Secrets of the Microwave Kiln

I’ve just bought my third Hot Pot Maxi microwave kiln. But, why buy yet another “toy” kiln when I already have a “proper” glass kiln of a distinctly robust and modern design?

I hope to answer that question by talking about the distinctive nature of firing glass in a microwave kiln, a little about the economics of using them, and a little about how they work and how they deteriorate.

I think I need to do all of this because I don’t see anyone else talking much about it.

Experiences With a Microwave Kiln

I still use a microwave kiln because I can melt a small arrangement of glass and have it back out and in my hands, fused and shiny, within about two hours. My “proper” kiln makes me wait a whole day (and night). So, speed and convenience is one reason.

The barely-controllable ferocious heating in a microwave kiln results in a greater risk of glass cracking at it heats up and the lack of processing temperature control means it’s not uncommon to find a mutant distorted blob of glass is the unexpected result of a firing. And of course the small firing chamber means we don’t get to make anything larger than a brooch or pendant. Sometimes this is not a problem.

The down-side of the almost uncontrollable heating, as I’ve just mentioned, is that the shape and form of the resulting glass masterpiece is rather unpredictable. This becomes a particular problem when the microwave kiln gets older and heats less evenly. I’ll be coming back to the “getting older” aspect of microwave kilns later as it seems to be a widely neglected topic!

Another characteristic of microwave kilns is the incredibly rapid cooling inside the microwave kiln. It has scant regard for “proper annealing”. You might think that this must lead to problems but in truth it very rarely does.

We are repeatedly told that it is important to properly anneal our work. From this we might suppose that the rapid cooling in a microwave kiln without “proper annealing” might cause us significant problems. In truth I find that breakages caused by poor annealing are very rare. If this is a surprise to you then consider the size of items being produced and realise there’s only so much stress and strain that can be built up and “stored” in such a small piece of glass. This is particularly the case for simple shapes like a blob of glass, a little decorated tile or a simple pendant – in other words, exactly the kinds of things that you’d use a microwave kiln for.

You can, of course, pop your microwave kiln masterpieces into a “proper” kiln to “properly anneal” them though we can take paranoia too far sometimes.

Another aspect of the rapid heating and cooling in a microwave kiln that I have not seen mentioned anywhere relates to devitrification. With a microwave kiln the processing time is so short that glass that is susceptible to devitrification rarely has time to devitrify. A practical consequence is that I am able to reliably produce recycle my otherwise unusable scraps of “ordinary” non-fusing glass into blobs with little risk of devitrification. You can see real examples in my Recycling Scraps of Stained Glass blog and you should bear in mind that every single glob you see in the picture is not fusing glass. For the lazy amongst you, and because it’s colourful, I’ll re-post the picture from that blog:

DSCF1857 Recycled Glass Globs

And here’s and interesting example that proves the opposite situation from my last blog. This little wonderous spiky blob of glass devitrified before it melted completely:

DSCF3010 Spiky Devit

The big surprise is that my “proper” kiln fails to produce shiny globs with “ordinary” non-fusing glass. Devitrification is always a problem. Processing time is important when dealing with glass that was not designed to be re-fired.

In other experiments, using a “proper” kiln, I find that most kinds of “ordinary” non-fusing glass can barely cope with slumping without devitrifying at least to some degree. Someday I’ll do a blog about this but I’ve not finished messing about yet!

Reasons to Use a Microwave Kiln

A consequence of the foregoing chatter is that I continue to use a microwave kiln in four very particular situations:

  1. I can quickly and cheaply perform a simple glass-related experiment in a microwave kiln. Firing-up a big kiln and wait a whole day to find out what happened can be too long to wait sometimes.
  2. Children visiting for a “smashing time” can arrive in the morning to make something small and simple in a microwave kiln then take it home that same afternoon. While they wait for the microwave kiln too cool down they can also make something bigger and more “special” that later will go into the big kiln. Immediacy is important for kids, as is the excitement of seeing seething red-hot glass when they “peek”.
  3. I can recycle scraps of non-fusing glass into blobs without devitrification problems and in turn it means I throw very little waste glass away.
  4. I can quickly make small quantities of frit balls (and other similar little things) when I run out of them which means I don’t have to suspend my project work for a long time. That they’re badly annealed doesn’t matter here because they will be fired again!

