Microwave Kilns: Great Tools for Testing Low-Fire Lusters, Enamels, and China Paints

Not just for leftovers anymore, a microwave oven can be used for low fore testing too!

Not just for leftovers anymore, a microwave oven can be used for low fire testing too!

Many people know that a microwave oven can be used to dry clay quickly when you’re in a pinch. Dielectric heating (the type used in a microwave oven) is also used in industry to fire ceramics for high-tech applications. This option is also available on a small scale to the studio potter, at least for firing tests and small objects using a microwave kiln. While the kilns don’t fire high enough to test higher temperature glazes or clay bodies, they can really help out if you are interested in low-fire decorative applications such as china paints, lusters, egyptian paste, overglaze enamels and many commercial decals.

In this post, Jessica Knapp tells you all about this alternate use for old microwaves! – Jennifer Poellot Harnetty, editor.


Fig.A

A microwave kiln is a container kiln consisting of a base and hood made for use in a household microwave oven. The kiln is made of a white insulating fiber and lined on the inside of the chamber with a black compound that absorbs the microwave radiation and heats up to 1650°F or approximately cone 010 (figure A).

The heat from this compound is then transferred to the chamber and to the piece being fired. It takes between 5 to 10 minutes to reach peak temperature ranges depending on the size of the microwave kiln, the work being fired, and the microwave’s wattage. After the firing, the kiln needs to cool for at least 20 minutes before being opened.

Fig.B

Fig.B

Designed for working primarily with small glass objects or precious metal clay, the microwave kiln is also suitable for firing small clay pieces, from beads and pendants to test tiles or small sculptural objects. The kilns come in three sizes. The small and medium size have a firing chamber that’s 1¾ inches in height, and a diameter of 2¾ inches and 4½ inches respectively. The chamber in the large kiln measures 61/4 inches in diameter by 23/8 inches high.

The optimum firing time for various materials and sizes needs to be determined through test firings. Spectrum Glazes, one of the kiln’s distributors, sells a range of glaze pens formulated specifically for the kiln (see figure A) but overglazes, glazes, and clays formulated for a range between cones 018 and 010 can be successfully fired.

Fig.C

Fig.C

Using the Kiln
First, cut a piece of kiln fiber paper to size. The fiber paper is usually provided as part of the kit (see figure A), but can be replaced by pieces of Bullseye Thin-Fire paper used for glass work. The fiber paper prevents anything from sticking to the insulating fiber base.Place your piece on the fiber paper and test fit the cover to be sure the piece does not hit the top or sides of the firing chamber (figure B). Place the base in the microwave and cover with the lid. Be careful; the black coating on the inside of the lid is delicate and cracks off easily if bumped. Set the microwave timer based on your tests. Once the microwave turns off, you’ll notice an orange glow coming from the vent hole in the top of the kiln.

Fig.2

Take the kiln out of the microwave immediately to prevent damage to the microwave’s ceiling from prolonged exposure to the heat from the vent hole. Wearing oven mitts or kiln gloves, grasp both the top and bottom of the kiln at the same time, taking care to not open it as you lift it up (figure C). Place the kiln on a heat resistant surface, like a brick or tile.

Fig.3

Fig.3

Leave the kiln closed for 20–30 minutes before opening. For slightly larger work, increase the cooling time to 35–40 minutes. The top of the cover and bottom of the base stay very hot for a long time, so keep at least a 6-inch clearance above and around the kiln. After cooling, gently brush the fiber paper residue off of the base with a soft brush. Note: Wear a respirator to avoid inhaling the dust.

Fig.4

Fig.4

When using this kiln, follow the same rules as you would when firing work in a larger kiln. Clay objects can be fired either green or after a bisque firing. If firing green, the work must be thin and dry. Dry glazed work overnight before firing. Due to fumes, the microwave needs to be vented properly or fired outside.

Take careful notes. As the microwave kilns are too small to insert a pyrometer, and, unless you buy the larger model, also too small for a cone pack, you’ll need to take notes on the time/temperature correlation for your own microwave.

Fig.1

Fig.1

Testing, Testing…
We tested the Spectrum multi pens (figure 1), a white gold and a mother-of-pearl luster, a purple overglaze enamel (figures 2–4), commercial underglazes (figure 5), an 04 transparent glaze (figure 6), and Egyptian paste (see “In the Mix” page 6). It took additional tests to find the correct firing time for the enamels and the lusters. The firings for the two overglazes produced noticeably more fumes and discoloration in the kiln than the other glaze firings.

Fig.5

Fig.5

As a general guideline, with a low-powered microwave, it took 3 minutes to fire a piece with luster or overglaze and 5 minutes to fire the glaze pens and underglazes. For clay bodies, two Egyptian pastes, fired green, a bisque-fired porcelain body, and a high-fired porcelain body. All of the clay bodies survived the thermal shock of the short firings (4 to 5 minutes for the Egyptian paste, and 5 minutes for the porcelain).
Fig.6

Conclusion
Though it won’t replace your need for a larger kiln, a microwave kiln can help speed up your glaze and clay body testing, or provide a way to make small-scale objects or even models for larger pieces.Microwave kilns are sold under three different brand names: MicroKiln, MagicFuse, and Fuseworks. Check with your local distributor for availability. All three can also be found online.

