One of the newer toys in our lab is a microwave kiln.    A microwave kiln is a device which you place in a conventional microwave and it acts like a kiln.  Microwaves kilns are commercially available from various places on the internet.

It is composed of a porous ceramic body (likely alumina) with a ring or inside coating of high temperature susceptor material (the black colored material).   Susceptor materials are materials that absorb microwave energy (like water, fats, oils, etc).   There are a wide variety of susceptor materials some of which reach very high temperatures -> 1800 degs F.     These materials are generally a patented or trade secret material combination – think graphite, magnetite, and/or various iron oxides.

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There propose along with the microwave oven is the bring the fired item in the kiln to high temperature in a short period of time (think 6-12 minutes).

We purchased one of these toys and have been exploring its usefulness.

We are using a standard 1000 watt home microwave.   We placed a fun-u-factured flower into the microwave kiln.  Closed the door.  Set the timer on 9 minutes.  We pressed start….

{Note the glow from the inside.}

Microwave kilns and microwave kiln processing of glass and ceramics have been around for over twenty years (thus its really not a new technology).  The availability of low cost ($75-$175) microwave kilns has only happened in the last couple of years.     We have found that there is quite a bit of variability in the results (there are some issues in getting repeatable results).

Lastly, we encourage you to try these out as they are reasonable in cost and may have a place in your lab/studio.   We caution you to read and follow ALL of the manufacturers instructions. Happy cooking!

48 Comments on Microwave Kilns

  1. Dawn says:

    I picked up a microwave kiln for $99 last fall – and had tons of fun violating all of the rules of glass…with help. To begin with, I have a 1200 watt microwave, and I also bought a fair amount of kiln paper in various thicknesses. I was able to create 1/2″ glass pieces that were fully fused within cut-outs formed from the kiln paper/padding, but stopped after only a couple weeks once I realized (after more online research) that the burn off from my microwave kiln might add itself to my food since I was using the same microwave for both.

    I do want to utilize the microwave kiln, and it works, but I’m holding off on any additional experiments until I pick up a microwave from goodwill that I can devote solely to glass fusing. I would advise that you do the same. That being said, “power” is not the big thing, I actually had to step mine down, as anything over 1000 watts, at least with the microwave kiln I purchased, actually seemed to be detrimental to the process.

    The cooling/annealing process is also less clear with microwave kilns. Upon removing the microwave kiln from the oven, I would place it on a “bed” of 2 bricks, and place a 3rd over the hole in the top – then leave it overnight. Not exactly the annealing process as outlined in your standard glass books – but I’ve dropped some of the pendants I made, and they haven’t even chipped.

    • admin says:

      Dawn, you REALLY never want to share/reuse anything that you use for food. We’re glad to hear some success stories for microwave-kilns. It has been iffy at best for us. Perhaps we shouldn’t have purchase a re-furbished microwave. You could use a fiberfrax blanket to cover your microwave kiln to slow down cooling and aid in annealing (although annealing has set temperature/time schedules). If you are using borosilicate glass, annealing may not be such a big issue (especially if you pieces are thin).

  2. fenn says:

    psst. it’s spelled susceptor.

    why is there a big hole in the top? to let out all the heat?

    • admin says:

      Fenn, thanks for pointing that out. I think the spelling checker didn’t like susceptor and suggested suspector (as perhaps an evil villain).

      As for the hole in the top, it’s a commercially sold device. Hmm? Letting out all the heat (you say)? Seems very likely. If one is processing glass, once the glass is above red glow, it starts to become conductive (thus becomes its own heating device) and thus heat loss may not be as big of an issue. Our biggest issue with the device is the inconsistency of the results.

      We have melted the rotating glass tray to the floor of the microwave (which was pretty interesting). Thanks for the assist.

  3. […] such as sugar, ceramic, and glass. Take a look through their archives. We found the post on microwave kilns interesting, as well as the writeup about Shapeways glass printing which is seen above. We’ve […]

  4. Maggy Rond says:

    The hole in the top is for allowing air, steam and vapours to escape and to allow air back in when it cools down. Quantum physics teaches us that this small hole releases only very little heat.

  5. ganter says:

    Smooth metal in a microwave is not a real issue (sharp or thin pieces of metal produces problems including wire). If you are looking for entertainment, check the web for “interesting things in your microwave”. That being said, you could get two pieces of boro glass to
    hot-tack state, open the microwave kiln and put the nichrome wire in the boro sandwhich.

  6. Vijayakumar says:

    Dear sir

    shell we make microwave kiln to fuse glass?

  7. RR says:

    Can you use recycled glass bits, such ass broken green, brown and blue beer bottles?

    • ganter says:

      Yes, you can use any type of glass. However, different glass bottles have different coefficients of thermal expansion which might make them incompatible with one another and cause cracking. You will need to do some testing.

  8. Karla says:

    I was wondering if anyone new of a really good home-made mix for the susceptor coating inside the microwave kiln at all?

    • ganter says:

      Karla, we don’t have one off hand but I can tell you a mixture of graphite, magnetite, and silicone carbide seem to work.

