Melting metal 'in the kitchen' .....

[Downunder35m]

I recently had to somehow fix an old part for an even older tractor - a broken part.
As it failed in action only one half could be found but there was a mirror one on the other side of the tractor.
Only one problem: I had to do it in brass to keep it authentic......

Long story short: The method I am trying to bring to you works for almost all metals and their alloys - ALMOST ALL
Aluminium, coppers, brass and such are no problem at all.
Steel however requires a bit more, still possible but more on that later.

Things you REALLY need:
And old microwave, preferably 1000W or more and with good size.
A roll or some sheets of ceramic insulation stuff as used for wood fire ovens and such.
Some furnace slurry, either commercial or home made.
Some old glass pane to be used as a sanding tool.
Silicon carbide powder, preferably two different grain sizes, like 600 and 1200 grit.
Kapton tape.
Waterglass - sodium silicate solution.

Optional things I highly recommend:
A good respirator as you really don't want to cut ceramic material without it.
Good work or rubber gloves - again for the ceramic stuff so it won't end your skin and from there in your eyes.
Some suitable moulds to make the crucible, be it 3D printed or just some old drinking glass that is wider at the top to make removal easier.

What is the goal of all this?
Of course to melt metal in a microwave !!!

[Downunder35m]

If you find a warning sticker on your microwave stating not to put metal into it then please take it off so it won't interfere

Ok, why would be so dumb to put metal into a microwave?
Because we know what we are doing....
Why does it work?
Because mircowaves provide an awful lot of energy if used properly.

Let's start with making a crucible....
Why not just buy one of these cheap thingies off Ebay or so?
Because they are designed to work in a furnace, with a hot flame, in a microwave the carbon used for them would just burn away and ruin your microwave.
So why use a crucible at all then?
Because we need it to heat and then melt the metal....
Confused much already ?
Let me fix this....
Silicon carbide is a semiconductor material and as such it reacts very well to microwaves by quickly turning insanely hot.
Since it has a much higher melting point than most metals it will transfer this heat to the metal and we get a nice puddle of liquid metal in a few seconds.....
Making your own crucible isn't hard and comes at a fraction of the cost of buying a pure enough silicon carbide crucible - if you can find one small enough to fit into your microwave....

Ok, let's get going here....
Start by mixing equal amounts of the silicon carbide powder together.
We use two different grid sizes as it provides a much stronger crucible due to the many gaps left being filled with smaller particles.
By volume you then want to add about 10% of the waterglass.
If you don't want to buy it from some glazing shop or such you can make your own from crystal cat litter - look it up on youtube.
You will need quite some time to mix it evenly and to get all the powder moist.
Since it is an abrasive you should prefer mixing things you don't mind getting badly scratched....
It will be tempting to add a bit more waterglass but try to resist this as you really want as little as possible here.
If you ever made green sand for a mould you know how it end up like.
On a warm day it really helps to put the waterglass in the fridge before using it as otherwise things might get dry and hard to quickly.

Let's assume you don't have a 3D printer and want to go basic:
Fill a glass (one you won't mind loosing) with the mix and compact as good as you can, meaning hard but without breaking the glass.
Now simply scrape the inside out to get your desired crucible size and shape.
Make sure to leave at least 8-10mm for the walls as otherwise you might break it during later use.
You can smooth the carved out areas with your fingers or use a spoon if you prefer.
When satisfied place in the oven at about 180-200 degrees Celsius for about 90 minutes.
Don't worry, the glass will crack eventually....

While the crucible is pre-cooking measure your microwave after placing it on the side - with the heavy part on the top.
You want to get the best dimension for cylinder to stand in there so you can still take it out without problems.
Let's say your microwave insides are 40cm high, 20cm wide and 20cm deep.
Considering you might have to use gloves those 20cm won't leave you much room to play with.
But with the doors now opening downwards and flat on the surface you can just use a hook going around to pull it out enough to grab it - be creative and try to maximise the volume for the padding.
Take another glass and use this to roll the ceramic sheet around it - it helps to cut the end at an angle with a sharp knife.
Make sure you cut the sheet so your roll is about twice as high as your crucible.
Roll until you reach the desired thickness to still fit into your microwave.
Secure with Kapton tape.
Now use this roll as a template to cut a bunch of disks - they will form the lid.
You also need to cut a bunch of smaller disks to be used inside our roll to form the bottom.

