Secrets of the Microwave Kiln

I ’ ve barely bought my third base 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 field glass in a microwave kiln, a little about the economics of using them, and a small about how they work and how they deteriorate .
I think I need to do all of this because I don ’ metric ton 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 glazed, within about two hours. My “ proper ” kiln makes me wait a unharmed day ( and night ). sol, speed and convenience is one rationality .
The barely-controllable ferocious inflame in a microwave kiln results in a greater gamble of glass crack at it heats up and the lack of processing temperature operate means it ’ s not uncommon to find a mutant deformed blob of glass is the unexpected resultant role of a fire. And of course the minor discharge chamber means we don ’ thyroxine arrive to make anything larger than a brooch or chandelier. sometimes this is not a problem .
The down-side of the about irrepressible inflame, as I ’ ve equitable mentioned, is that the form and phase of the resulting methamphetamine masterpiece is preferably unpredictable. This becomes a particular trouble when the microwave kiln gets older and heats less evenly. I ’ ll be coming back to the “ getting older ” aspect of microwave kiln belated as it seems to be a widely neglected subject !
Another characteristic of microwave kiln is the fabulously rapid cooling inside the microwave kiln. It has skimp regard for “ proper annealing ”. You might think that this must lead to problems but in truth it identical rarely does .
We are repeatedly told that it is crucial to properly anneal our work. From this we might suppose that the rapid cool in a microwave kiln without “ proper annealing ” might cause us significant problems. In truth I find that breakages caused by poor annealing are identical rare. If this is a surprise to you then consider the size of items being produced and realise there ’ s only sol much stress and strain that can be built up and “ stored ” in such a belittled piece of glass. This is peculiarly the case for simple shapes like a spot of glaze, a little dress tile or a simpleton pendent – in other words, precisely 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 besides far sometimes .
Another view of the rapid heat and cool in a microwave kiln that I have not seen mentioned anywhere relates to devitrification. With a microwave kiln the action time is so short-change that glass that is susceptible to devitrification rarely has time to devitrify. A hardheaded consequence is that I am able to reliably produce recycle my otherwise unserviceable scraps of “ average ” non-fusing glass into blob with little risk of devitrification. You can see veridical examples in my Recycling Scraps of Stained Glass blog and you should bear in mind that every individual ball you see in the mental picture is not fuse glass. For the faineant amongst you, and because it ’ sulfur colorful, I ’ ll re-post the picture from that blog :
DSCF1857 Recycled Glass Globs
And here ’ sulfur and interesting exercise that proves the reverse position from my last blog. This little wonderous peaky spot of looking glass devitrified before it melted completely :
DSCF3010 Spiky Devit
The big surprise is that my “ proper ” kiln fails to produce glistening globs with “ ordinary ” non-fusing field glass. Devitrification is constantly a trouble. action meter is authoritative 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 “ average ” non-fusing looking glass can barely cope with slumping without devitrifying at least to some degree. Someday I ’ ll do a web log about this but I ’ ve not finished messing about so far !
Reasons to Use a Microwave Kiln
A consequence of the forfeit 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 waiting 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 topic how hard we try to make consumption of smaller pieces of glass we end up with little scraps that are unserviceable. Where possible unserviceable scraps get melted into ball. It makes environmental common sense through I doubt the time and feat to make them is commercially viable .
Some of the smaller ball ( 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 foreground in a wooden decoration for example .
Larger ball ( typically 6-10 grams ) are supposed to be sold though I tend to give away most of them. My principle is simple – kids who show an interest in my work can have one free but atrocious kids have to pay for them. There has to be a wages for being “ courteous ” .
Microwave Kilns Deteriorate
I ’ ve already mentioned that I ’ molarity now on my third gear Hot Pot Maxi microwave kiln. What happened to the other two ?
As battered and bruised old-timers the previous microwave kilns have been retired. They now live in landfill. The dull accuracy is that they ’ re flimsy, get damaged easily and actually do get old and banal .
I should now explain how a microwave kiln works ( in abbreviated ) 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 sparkle and reduce the ceramic material is, it performs signally 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 mismatched inflame will be caused by subtle differences in the mixture of materials and their thickness. With time the degree of spotty heating gets worse for reasons that follow…
insistent heat and cool causes insistent expansion and contraction which will result in hairline cracks. The brittle nature of the ceramic material ( and the at heart coating ) of a microwave kiln means it starts preferably soon and gets increasingly worse the more you use the microwave kiln .
precisely where the hairline cracks appear depends on the ineluctable “ defects ” of manufacture and some basic physics. That the cracks constantly seem to run from top to bottom is strictly down to the combination of geometry and coefficients of expansion – the inside gets hottest indeed wants to expand proportionately more than the external. The reverse happens when cooling. This difference causes stresses and strains which result in hairline cracks appearing. so, don ’ metric ton be unduly concerned by hairline cracks because they ’ re a natural consequence of the heating and cooling system 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 radiotherapy ?
