Kentucky-Fried Induction Furnace

[John] and [Matthew] built an induction-heater based furnace and used it to make tasty molten aluminum cupcakes in the kitchen. Why induction heating? Because it’s energy efficient and doesn’t make smoke like a fuel-based furnace. Why melt aluminum in the kitchen? We’re guessing they did it just because they could. And of course a video, below the break, documents their first pour.

Now don’t be mislead by the partly low-tech approach being taken here. Despite being cast in a large KFC bucket, the mini-foundry is well put together, and the writeup of exactly how it was built is appreciated. The DIY induction heater is also serious business, and it’s being monitored for temperature and airflow across the case’s heatsinks. This is a darn good thing, because the combination of high voltage and high heat demands a bit of respect.

Anyway, we spent quite a while digging through [John]’s website. There’s a lot of good information to be had if you’re interested in induction heaters. Nonetheless, we’ll be doing our metal casting in the back yard.


34 thoughts on “Kentucky-Fried Induction Furnace

  1. Horrible idea inside your home ( I hear no fume extractors). Potential for fire is very high as well! In industry we like to call risk reduction engineering control. /sarcasm But, seriously my buddy was using electromagnetism to snuff fire from candles and ended up setting his apartment on fire. (yah I know stupid shit but it happens when you don’t plan for worst case scenario)

    Details people, der tuefel ist en die Einzelheiten.

    Other than that sweet ass set up.

    1. In my industry (I’m a retired nuclear engineer), we call it Probabilistic Risk Control. Let’s take a quick look.

      The vent fan which is vented to the roof was running so the very few fumes this process generates is vented.

      There are 5 lbs of aluminum in the crucible. Accidentally dumping this charge is the highest risk. If the crucible fails and the aluminum runs out onto the stove, no problem. There is a catch-basin under each burner. If it dumps out on the floor, it gets quenched with the water. If a fire starts, the fire extinguisher is at hand and there’s another larger one out of sight behind Matt who was operating the camera. And no, the water won’t cause a steam explosion. There’s a guy on youtube (myfordlover, I think) who disproves that myth with molten iron, pouring the iron into water, pouring water into a ladle of molten iron and so on. We’ll be happy to do a video demonstrating this with aluminum if so desired.

      Another risk that you didn’t mention is that I wasn’t dressed up in a fire-proof suit. I assessed the risk and decided that 5 pounds of aluminum didn’t justify the hassle of the suit.

      OK, so one probably doesn’t want to do sand casting in their kitchen. We did the video to demonstrate just how clean and safe induction melting is. If we can do it in our kitchen then you can do it in your garage, shop or back yard.

      And did I mention the energy savings? That run used 2 kWh or 24 cents at our rate.

      Finally, Rev 2 of the Kentucky Fried Furnace with vastly better refractory (and no crack :-) will appear at in the next few days.


      1. I’m no expert by any means and i know that the steam explosion is very unlikely. i’m just curious if the molten aluminum would not react violently to water and produce hydrogen and alumina and therefore pose a possible explosion hazard? don’t really how the passivation layer reacts in molten state….

        1. I work in an aluminium semi-fabrication factory, we have several aluminium casting facilities. We go to great lengths to prevent our scrap from retaining water for exactly this reason. I’ve seen videos, the explosions are devastating.

          We’ve just started a beverage can recycling plant, the majority of the process is dedicated to drying kilns. I get inspected every time I enter the plant to make sure I’m not carrying any bottles or containers that could retain water residue and then end up in a scrap bin. We sued a supplier out of business for supplying us with scrap bags that had been stored outdoors and exposed to moisture.

      2. I do sand casting at home and have built a [casting] furnace or two.

        Try refractory cement mixed with perlite in a 4 (perlite) to 1 (cement) ratio. Perlite is effectively volcanic rock with the consistency of styrofoam, and works well as a high-temp insulator. Don’t skimp on the proportions, err slightly on the side of more cement if need be.

        I’ve made two furnaces with this material and haven’t had a problem. I suspect the “styrofoam consistency” of the material helps to relieve cracking stresses.

        You might also try embedding a refractory brick in the bottom of the furnace to make a flat surface for the crucible to sit on.