What to look for before you start cutting:
We will be using this insulated chamber UPSIDE DOWN once finished, meaning our 'lid' will be the bottom.
You want the roll be long enough so you can add enough disks to fill about 4cm on one end.
The remaining space then still should be long enough for the crucible to fit even if some metal pieces would be sticking out slightly.
Plus: you will need a few mm in order to being able to sand it flat and smooth....

Don't worry if you struggle a bit and the rolled ends look a bit ugly.....
Use some more kapton tape to fully cover the side and closed end of your roll - you don't want any of the ceramic stuff visible except whe the crucible and lid goes.
If you do have the room in your microwave then try to make the lip at least 5cm thick.
Now for the fun part, sanding it smooth....

[Downunder35m]

By now your crucible should be bone dry and a bit more sturdy.
To make sure let it cool a bit, then place a metal plate over the glass for a moment - if moisture condenses put it back in the oven for a bit longer.

Since you already have a powerful abrasive at your disposal use it with some water and piece of wood or sturdy cloth to make the glass really dull and rough.
Oh, you wonder why you don't use sandpaper instead?
Well, try it, then you know
The glass should look really foggy and have a rough feel to it now.
Time to wet it and to do the same with the open end of your roll.
Use a spray bottle but only WET it, do not soak the roll.
Using a circular motion and not to much pressure you move the roll over the glass but once done and you want to check do not stop, move it off the glass - otherwise you will just lift out more and more fibres and sand forever.
The roll will get really stuck on the glass once it starts to get smooth.
Don't try to get it perfect, the goal is to get it perfectly flat and smooth enough to hold our slurry later.
For the lid, since it is already a flat face of the sheet you won't need as much work and it really is enough to get it properly flat - you want a goot seal when you put it together to melt metal.

You will notice that your carefully created hole is now pushed in.
What a bummer - but now you know why I said it does not matter earlier
Time to try this microwave....
Place the roll or the lid inside and use the microwave to dry things out.
About 2 minutes and the inside should be quite wet, so wipe it dry and do it again until no condensation happens.
Once you dried the roll use a SHARP knife to cut the pushed in bits away but be careful!
Cut to the INSIDE and DON'T pull the knife out to get a cleaner cut.
The later would mean you ruin your flat surface and need to sand it all over again....
Check if the glass still fits.
A bit of light rubbing is ok, if more use the knife to scrape off what needs to go.

Chances are your kapton tape now needs some trimming as well.
This is not easy and IMHO a very sharp pair of scissors works best as you use them to cut right along the edge.
As things will get very hot whatever you created so far won't last forever.
But you extend the lifespan by adding some furnace slurry to the mating surface of the roll and the lid.
The ceramic material is a great insulator and can tolerate the heat created by the crucible.
On the other hand it is VERY fragile once fully heated.
The slurry is not much better but has more resilience to withstand the constant rubbing we can't avoid when handling and using our microwave furnace.
So once it starts to wear down or you get a dint where the crucible is supposed to have a firm stand you just re-apply another coat or fill the hole with a few coats.
Just don't forget to cure or at least properly dry it in the oven before melting metal - water and melting metal is not a good mix...

Oh, you have no clue where to get proper furnace slurry or such?
Get creative
You already have the waterglass, so you can make your slurry by adding magnesium oxide, aluminium oxide, vermiculite - or those pesky leftovers from cutting the insulation pieces, just break them up in a mortar or such....
You can also use some clay (mud) from your garden you ran through a fine sieve and burnt properly with a torch to then grind it into a fine powder...

[Downunder35m]

Up until know your only worry were gloves and a good respirator.
Now let the fun begin.

I assume that you confirmed the entire insulation pack is now bone dry....
If the glass already cracked in the oven then it is now time to carefully remove it all to free the crucible.
Don't do this if there is only a few little cracks as chances are you will crack the crucible in the process.
You can remove the more cracked glass after the first heating round in the microwave.