You may be surprised to learn that point (3) is what my microwave kiln gets used for most of the time. Let me explain…

No matter how hard we try to make use of smaller pieces of glass we end up with small scraps that are unusable. Where possible unusable scraps get melted into globs. It makes environmental sense through I doubt the time and effort to make them is commercially viable.

Some of the smaller globs (under 6 grams) I use in my own copper-foiled work or give away to other crafters when we meet at events. They might end up as a glass highlight in a wooden decoration for example.

Larger globs (typically 6-10 grams) are supposed to be sold though I tend to give away most of them. My rule is simple – kids who show an interest in my work can have one free but horrible kids have to pay for them. There has to be a reward for being “nice”.

Microwave Kilns Deteriorate

I’ve already mentioned that I’m now on my third Hot Pot Maxi microwave kiln. What happened to the other two?

As battered and bruised old-timers the old microwave kilns have been retired. They now live in landfill. The blunt truth is that they’re fragile, get damaged easily and really do get old and tired.

I should now explain how a microwave kiln works (in brief) and then pull-in information to explain how and why they deteriorate and get old.

The body of a microwave kiln is made of a light and brittle ceramic material. Considering how light and thin the ceramic material is, it performs remarkably well as a thermal insulator.

With a new microwave kiln we can expect the grey heating material to heat up reasonably evenly. The relatively small degree of uneven heating will be caused by subtle differences in the mixture of materials and their thickness. With time the degree of uneven heating gets worse for reasons that follow…

Repetitive heating and cooling causes repetitive expansion and contraction which will result in hairline cracks. The brittle nature of the ceramic material (and the inside coating) of a microwave kiln means it starts rather soon and gets progressively worse the more you use the microwave kiln.

Exactly where the hairline cracks appear depends on the unavoidable “defects” of manufacture and some basic physics. That the cracks always seem to run from top to bottom is purely down to the combination of geometry and coefficients of expansion – the inside gets hottest so wants to expand proportionately more than the outside. The reverse happens when cooling. This difference causes stresses and strains which result in hairline cracks appearing. So, don’t be unduly concerned by hairline cracks because they’re a natural consequence of the heating and cooling and the materials being used.

We now need to remember some high school physics. Do you remember that heat can be transferred by any combination of conduction, convection or radiation?

The hairline cracks will cause uneven heating because areas that heat up fastest can not conduct some of their heat to cooler areas because of the barrier caused by the cracks. So, any minor differences in one area heating up faster than another due to original manufacturing “defects” is made more pronounced when hairline cracks come into play. As the size of the firing chamber is so small we can assume there is no heat transfer by convection. There will however be some heat transfer by radiation because that’s what we’re using to heat up the glass in the firing chamber.

So, uneven heating becomes an unavoidable and noticeable problem once the microwave kiln starts to develop hairline cracks. This in turn adds to the unpredictability of what you can produce in a microwave kiln. A partial answer to this uneven heating is to pause the firing mid-way, have a peek, rotate the lid by half a turn, then continue to the firing. With practise and good timing this can almost negate the effects of uneven heating.

Glass slippage is another problem because. It is very easy to accidentally nudge the lid of the microwave kiln and cause the glass pieces inside to slip. A microwave oven platen that rotates badly (wobbling or shuddering) can also cause glass to slip. Heating too rapidly may cause glass to crack and move, so is another form of slipping. Any of these (and other) mishaps may result in hot glass “gluing” itself onto the base or the sides of the microwave kiln. You can also achieve the same effect by over-cooking the glass such that it becomes very fluid and “runs” to the side of the kiln to glue the top and base together.  Yes folks, I confess. I’ve experienced all these mishaps.

The trouble with glass fused onto the ceramic material is that you will find yourself gouging a big hole into the base or sides of the microwave kiln in your attempt to remove the glass. It is rarely possible to remove the glass without damaging the ceramic material, even if you have use kiln wash to protect the kiln base. Such mishaps tend to shorten the life of a microwave kiln, either because you find yourself with a kiln base that resembles the aftermath of World War I trench warfare, or sides where big chunks of the grey heating material are missing.

Using kiln wash and fibre paper can help deal with some of the problems some of the time but in my experience they will only reduce the rate of kiln destruction!