  9. thirteenfile says:

    More information here about microwave kilns: microwavekiln.wordpress,com

    they’re a lot of fun!

  10. jj says:

    Try YouTube video of a chap making his own kiln for £5 using fire cement and vermiculite… just started making mine using a cake tin covered in plastic bag and vaseline as a mould. Silicon carbide comes from lapidary suppliers. I am hoping to fuse glass for Jewellery making.

  11. joy miller says:

    I just tried to use this kiln followed directions and after taking it out of the microwave the kiln paper was brown and none of the glass had melted at all. Tried doing it again and it did the same thing. What am I doing wrong?

  12. lou*pea says:

    try fusing for a little longer. i just got a microwave kiln and am having a fair bit of trouble with it in terms of some stuff doing exactly the same. ie0 nothing !! no fusing at all. i’m not sure why. i have a 700W but it was taking 15 minutes to fuse, but now its not really fusing…and the paper stays brown. not sure if i’ve broken the microwave. i noticed reactive bullseye glass did not seem to fuse.

    • Betty says:

      Hi,
      I am having the same trouble. Did you get an answer to why the paper is burning and the glass not fusing.

  13. bonnie says:

    can you put sterling silver or gold filled wire in with the glass to be fused?

    • ganter says:

      Bonnie, perhaps, BUT there are issues with metals in the microwave (especially sharp, thin or
      pointed pieces of metal as they tend to arc). Therefore, smooth pieces should be okay.

      For fun there is a website dealing with “weird” things in the microwave to give guidance of
      what NOT to do!

  14. Doug says:

    For those concerned about putting metal into a microwave. You’re actually putting this into a kiln where the microwaves are being efficiently absorbed by a susceptor. The other point to consider is that hot glass, by itself is a microwave susceptor. Once glass gets hot, it will absorb microwaves and can ultimately turn the glass red hot.

    Since the top and bottom of these kilns appears to be susceptor free, there likely will be some microwave penetration, but little compared to putting a piece of wire into a microwave oven.

    Doug

    • Sandie says:

      Thanks Doug and Ganter!
      Does this mean that my microwave kiln is still good to use, even though the inside of the base where I put the kiln paper is blackened and it smells nasty when I try to use it?
      Thanks for helping me. I’m a rookie at this if you couldn’t already tell.

  15. Sandie says:

    I have had success using a microwave kiln. I recently tried fusing an image with fusing paper in between two pieces of glass (I think sandwiching them in the beginning is where I went wrong). The glass broke and the bottom of my kiln is blackened and smells when I try to use it. Is it ruined? Can I fix it somehow?
    Also, should I use a separate microwave than the one I use for food? The instructions say to clean it before/after use. Why is that? Is it fumes that risk contamination?
    Any input would be greatly appreciated!

    • ganter says:

      Sandie, microwaves are low in cost, please use a separate one. Get one from a thrift store.

      You can make your own suseptor material (go searching both the web and patent databases).

  16. Andrea says:

    Would a microwave kiln work to anneal the edge of a bracelet cut out from a wine bottle? Thanks Andrea

    • ganter says:

      Andrea, please check some glass reference books for an annealing schedule of the appropriate thickness. Generally annealing is a combination of temperature and time.

  17. Robin says:

    I got a micro kiln for Christmas and have been doing very well with it. I think key is to play with scraps till you figure out the ideal time for your kiln/ microwave combo. Then, once you get that figured out, do not be afraid to experiment! My next experiment is going to be using a frit mold in the micro kiln….just as soon as Fed Ex drops the thing off! *notsopatientlywaiting*

  18. findoctr says:

    I have used them for years in a $5. yard sale oven to test fused glass combinations and also to produce over 200 hundred pendants and earrings before the susceptor coating disintegrated.

    Although I prefer to use glue on bails, I have fused wire into the glass with no problems at all.

    On annealing, a single piece of glass size of a dime-quarter will anneal just fine in about 2.5-3 hrs. In a normal size kiln you have much more area that heats and cools differently from center to perimeter so annealing needs much more controlled cool down to insure consistency.

  19. Jenny Capra says:

    My microwave kiln works great but very fine pieces are coming off the bottom piece ,it appears to be made from Hebel material but not sure and I get a lot of dust in the microwave I do wear a mask but I’m concerned about health issues with the crumbling and the kiln paper. Any ideas please

    • ganter says:

      Jenny, your kiln could be made of a WIDE variety of materials depending on the location of its manufacture. I would guess
      that the dust is something to always take reasonable precautions. Kiln paper should be alumina (Al2O3) based again fine particles
      are never good. Try coating the bottom of the kiln with refractory sealant. OR simply get a new microwave kiln as they are not
      too expensive.

  20. Carol says:

    Wondering if anyone has any experience with using this type of kiln for small ceramic pieces versus glass? Any advice would be greatly appreciated.

    • ganter says:

      Sorry we haven’t tried firing clay. We did fire some rocks. The issue that I see is the susceptor material has a finite life. The proper firing schedule for clays/ceramics might be longer than the susceptor life.