        Also also, if you make a new version consider making it as 2 separable pieces: a round base, and a hollow tube core containing the heating element. That way if the crucible breaks or you spill something in the furnace you can take the two pieces apart for easy cleaning. Also, if an the worst happens it’s easy to replace a damaged base and not have to build the new body.

        You can get perlite in the garden section of WalMart and refractory bricks from Home Depot. See this website for more info:


      3. You’re right about the steam explosion, but only partially. It happens when water is vented underneath the molten metal, such as a damaged furnace with water cooled coils. The molten metal contacts and eats through the copper tubing until it either opens a hole or superheats that particular section of tubing. There’s no direct access to open air for the resulting steam. THEN you get a steam explosion, or what is effectively high speed aluminum bubble blowing. It’s more common with steel and iron due to the ability to melt copper.

      4. Sorry John did not hear the fan running thought it was an older home without a hood. I understand you had a fire extinguisher. So I was wrong you got those bases covered.

        Thinking it is important to mention however so when others replicate this they take the same safety precautions.

        I see also you fixed the foundry as well so no cracks and less possibility of getting molten aluminium on the legs hands or feet.

        Again sweet ass set up, I just think the safety precautions were worth mentioning. As to Mike below you really think I meant this comment as an I know better comment, it wasn’t I just was concerned with the persons well being.

      5. A guy I worked with always had a really red mottled face. Cool guy.
        I always assumed genetics had done this, or too much sun.

        Just before I left that job he told me that he’d gotten that from an aluminum casting gone wrong. He had a face shield on but the pour (a small one) had been into a mold with (you guessed it) some residual moisture. He told me he had carefully dried the mold beforehand.

        He never saw it coming. The explosion basically went up his chest, behind the mask and cooked his face (scalding). He wasn’t horribly disfigured, but it made him look permanently embarrassed and forever ruined his chances to pick up age inappropriate women in bars. This is something of a serious handicap for the average married man, but he survived.

        And all of this was from an imperceptible amount of moisture.
        It could have been worse.

        I’ve done a lot of stuff that could have turned out much, much worse with lethal consequences. Luckily, as we age, we get the ability to more properly assess risk and mitigate danger… or at least some of us.

        My father was not so gifted – he was quite happy to smelt lead for weeks on end in our tiny kitchen, with only a tiny vent fan… while he made and sold dive weights from cast off lead wheel balancers. I shudder to think of my family’s lead exposure, but it probably explains a lot.

    2. I just finished version 2 of my own induction heater, based on this circuit:

      This website has a pretty good explanation of the principles involved:

      The Royer oscillator is blood simple to build and always oscillates at resonance no matter what the resonant frequency of the LC circuit is. That means that you don’t need a complex resonance-seeking system such as a uController – anything you put into the coil will change the inductance (even the temperature of the heated item has an effect), but it will still oscillate at resonance.

      The transistors switch at the voltage zero-crossings, which means that they don’t shunt appreciable current while switching. The transistors don’t dissipate much power, and with a properly adjusted system will barely get warm even when controlling massive amounts of power.

      My first version had the components in open air, soldered onto the thick copper of the coil winding (and using the same thick wire for the current-carrying path). Worked like a peach.

      I’m working on version 3 now. It’s tough to get a power supply that works. All of my high-ish voltage supplies (50 Volts) have over-current protection and will quickly switch off. For the moment I’m using a portable car starter (12 volts at 200amps), but I’m building a linear
      supply (30 volts at 50 amps) from a microwave oven transformer which should do the trick.

      Really simple to build, reasonably safe (electrically), and lots of fun.

      (I know I know, the MOT is a poor choice for various reasons. For the near term, I’ll just use a low duty cycle and not let the transformer get too warm.)