Another aspect of the deterioration relates to the heating ability of the dull grey gritty substance on the inner surface of the microwave kiln’s lid. It’s the heating element. The dull grey gritty substance is something I’ll talk about in more detail at the end of this blog so for the moment just accept that it is chosen for its ability to absorb microwave energy and re-emit that energy as heat. In other words, a microwave kiln works because of a peculiar characteristic of the grey material.

I am not sure why, but the effectiveness of the grey “heating” material seem to deteriorate over time, partly because of minor mechanical defects such as hairline cracks, but also because it seems to take longer and longer to heat up as the kiln is used more and more. This is something I noticed with my first microwave kiln but I hadn’t been keeping any records.

The fact it takes longer and longer for the microwave kiln to heat up with age implies there is some form of chemical deterioration in the “heating element” part of the microwave kiln. Anything that’s hot and in air tends to get oxidised as a matter of routine. This is perhaps most familiar to you if you’ve ever put some lovely salmon-pink shiny copper elements in your kiln-fired work and was disappointed to discover they came our red, purple or even black as heat and oxygen progressively turned the copper to copper oxide. This is what heat and oxygen routinely do to most things around us. This is what I suspect is happening to the “grey stuff” in the microwave kiln. But I suspect there are two other possibilties.

One of the alternative possibilities is that metals in coloured glass are “firing off” and reacting with the heating element. The other possibility is that the mixture of materials in the heating element react with each other causing chemical changes.

Whatever the cause, the effect is that the heating element becomes less susceptible to microwave energy so is not able to re-emit heat so effectively.

Firing History

My first microwave kiln told me that there was deterioration. So, for my second microwave kiln, I kept a record of each firing. Not much more than the date, what kind of task and how long it was “cooked” in the microwave. What you see in the graph below is the result of my nerdy record-keeping. Have a look at the graph then I’ll explain what it all means.


The graph shows that my second microwave kiln didn’t quite make it to 300 firings before I felt it was time to throw it away. The exact number of times was 283.

You can also see from the graph that the jagged curve runs from the lower left (the first few firings) to the upper right (the end-of-life firings). Notice also that the curve is steeply upwards on the left and goes shallow on the right. This curve tells us that a new microwave kiln is much quicker than an old one and that the super-duper performance of a new microwave kiln doesn’t last long.

Notice that I’ve scaled the processing time so that 100% represents how quick the new kiln was. This means that when I threw it away it was taking almost twice as long to do exactly the same job – over 180% of the original firing times. Notice also that the graph shows us that the rate of deterioration slows down and seems to be levelling out at around 180%.

There are consequences for this “deterioration”. One is that it takes more time and energy with an older microwave kiln when compared to a new one. The other is that there’s no point in relying on detailed accurate firing records with a microwave kiln because its behaviour changes over time.

I’ll now reinforce that last paragraph in a different way. If you use a firing time from an old microwave kiln to guide to what you should do with a new microwave kiln you will likely “double blast” your glass. It will be “double processed” and you may end up producing an very runny pool of molten glass. And runny molten glass flows rather well if a surface is not exactly level. This is how I managed to “glue” the inside of the lid of a microwave kiln onto its base using molten glass. Don’t be as stupid as I can be. Consider yourself warned!

Now that we have some evidence about how microwave kilns deteriorate, and why, lets look at the economics of using a microwave kiln.

Microwave Kiln Running Costs

There are different brands of microwave kiln and some brands come in different sizes. The kind I’m using has a firing chamber that is about 10cm in diameter and cost about 50 GBP. Knowing that your 50 pound investment will deteriorate and may be ready for landfill after about 250-300 firings is something to think about. So is the ever increasing cost of the electricity, the kiln wash, fibre paper, currency exchange rates etc.

So, how much does it really cost to fire-up a microwave kiln? Lets find out…

I’ve already mentioned 50 pounds Sterling (notice it’s “Pounds Sterling”, not “English Pounds”) as the purchase price of my new microwave kiln and that I got 283 firings out of my second microwave kiln. So, that’s about 17.6 pence per firing due to the kiln cost.

But electricity also costs money. I am using an old 650W microwave oven. The 650W measure is the microwave output, not the electricity consumed. From the technical information at the back of the microwave’s manual I see it consumes 1.1kW per hour. So that’s about 60% efficient. My electricity costs around 16 pence per kWh and I’ve factored-in a proportion of the standing charge. We end up with just a few pence of electricity per firing which I can now plot on a graph.


