.

Once you start using a crucible, it will stick even more.

I will demonstrate 2 ways how you can make a silicon carbide crucible.

You'll need a fine grit silicon carbide.

You can use medium grit, but I find that fine/powder grit will produce stronger crucibles.

For the glass method, you'll need a drinking glass that will be our mould.

Just make sure the glass is not too thin.

First we’ll fill up the glass with silicon carbide to the desired point.

Then we'll transfer it to another container and add between 8 - 10% sodium silicate.

Sodium silicate is also known as water-glass. You could say that it's a high temperature resistant glue.

The water-glass I used is a 37% solution.

Anything between 37% - 40% should give similar results.

The reason I am mentioning it is because there are products out there that are very diluted, yet people think it’s the same thing.

Usually those products are not advertised as water-glass or sodium silicate. They might be advertised as a “Gasket Sealer” or something like that.

Your neighbour might advise you to buy the “Gasket sealer” because it’s the same sodium silicate, but much cheaper than the product on the web that is advertised as sodium silicate.

He might be right or wrong.

Check the label. If it does not say that it’s 37% - 40% concentration, look for other options if possible.

Mix it thoroughly and fill up the glass step by step pressing down as good as you can with a tool of your choice.

I prefer to use a 3D printed press, because you can make it the perfect size.

Now we can carve out the middle.

I also like to use my finger to shape the crucible (if I can reach)

We'll put the glass in the microwave (on top of the base or ceramic fiber sheet) and microwave on full power until glass shatters.

Consider wearing eye protection when checking on progress

Once cooled, free the crucible from the glass mold.

You can use different sanding methods to keep shaping the crucible if necessary.

At this point the crucible has reached...what I like to call - Stage -1

You could use it at this stage, but maybe you should not.

I don't want to scare you. I use crucibles at this stage more often than not, but I've also practised a lot.

Crucible at this stage is not as strong. There is a possibility that it could split in half once you use it.

It might NOT happen the first time you use it, but it could later down the road.

Crucible at this stage could "naturally" become harder once you star using it.

Heat, fluxes, different metals used, contact with ceramic fiber...........can all affect the crucible.

I personally use smaller crucibles at this stage more often than not and I pay attention to the state of the crucible before I use it.

I am always ready for the worst. Just like with all metal melting - same rules apply!

You don't hold crucibles full with molten metal over your head, legs etc.

Wear safety equipment.....etc.

So, can you use the crucible at this stage? Yes! But it's better not to.

Just remember the dangers!

If I said that the crucible had reached Stage -1, then there must be Stage - 2

This is how you can make the crucible much stronger (and uglier) by hardening wrapped in ceramic fiber.

We'll wrap the crucible in ceramic fiber sheet. I usually use a half sheet as it's easier to wrap.

As you can see, I also covered the top and the bottom with corresponding ceramic fiber circles.

Then we use 2 rubber bands to keep the sheet from unwrapping.

We'll microwave it (away from the door) until the crucible glows red.

It's important for the crucible to reach glowing red temperatures, not just....Oh, I think I see a bit of red.

If the crucible is not heated enough, the hardening will not happen!

Rubber bands will break and shoot away from the crucible.

Tip: Don't use rubber bands that are too big. What I mean, don't do the old twist, twist, loop.. now it holds trick!

If you do that, rubber band will break, but fail to shoot away. It will stay stuck to the ceramic fiber producing smell of a burning rubber.

Once cooled, take it out from ceramic fiber sheet.

Careful, the crucible might still be hot despite the ceramic fiber feeling cool.

Scrape off ceramic fiber and you're done!

I usually start with a popsicle stick or a spatula and finish with a wire brush.

Warning: Scraping OFF ceramic fiber will produce more dust than any other step we did before.

Make sure to wear a respirator.

That's it! The crucible has reached Stage - 2 and it's much stronger!

You can download 2 different size molds and corresponding tools that you can 3D print.

You'll need to tie a rope to one of the 3D printed parts.

Then screw it onto the other part and assemble rest of the mold.

We'll mix silicon carbide with water-glass just like before.

For the small mold you'll need approximately 100g of silicon carbide.

So we need to add between 8 - 10% of sodium silicate, or 8 - 10g in this example.

Use 3D printed tools to press silicon carbide into the mold nice and tight.

Put the mold in the freezer for a few hour.

Download files for the small crucible under..........

We'll use a Hex/Allen key to unlock the inside mold.

Keep twisting the key and pulling until the inside mold is removed.

Then pull the rope to remove the lid.

Take out the crucible and put it in the hot oven.

I usually preheat the oven and then turn it off because I don't want the aggressive heat from the heating elements.

After 10 - 15 minutes, the crucible is ready!

Another option is just leave it to harden on its own.

After a few hours, put the crucible in the microwave and heat a little bit to finish hardening.

You can also microwave the crucible from frozen (if you're brave).

Microwave on low power bit by bit.

For example: Microwave 10 sec, stop and wait for 30 sec, repeat.....times are only example

I usually cover the mold with a plastic cup that has a hole cut in it. It seems to help.

I remove the cup after a few Start - Stop cycles.

It works, but takes some practise.

Microwave it too aggressively and the crucible will deform.

From all these methods I recommend the oven.

Download files for the big crucible under..........

If you can't remove the lid with the rope easily, just leave it in the crucible. It will release itself once the crucible defrosts and hardens.

If you don't own a 3D printer and somebody is printing the mold for you, ask to print a few extra pieces of the lid-with a rope.

That's the only piece that can sometimes break. That's why I don't force it too much and leave it in the mold if it does not come out easily.

The crucible has reached Stage-1. Just like before, if you decide to use the crucible at this stage, heat it in the chamber before the first time of use.

To take the crucible to Stage-2, microwave wrapped in ceramic fiber sheet.

That's it!

Now you can melt metals in the microwave!

Watch the video for microwave metal melting examples.