      1. Nah, the MOT is perfect for your power supply. Just rewind the secondary to 30 or 50 volts output and add diodes and filter capacitor to output and variac on the input. If you’ve only used 12v into the Royer so far, you’re gonna be amazed at the power you get from 30 or 40v. I’ve actually run close to 60 or 70v into a IRFP260 based Royer with over 1kw output through a flyback. Great little circuit!

    b) If you shake some baby powder in those cupcake tins, they aluminum will just slide out, otherwise you will be beating your tinns to death trying to get your muffins.
    c) How come people say my molten lead pot was dangerous when that molten aluminum was teetering on a brick with two small supports under it? IN THE KITCHEN!?
    d) Sodium free tablesalt…
    e) Aparently aluminum gives off unhealthy gasses, I know some of the stuff in the alloys turns into evil fumes. If your doing this in the house, please use an operating fume removal device…
    f) what!? no homebrew induction inverter!? ;-)

  3. Let’s start backwards.

    F, The Roy is our flagship induction heating product. We have a much more elaborate lab for electronics than we do for foundry work. There is a link to the open source induction heater in the write up. We had done melts with the OS heater as well but we wanted to get solid data on how the Roy 1500 performed this instance, so it was featured in the video.

    E. The stove vent was on, and this is an old school vent that actually vents to the outside. Not that I smelled anything too toxic coming off the melt anyway.

    D I have no idea what you are talking about here. It seems pretty common to use salt for a flux. We have gotten a more proper flux now. We also have quite a few more melts under our belts.

    C The furnace was sitting on two large light weight firebricks. It was stable there with two adults who understood that there was a vat of molten metal sitting on the stove. If you have kids or pets that might not be the best set up for you.

    B Yup, we learned that after that melt. We got some strange looks at the save a lot picking up a make up brush and looking over all of the BB powders. FYI, you want to get one that does not have cornstarch in it. The one we picked out has talc and zinc oxide. It seems 100% talc is hard to find in a small town supermarket.

    A No idea about the video stabilization. The video is pretty much shot on with just a few gaps where it was just boring or we needed two sets of hands for a few minutes.

    And going a few comments up… The Royer is simple, but it is far from bulletproof. When you start getting up into the hundreds of watts plus power range you will start to understand. You may want to join the induction heating mailing list on John’s site. There are lots of people working on heaters there. The major difference between the OS heater and the commercial Roy 1500 is the Roy 1500 has a micro that monitors and supervises all aspects of it’s operation. The Roy 1500 is darn near indestructible, and we have done a lot of in house torture testing and we get almost zero returns from the field.

  4. Various clarifications and expansions.

    First, it’s vitally important to understand the difference between “showing you what we did” and “advocating that you do what we did”.

    Second, my professional specialties involved dealing with extreme levels of nuclear radiation and very large amounts of radioactive materials. Matt is a licensed pyrotechnician – he makes and fires professional fireworks. In other words he deals with some of the most sensitive explosives there are. In both our cases, our professions have shaped our perceptions of risk. Compared to dealing with a megacurie Co-60 source or a pound of flash powder, a few pounds of molten aluminum just can’t get us excited.

    Rue_mohr, F: To amplify Matt’s comments, the Royer oscillator in my open source kit and the one in our commercial Roy product are in absolutely every way identical. The difference is that since Royer power oscillators are very easy to kill, the Roy contains a microprocessor that supervises the Royer and protects it from itself and users. If you like I can make another vid using the open source unit. Same results.

    Rue_mohr, E: Not at all. Unlike say, bronze, aluminum does not boil off any of its constituents unless grossly overheated. The almost universal sand casting alloy is A356. It contains about 7% silicon and 0.3% magnesium. Nothing there to get excited about. Some foundrymen add some copper to the mix to reduce the viscosity to make the alloy pick up finer details from the mold but still nothing to get excited about. The only fumes produced were the oils and other contaminants and they were extremely minor.

    vitomakes: You guessed it. No wifey. She did me the favor of running off with my best friend about 10 years ago :-) Now my whole house, upstairs and down, is a hackerspace.

    Okian Warrior: Please quit wasting your time on that diode coupled direct Royer. For reasons more complex than I care to write up in a comment, that circuit will never go above 60 to 80 volts. In the beginning I tried that circuit but quickly figured out its weaknesses. So I designed the digital Royer that is my open source heater. Go ahead and use it – that’s why I put it out there. I know that the circuit as presented, if carefully matched to the load, properly heat sinked and carefully monitored as described in my article, it will run at least 3kW. Add a third paralleled transistor to each side and it’ll do near 6kW.