Either way, place it on the LID and then add the roll on top.
Do one last check that all looks good and place it your microwave.
If you paid attention above then it is now standing on the side with the magnetron firing down.
If it has some low power settings then use them!
You want to heat the crucible SLOWLY this first time around as there is still no complete sintering yet.
For example set it to 10% and let it run for 30 minutes.
Take a sneak peak after but double check your rolls and lid are cold enough to touch - if in doubt wear oven mittens...
There should be no steaming when you lift the roll up and with bit of luck the crucible will be bloody hot.
DO NOT TOUCH IT! Even if it might not be glowing yet it WILL severely burn your fingers!
If you notice some bubbles have formed then let the thing fully cool down to then carefully scrape those bubbles of, a slight tap is often enough to break them.
Now do it all again but this time we go for 50% and 5 minutes to see how well this old microwave is still doing.
If you see it glowing red when you lift the roll we have a winner.
In case there is no glow at all repeat once cooled at full power for 5 minutes - this should give you a critical orange glow on a 1000W model.
Prefer to be patient and to start with short times until you figured out how much your MW with your empty crucible takes to get just past a red glow.
This time gives you the indication of the minimum required to get things to temperature.
Knowing this time for your crucibles allows you hone in on required melting times based on the metal you use and the weight you put into your crucible.
And if you did this BEFORE trying to melt any metal you make sure your crucible is properly sintered and hard.
You should hear a distinct difference to after the oven treatment and after the microwave full heat treatment when you tap it with something hard.

[Downunder35m]

Can I melt something now, can I ????
Nope, first you need a good drink to celebrate

I suggest to start with Aluminium as it is cheap and comes with a low melting point.
Prefer solid chunks over crap like old beer cans or already badly oxidised stuff.
BE VERY CAREFUL!!!
Molten metal is very dangerous, especially if it comes into contact with water, even a moist casting form can spill a disaster!
ALWAYS use proper PPE!
You can add more metal once the first bit is molten.
It will take far less time to melt than the first batch, so don't overdo it.
I hope that at this point you won't have to start swearing because you forgot that you some suitable tool to grab and handle this glowing crucible.....
If not, well, carefully tip it over to get the metal out then make a proper tool, like by using some old pair of pliers and welding on some sturdy wire you can for to grab around the crucible....

If your microwave managed Aluminium then other soft metals are no issue either, just a few seconds longer to 'cook'.....
Steel is a tricky one as said, mainly because the sodium silicate in our crucible melts at a much lower temperature.
Prefer not to melt steel....
If you really have to then try a new crucible and add about 5 to 8% of fine quartz sand plus about 3 to 5% of baking soda to the mix.
Try to only use a few more drops of the waterglass then the measured 10%....
Treat in the oven as above but do the microwave heating extra carefully.
After a few cycles of getting it dark red double this time to do 5 to 7 cycles of getting it to a nice orange colour - this can be best judged if it is dark enough....
Right now all of our waterglass should have combines with the baking soda and sand to well, form actual glass.
It is not really usable like this though as it will just crack once you try to steel in it.
You need to do one last cycle in the microwave now where you get the crucible to bright orange, close to a yellow.
Keep in mind that it always takes a little moment to get things out and to see the crucible!
If you 'boil' it for too long the formed glass on the surfaces will get runny.
Not a big deal but something to consider as it might crack and end in your metal castings, so double check the thing a few times or just do some ingots....
Of course you also need to make a suitable lid as otherwise you risk the steel burning off with the oxygen inside - or of too hot once you remove the roll to check things.
As said, prefer to stick with soft metals....

[Downunder35m]

Ok, you ruined an otherwise perfect microwave, created some stuff with your molten metal and now you are sick and tired of having such a hard time....

A dead simple crucible as we made is great fun and more than enough for do your gold nuggets or some little castings for your things...
But of course it is far from ideal, hence the initial mentioning of 3D printers.
It is quite easy to print some multi part forms to make a great mould for a crucible and if need be a lid.
And if you are not that good there is plenty of free templates on the usual sites you can just download and print.
On the other hand I find it quite easy to just some basic scraping and smoothing tools to carve out a good mould from just the compressed mix.
The great thing about waterglass is that it slowly cures in air by just drying out.
So if you think you might ruin it because the last one was so soft then just let it air dry a few hours and try then