  21. Colin M says:

    Hey guys thanks for all the great tips when it comes to fusing glass in the microwave kilns. It is really cool to know you can fuse metal pieces into your glass in one of these microwave kilns and in general. I am trying to make my own kiln dirt cheap. At this very moment I am trying to find a good susceptor mix.
    The materials I’m working with currently: Some black semi-shiney powder that came from burning the black tape found in a VHS video tape with a propane torch. I just did it in a used bread tin. The powder should be magnetite (and a bit of carbon left over from the burned out plastic tape).
    Also I have a deep reddish powder extracted from a black plate found inside a lead-acid battery. I’m assuming its some kind of carbon(close to graphite)? mixed with an iron oxide. It was black at first until it touched water. I honestly don’t know what it is but I am careful not to get it on my hands or kicked up into the air. I rinsed it with water after crushing to obtain a fine powder.

    Anyways! I put both powders in the microwave and ran a few tests with them placed on a slab of Autoclaved Aerated Concrete (AAC, a building material, highly insulating). The black “VHS tape” powder caught on fire with a clean burning flame and then glowed for a little bit while running the microwave but it started to cool down for some reason. When I opened the microwave though I scraped through the pile and the brick was glowing underneath which is a good sign. I was surprised the whole thing didn’t glow. The reddish powder did not glow and I didn’t add enough for form a pile as large as the magnetite.

    If you read all this I hope it gave you some decent info to work with.

    Now if anyone has any ideas for a cheap susceptor and any alternatives to silicon carbide, magnetite or graphite, please share it! It would be wonderful to have more info. Thanks.

    • ganter says:

      You need to use the proper susceptor materials. They really aren’t that expensive or hard to obtain.

    • Rip says:

      I disagree, many materials are susceptors and there is nothing wrong with creating your own from waste materials. Very green.

      • Colin M says:

        Thanks for the encouragement Rip! I’d find it hard to believe that there are only a few available useful susceptors. With everything I do I like finding the most affordable/efficient methods that I possibly can so that I can spread those ideas around to other people and help them be a bit more free.

        Not that graphite/silicon carbide/magnetite aren’t affordable but how low can we go (in $) and if I wanted to make something fast and I didn’t have those raw materials in my area what else could I use or where could I obtain them in non-conventional but readily obtainable ways? 🙂 I think those are good questions to ask especially since 2/3 of the world is poor and I for one love to spread useful information that builds up. I’m just passionate about that, not trying to brag or pick at anyone.

        -Colin

  22. Tracey says:

    Hi, I bought one online and it arrived today and I couldn’t wait to have a play. I’m very disappointed tho as its not working 🙁 I see the glow after about 2 minutes so I lifted the lid just to have a quick look (as I’ve seen them do on videos) Anyway it isn’t the glass that’s glowing, there’s a patch on the wall, about an inch in size that’s glowing, I can even see the glow on the outside of the kiln. Does this mean I have a dodgy kiln?

    • ganter says:

      Tracey, our experience is that you can see glowing outside. We had more than a small spot glowing. Glass will
      heat itself via microwave once it gets above a certain temperature (basically the microwave kiln just gets the glass
      up to temperature and then the glass can heat itself). Yes, you may have a dodgy kiln. We have seen variable
      performance based on cost. Good luck and keep us posted.

    • Colin says:

      Sorry to hear that. Do you know the wattage of your microwave? Knowing the wattage tends to give you an idea of the power of the microwave. Maybe check to see what wattage of microwave the manufacturer recomends for the kiln? If you know your microwave is plenty powerfull then I would doubt it is a lack of heating power then. Also if I were you I would consider getting some silicon carbide powder and some graphite powder online, mixing the two together with water and suger (using the suger to make a sticky paste) or something else to bond them together and then painting that mixture on the walls of the kiln. Let it dry before firing if you do this. Maybe it will give it the boost it needs?

  23. Barb says:

    if bottom of my micro kiln is burned brown, (from using the wrong fiber paper) and I scrape most of it off, but a brown stain remains, can I cover it up with repair receptor, or do I have to scrape down more? I’m afraid I’ll have to scrape so much, most of the ledge will disappear, in order to get it all back to white, so that’s why I’m asking if I can spread repair receptor on it while still stained a little brown. Thanks1

  24. Kathy says:

    We use our microwave to melt glass does anyone know if the ordor toxic?

    • ganter says:

      Kathy, I have no way of knowing the source of the smell. I would alway employ
      a good ventilation system (similar to what is common over electric kilns).

      Melting glass in a microwave should be similar to melting glass in kiln. However,
      the microwave method may stress the microwave itself (and cause unknown chemistry
      to happen).

  25. ganter says:

    You can use plaster of paris to make your mold.

  26. Jeanne says:

    just a quick question from a newbie. after i have it “fired up” (The middle circle is red) I am still suppose to heat it more for the fuse correct? The red circle just tells me it made it to cooking temp?

    • ganter says:

      you will need to experiment. each microwave is different. each microwave kiln is different. Try several tests and KEEP GOOD NOTES.

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