    Just don’t substitute the specified transistor. We collectively spent man-months finding the one IGBT in all of Creation that will reliably run up to about 100kHz. OTOH, FETs switch too fast which stimulates parasitic ringing that must be addressed with snubbers.

    Murray: Thanks for the sexy comment :-) Once we decided we wanted to do a video indoors to demonstrate the cleanliness of induction heating, we looked around. The concrete floor in the basement is a BIG NO-NO. Molten metal on concrete causes steam spalling which violently propels the metal into the air. That left the upstairs area and the only fireproof place in my house is the stove. It was purchased in the year of my birth, 1954 and is made out of real steel. It’s so heavy that I have to use a hand truck to move it out to clean under it. There are heavy steel catch pans under the burners. So if the furnace failed or tipped over, most or all of the aluminum would be caught by the stove top. If any had spilled on the floor, we had the pot of water, two fire extinguishers and a pressurized 1″ hose just outside the door.

    I got involved in sand casting back in the 80s but gave it up because of all the hassles and the filth associated with running a combustion furnace. I’ve spilled molten aluminum on my clothes several times. I’d much rather it be aluminum than solder! Aluminum is light and has a low specific heat so unless one pours it down his shoes or something, one has to work at getting hurt.

    Looks like the sun’s about to come up so it’s time for me to go to bed :-) I’ve enjoyed this exchange. I hope our articles and vid will get some of you involved in sand casting. It’s the most fun I’ve had in decades. And do stay tuned to The KFF Rev 2 will appear shortly, followed by our 10 and 20 lb furnaces. All will be both open sourced and available ready to go from Fluxeon.


    1. Wow, you guys brought out all the arm chair experts with this one (great job by the way), I have thought about trying some aluminum casting but would never consider trying it in my own home (Fortunately I have a warehouse I can use for that), at least my wife would never let that happen in the house. Thanks for showing how I should not do it.

    2. Um… no offense, but duplicating your circuit wouldn’t meet my goals, which is to learn about induction heaters. I could just *purchase* a unit and it would save me a whole lot of time – and teach me nothing. I want to be a hacker, not a user.

      My goal isn’t to have the biggest power output possible. I’ve already got a casting kiln, I’m designing the circuit for another purpose.

      I was hoping to encourage and inspire others by showing how easy it is to start.

      If you want to be a user, then by all means go duplicate John’s work – he’s done all the learning for you, and there’s no more to be said about the issue.

      Alternately, if you want to be a hacker… start simple and work your way up.

      1. There is no ‘hacker’ ruleset that says you can’t buy something then learn how it works from that. there isn’t always something to be gained by building it from scratch, pick your problems.

        It is great that approach works for you, but don’t diss everyone else not following your closed mindset.

      2. my my, aren’t we full of ourselves. you know what you said is wrong, you can be a user without being a hacker, but you can’t be a hacker without being a user, well not a good one anyway. You have to use what you hack in order to test it. I think you’re being a little too reactionary to the situation. chill.

  5. I couldn’t help but giggle at the title. Then in the last sentence of the mini article I couldn’t help but laugh out loud.

    I’ve smelted aluminum, we use an out door brick oven using propane + electric blower motor. And I agree, the kitchen is no place to do this . . .

  6. Bad idea to do this in your house. I have poured aluminum in a foundry and will only pour in a foundry with a grated floor. Water is a no-no around molten metal in a foundry. Yes I have seen a molten aluminum explosion. Lucky everyone had on fire suits, leather hoodies with face shields. I bet his home owners insurance would not cover this if he burned down his kitchen. Just because I can melt aluminum in my kitchen doesn’t make it a good idea. Why not do this outside or in a shed?

    1. He has a steel liner between the work coil and the aluminum which is embedded in the refractory. So the Induction coil is primarily heating this steel liner and convection/conduction/radiation from that to the aluminum does the melting. (some magnetic fields will make it there but much much lower than a direct induction system.)

  7. This link is to a web site in Australia he has been melting cast iron for 25 years this video describes a light weight furnace using ceramic fiber coated with zirconium silicate solution with properties similer to ITC 100 I would think. A mixture of the oxide or silicate in a ridgidizer should work great for your purpose. When I build mine this is the ave. I will take. Hope you find this useful. Jim

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