I see that the cost starts somewhere between 3 or 4 pence, quickly rises to nearly 5 pence, then slowly drifts upwards to a little over 6 pence per firing. If you compare this graph with the previous one you’ll see exactly the same shape but a different Y-axis scale. This is because we’re doing nothing more complicated by converting a Y-axis in units of time into units of pence by multiplying by a constant value. For full marks in a mathematics exam I should have perhaps chosen a Y-axis starting at 3p rather than zero to make better use of the space.

Other Running Costs

There are other running costs that were not included in the previous graph. We tend to use some kiln wash to protect the base of the microwave kiln. A tiny fraction of a penny per firing for kiln wash is negligible compared to the cost of your time and the other costs associated with running a microwave kiln.

You, like me, might also use Bullseye’s thinfire paper between the glass and the kiln surface. It’s expensive and it can’t usually be used more than once. But how expensive is it?

If you’re lazy you’ll buy ready-cut 10cm squares at around 11 pence per firing, such as from here at Glass Studio Supplies in the UK but if you compare the price for buying 100 big sheets, such as from here from Warm Glass in the UK, you find you could instead be paying around 6 pence for the same amount of thinfire paper. All it takes is the will to buy in bulk, a pair of scissors and a few minutes of your time.

And finally, we need to remember to allocate a portion of the cost of buying the microwave kiln to each firing as well as the electricity cost, both of which were calculated in the previous section.

Overall Running Costs

My second microwave kiln tells me to expect a lifetime of about 250-300 firings, or maybe more if I treat the microwave kiln with more respect and care. As most of my use of a microwave kiln is to produce circular blobs of glass, we’re talking “full-fuse-plus”. We might therefore reasonably expect a longer life for the kiln with profile fused work.

Record-keeping may be boring and nerdy but it clearly has its uses. I now know the lifespan for my second microwave kiln and how it has behaved from new until the time I threw it away. Combining all the information at current (2016) UK prices tells me that the total per firing will be somewhere in the region of 25 to 35 pence, depending on how old the microwave kiln is, the kind of work being done, and whether or not I am prepared to buy raw materials in bulk.

You might like to think about how costly it is to fire-up your “big kiln”. The same ideas and methods apply, but the numbers will be bigger.

How Microwave Kilns Work

If you’ve got this far and have an urge to find out more about how microwave kilns work, and would also like to know how you can make your own, you’re in luck. I’ve gathered together a few links below which I’ll pad out with some commentary.

When you hunt around the Internet you’ll maybe find some misleading information about “the grey stuff” in a microwave kiln. The grey material is not granite, nor is it graphite. It is a mixture of silicon carbide and sodium silicate. Notice I say silicon and not silicone. Silicon is a shiny silvery metal. Silicone is a kind of plastic used for waterproofing products, breast implants and more besides. Silicon and silicone are not the same things.

You will be familiar with silicon carbide as an abrasive if you’ve ever tumbled rocks and minerals. You will also be familiar with sodium silicate though it’s unlikely that you realise it. Both are inexpensive chemicals that you can buy on eBay and I’ll give you a couple of links later that tell you more about both of them.

Once upon a time I found a reference to both these materials when I was reading something about LVR Products’ Micro-Kiln EZ-5 and Micro-Kiln No 9. I made a note that in their parts list it said there was a ‘Repair Solution Set’ which consisted of Silicon carbide (solution A), Sodium silicate (solution B) and a Brush. I forgot to make a note of the URL and I can’t find with Google any more, so I’m sorry I can’t give you a link to this evidence. But not to worry. I have more sources of information that should reassure you I’m not talking out of my backside.

Silicon carbide is used as the heating element because it has the interesting property of absorbing microwaves and re-emitting the energy as heat. You can find out more about this grey “heating” chemical at Wikipedia’s entry for Silicon Carbide (especially in the Heating Elements section). You will also find silicon carbide mentioned in some of the links listed below.

To “glue” the silicon carbide to the microwave kiln lid requires a binding agent and although there are several possibilities, you will you find that the commercial repair kits seem to use sodium silicate. Find out more about this “binding” chemical at Wikipedia under Sodium Silicate (especially in the Refactory Use section).

Over at Paragon you will see repair instruction that mention a silicon carbide layer. Actually, this is a very useful little instruction manual for any microwave kiln user, not just the Paragon MagicFuse microwave kiln.