For me I try to keep things simple and I don't really do much melting these days as I can't forge my knifes without a working shoulder....
When I do need to something a lot of times though I want things to work properly....
I tried quite a few designs of tools meant to securely handle a glowing hot crucible....
Never really liked and of them much.
If you make a cylindrical crucible you can use the basic double finger approach.
A thick wire ending in two that form a perfect match for the diameter of the crucible.
The slight flow of the wire make sure you won't accidentally break the crucible.
And they work just great - until they don't and you have big problem on your hands....
Much better are those designed for the grooved crucibles, here use one finger grabs the thing but it sits in a nice groove so nothing can slip out.
Problem is that these crucibles come with rather thick walls and usually for something in the above 1kg range for the filling.
On our scale you really need a large microwave, preferably a commercial or restaurant size model.
You need a good insulation and it has to be thick enough so the kapton tape won't fail, so around 240 degrees Celsius to be on the safe side.
I found a much easier way:
Have a conical (at least slightly on the outside) crucible and make sure the top lip is at least 10mm tick.
Make a wire ring that allows to fit over the bottom of your crucible.
Now cut enough of it away that you can push it over the crucible - it should start to grab about half way up if the conical shape is right.
Like with the normal finger thingies you now add one more wire to your pliers.
But so that it closes from the top of the crucible.
Like a finger pressing it down into the open lop.
Can't fall out, can't make a mess, just a bit weird to grab something this way.
But if you hold so you press with your thumb down it comes almost natural once you start pouring things.

[Downunder35m]

What about flux ?

Tricky question....
Sometimes we can't do without it if we want a nice a clean result.
Especially if you works with aluminium and find you struggle to make to to your form quickly enough after scraping all the crap off.
The crucible is a consumable anyways and you should have at least one spare around at all times.
So using a flux should not be much of a problem unless you use something rather aggressive or way too much.
Some fluxes can bond with your crucible and it is not always a good idea to scrape them off.
You try while still hot and soft enough but that's it.
Means if you need CLEAN metals you should have a dedicated crucible for each - just in case....
I found that thanks to the fast heating if a lid is used the problem of oxidisation and such is already minimised.
That is course is you start with CLEAN metals....
Some filthy brass with teflon tape left on thread is no good for the crucible an certainly no good in terms of the resulting sludge.

Speaking of.....
If you have a lid, then how to deal with that one without risking to drop and break it ?
Just slide it onto a fire brick or some leftover insulation sheet ....
As said, be creative as not always things require an expensive solution....

[Downunder35m]

What is the biggest crucible size I can use?
We CAN go tall is we have to.
But in terms of diameter it really comes down to your insulation.
The last thing you want is the inside of your microwave getting too hot....
Considering it is designed to cook food we can assume a short term kick of around 120 to 140 degrees should not harm it too much.
The insulation sheets are not really expensive, so you try to check what the minimum thickness is to stay of the safe side for the tape.
Always check this with enough extra 'cooking' time and some metal in the crucible.
Ideally you should always be able to take the thing out with your bare hands- in terms of temperature as you shall always use some proper gloves here.
And if you plan on taking shortcuts on the insulation then forget it!
Firstly you loose too much heat to melt the metal and secondly you risk the microwave overheating.

I know what you are thinking now....
I just grab some glass cord or twine from some old fire blanket and use that to secure the roll and lid.
Then I add a good layer of the slurry or clay and Bob's your uncle....
Might come slightly cheaper than the tape but how do you handle the thing ?
It becomes quite heavy and what is inside is quite soft, so you risk cracking it.
Once it does you end up with fine ceramic dust in the air - or right in your face when you open the microwave...
If you really wanna go this route then don't roll and use disks instead that have a suitable cut out for the crucible.
Using a soft brush apply some waterglass on all mating surfaces then place in the oven at 200 degrees Celsius to cure and set.
Sand the mating surface for the lid and the outside of it like before on the wet glass plate.
Dry and fix and so on, then apply some waterglass on the outside and again cure in the oven.
This time tough use enough or enough coats to let it soak in.
It will bind the ceramic stuff and should provide enough stability.
A waste of time though is you ask unless you really need to make larger casts....

[Downunder35m]

Confusius, the wisest camel in the frozen desert says:
Never assume, never risk thing, never let your concentration drop when handling hot or molten metal !!

Aluminium, if hot enough can run over your DRY hand without causing any real harm.
Because it simply won't stick to your skin.
Copper or brass though will ruin things for your for a long time.
And since you can't really be sure your skin is dry enough you should never handle things without gloves and the rest of the PPE.
A proper face shield is a must have as well as googles with GLASS lenses, even if it is just a pair of old welding googles.
You can only loose your eyesight twice, so why risk it....

If your crucible start to show too much wear and tear consider tossing it or breaking it to grind it up in a ball mill to be used again as an addition to keep the carbide use low - recycling is good....
Having it fail and molten metal going everywhere is no fun....

[Orngrimm]