A special thank you to Spectrum Glazes (www.spectrumglazes.com) for providing a MicroKiln for use in testing clays and glazes for this article.

Comments
  • I use and sell the MagicFuse by Paragon and have fired raku as well as enamel and glass. They are fun and quick for those of us that are teachers and need a quick demo.

  • Raku with these small kilns? Tell me how that works, I’m really curious.

    Kate

  • Tried drying clay piece in microwave and it just cracked to pieces. Can’t imagine how this could work. (the drying, not the little kiln)

  • May be use a low power programm during few short times

  • Interesting for glaze testing, bead making and the like.

    But me too, I’m really puzzled as to how you can dry clay in a microwave oven, since all a microwave oven does is agitate water particles to heat the food of which the water is part. How can it dry the clay, that is, evaporate the water ?

  • Dear Kate
    I also use a Microkiln for raku firing. Good alternative for the “real thing” when you live in a busy city. I use a small pan for reduction after firing and this hardly causes any smoke nuisance.
    In case of raku firing you should open the kiln directly after you remove it form the microwave oven. Very hot so be careful! If you want decent cracks in the glaze you have to hit them pretty hard with the pliers you use. Have your reduction pan ready, because small pieces cool off really fast. Leave them in there for about 15 minutes. Then you clean and say WOW! or …. (ehm) DARN!!

  • Really enjoying this site and am getting really enthused even though I am in New Zealand.

  • am very interested has anyone tried to fire o6 glazes or attempted to build a larger kiln height?

  • Dear Neera,
    I got all this information from a book called “Raku Varia 2”. This will give you all you want to know about raku firing, and more. It is written by a couple from Holland (I am Dutch, too) named Ed and Ine Knops. The book is also available in English, but I don’t know if you can get a hold of it in the US. Give it a try though, even if you have to get it from a second hand book store online, it is definitly worth the trouble! ISBN 90-77266-03-8. And of course I tried it myself and learned a few things along the way. Good luck!

  • How much does one of these little kilns set you back? I am intrigued……

  • Microwave Kilns???!!!! WHERE can I get one of these??? I need to read more about them. I just found this on my spam page…How much do these microwave kilns cost? I used to do a lot of luster firings but don’t want to do so much anymore and a microwave kiln would be PERFECT for what I am doing these days!!! WOW! what will they come up with next?????

  • Oh from what I just read, it looks like we have to make our own little housing to put in the microwave…I wonder though, what does this do to the microwave itself…how safe is it to use for food items after using it for lusters or china paints…MAYBE it’s a good idea to buy a microwave JUST for luster firings? Anyone got any ideas on this? Anyone out there tried this yet? Microwave ovens aren’t so expensive these days. I might give this a try.

  • If you want to buy one google on “glass fusion microwave kiln” as these kiln are really meant for glass fusion. I found them on delphiglass.com, microwave-kiln.com, paragon.com and ramstainedglass.com. Prices vary from $ 89.00 to $ 271.00.
    Don’t use your kitchen microwave in which you prepare your food for this game! If you want to use it for raku firing you should fire it outside anyway. I bought a second hand microwave. You should get one (750 Watts) with a rotating glass plate and place the kiln towards the edge of the plate, on stilts. This way the kiln rotates in a larger circle and will not damage the ceiling of the microwave with the hot air coming out of the hole in the lid. Moreover the plastic spindles on which the plate rotates won’t melt. Give it a try, it’s great fun!

  • Hello, I am a second year ceramic student at the University of Cumbria, U.K. I am currently developing my dissertation proposal, which I have decided to produce a practical research document exploring traditional and non tradition Raku firing techniques, there doesn’t seem to be a lot of information available about microkilns; I was wondering if anyone knew of any sources of information from texts to websites that might be able to give me a little more about where it is in industry and its technical notes? Also if there are any ceramicists who may be interested in talking to me about their work with a mircokiln, I would be really thankful.

    Sarah

  • Dear Sarah,
    If you want to know more about how I use the microkiln for raku firing, you can contact me on info@ceramiga.nl. If you would like to see some of my work, check out my website http://www.ceramiga.nl. Although it is all in Dutch and my website is not finished completely, you can always look at the pictures: all the small animals are fired in the microkiln.
    Carolien

  • For the price you’re better off with a small electric kiln. These things are tiny and very chancy. Results are highly variable, and having to invest in a microwave for the sole purpose of using this expensive little device pushes it over the top.

    Just buy a small electric kiln that runs off household 115/120v power. You’ll be way better off.

  • Zen,
    If you want tomtest ordinary glaze I think you are right, but if you use the kiln for raku the results will be just as variable as in a big raku kiln fired with gas or wood. The most important point is, I think, that it is a lot quicker than firing an electric kiln, therefore cheaper and maybe even ‘greener’?
    I bought a seond hand micro wave for € 25 and the microkiln set me back € 50. Isn’t a small electric kiln more expensive?

  • I have tried a few times using 50% for 5 minutes, then full for various different times (in an 800 watt micro) and get very different results, so it is very frustrating! Have given up for the time being! Wish someone could come up with programmes that work consistently!

  • Hola¡¡ desde España una consulta, a ver si alguien me entiende y me puede resolver una duda.
    Con este pequeño horno se pueden bizcochar las piezas primero y luego esmaltarlas o solo sirve para esmalte?

    Gracias

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