You can get a really good insight into how microwave kilns are made by watching a YouTube video called How to make a microwave kiln (Furnace) from scratch for £5. The audio is not good but it is worth the struggle. Not only will you see a microwave kiln being made but you discover silicon carbide is just one of many “susceptor” chemicals that can be used as a heating product and that there are different binders, not just sodium silicate. Also interesting in the narrative is an explanation of how the same heating method is used to cook microwave chips.

You can find out more information about microwave absorbers here though in a completely different context.

If this isn’t enough for you then there is an old technical reference about “self heating” ceramic crucibles for microwave melting of metals and nuclear waste glass  at the Office of Scientific and Technology Information in the USA which is not as irrelevant as you might initially suspect. Vitrification has for many years been considered as a “safe” method of disposal for nuclear waste materials.

For the fearless amongst you, I have found some rather technical references. I can promise you an especially dreary read with this patent. If it is too much for you, I suggest try the readable article here because they’re both about the same thing.

Are We Being Ripped-Off?

And finally, we should give some thought to whether microwave kilns are good value or not.  The same applies to repair kits that you might encounter.

For almost the cost of buying a replacement microwave kiln you can buy a microwave kiln repair kit. One example is here. I am always suspicious of spare parts and repair kits that cost almost as much as the original item.

If you have a look in eBay (or elsewhere) you’ll discover just how cheap silicon carbide and sodium silicate really are. This should make you wonder why there’s such a big difference between the price of these raw materials and the price of a commercial kit or a microwave kiln.

If you understand the instruction in the YouTube video I mentioned in the previous section you’ll begin to understand that 50 GBP is ten times the cost of making your own. Again, this should make you wonder why there’s such a big difference between the price of the raw materials and the price of a commercial microwave kiln.

Yes folks. Information is power. The power to exploit. And now you know their secrets they can’t exploit you so easily. But you can exploit what you know. You too can make a microwave kiln. You too can buy the materials you need to make your own repair kit.

If you enjoy making things and you don’t have a microwave kiln then making one is surely a candidate for the top of your “Things to Make” list.

Bye for now. Tomorrow I’m going to make some rainbows. How about you?


About chatterglass

Maker of stained glass frippery.
This entry was posted in Devitrification, kiln schedule, Melting Glass, Microwave kiln, temperature curve and tagged , , , , , , . Bookmark the permalink.

4 Responses to Secrets of the Microwave Kiln

  1. Suzanne Wade says:

    Please unsubscribe me since your Web page won’t let me do it myself. Thank you. [Edited to remove personal information]

    • chatterglass says:

      I am not able to unsubscribe you Suzanne. I was however able to edit-out your email address so that it is not “captured” by “spambots”, causing you to get loads of spam emails.

      If you are an email subscriber then I believe there will be an Unsubscribe link at the bottom of your emails.

      If you have a account then I believe you can unsubscribe at or you can go to my blog and there should be a follow/unfollow button somewhere on the page – in the right-hand menu I guess.

      If all else fails, try contacting the WordPress support people though I don’t know how to do this.

  2. Peter Evans says:

    Thank you fro sharing your experience. This page is a revelation to me and answered a lot of questions I had. I find it fascinating that its possible to fuse glass in the microwave and I was looking around for more info. I wanted to know what you can and can’t do so I learned a lot here. Its good to know the limitations pros cons etc versus a real kiln and I enjoyed your detailed, scientific approach. Geeky but good. I was toying with making my own after I also came across and watched the same video you refer. I had wondered if the commercial micro kilns degraded over time too so you answered that question perfectly. The graphs made it nice and clear what to expect. I’ll take a look around thhe rest of your blog , It seems like a mine of good info.

    • chatterglass says:

      Hello Peter. Thank you for the kind remarks. I’m most pleased to have helped you and hope to do so for many years to come.

      In times of old there was no rigid separation of the Arts from Sciences like we find nowadays. I believe the separation limits our ability to reach our potential. A quote from John Constable, the painter, sums up my thoughts: Painting is a science and should be pursued as an inquiry into the laws of nature. Why, then, may not a landscape be considered as a branch of natural philosophy, of which pictures are but experiments?

      This is why I try to cut across the grain by being geeky, as you put it, including a little maths, a touch of science and a less “arty” way of presenting information. And again John Constable has me “sussed”: We see nothing truly until we understand it.

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