The hairline cracks will cause odd heat because areas that heat up fastest can not conduct some of their estrus to cooler areas because of the barrier caused by the cracks. then, any minor differences in one area heating up faster than another due to original manufacture “ defects ” is made more marked when hairline cracks come into play. As the size of the arouse chamber is indeed small we can assume there is no heat transfer by convection. There will however be some hotness transfer by radiation because that ’ s what we ’ ra using to heat up the looking glass in the burn chamber .
thus, spotty heating becomes an ineluctable and noticeable trouble once the microwave kiln starts to develop hairline cracks. This in turn adds to the volatility of what you can produce in a microwave kiln. A partial answer to this spotty inflame is to pause the dismissal mid-way, have a glance, rotate the eyelid by half a turn, then continue to the dismissal. With practice and commodity time this can about negate the effects of odd heat .
Glass slippage is another problem because. It is very easy to incidentally nudge the eyelid of the microwave kiln and cause the glass pieces inside to slip. A microwave oven platen that rotates ill ( wobbling or shuddering ) can besides cause glass to slip. Heating besides quickly may cause glass to crack and move, so is another form of slipping. Any of these ( and early ) mishaps may result in hot glass “ glue ” itself onto the al-qaeda or the sides of the microwave kiln. You can besides achieve the same consequence 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 methamphetamine 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 undertake to remove the glass. It is rarely possible to remove the glaze without damaging the ceramic material, tied 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 al-qaeda that resembles the consequence of World War I trench war, or sides where big chunks of the grey heating material are missing .
Using kiln wash and fiber paper can help deal with some of the problems some of the prison term but in my feel they will only reduce the rate of kiln end !
Another aspect of the deterioration relates to the heating ability of the dull grey game means on the inside airfoil of the microwave kiln ’ mho eyelid. It ’ s the heat component. The boring grey farinaceous means is something I ’ ll speak about in more detail at the goal of this web log so for the here and now merely 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 curious characteristic of the grey fabric .
I am not sure why, but the effectiveness of the gray “ heat ” material appear to deteriorate over time, partially because of minor mechanical defects such as hairline cracks, but besides 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 ’ deoxythymidine monophosphate been keeping any records .
The fact it takes longer and longer for the microwave kiln to heat up with age implies there is some shape of chemical deterioration in the “ heating component ” part of the microwave kiln. Anything that ’ s hot and in air tends to get oxidised as a count of routine. This is possibly most familiar to you if you ’ ve ever put some cover girl salmon-pink glistening copper elements in your kiln-fired influence and was disappointed to discover they came our bolshevik, empurpled or flush black as heat and oxygen increasingly turned the bull to copper oxide. This is what heating system 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 tinge glass are “ firing off ” and reacting with the heating system element. The early possibility is that the mixture of materials in the heating chemical element react with each other causing chemical changes .
Whatever the cause, the effect is that the heat element becomes less susceptible to microwave energy so is not able to re-emit estrus sol efficaciously .
Firing History
My first microwave kiln told me that there was deterioration. then, for my second microwave kiln, I kept a record of each dismissal. not much more than the date, what kind of undertaking and how farseeing it was “ cook ” in the microwave. What you see in the graph below is the solution of my nerdy record-keeping. Have a look at the graph then I ’ ll explain what it all means .
firing-times
The graph shows that my second microwave kiln didn ’ triiodothyronine quite make it to 300 firings before I felt it was time to throw it off. The accurate count of times was 283 .
You can besides see from the graph that the jagged curve runs from the lower left ( the beginning few firings ) to the upper berth correct ( the end-of-life firings ). Notice besides that the wind is steeply upwards on the leave and goes shallow on the right. This swerve 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 ’ thymine concluding long .
Notice that I ’ ve scaled the march time so that 100 % represents how quick the new kiln was. This means that when I threw it away it was taking about doubly ampere hanker to do precisely the lapp job – over 180 % of the master ignition times. Notice besides 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 fresh one. The other is that there ’ s no luff in relying on detailed accurate fire records with a microwave kiln because its behavior changes over time.

I ’ ll now reinforce that final paragraph in a different way. If you use a fire time from an old microwave kiln to guide to what you should do with a raw microwave kiln you will likely “ doubly blast ” your glass. It will be “ double over processed ” and you may end up producing an identical fluid pool of fade glass. And fluid molten glass flows quite well if a surface is not precisely level. This is how I managed to “ glue ” the inside of the lid of a microwave kiln onto its floor using melt glaze. Don ’ t be deoxyadenosine monophosphate unintelligent as I can be. Consider yourself warned !
now that we have some evidence about how microwave kiln 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 ’ molarity using has a open fire 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, fiber paper, currency exchange rates etc .
therefore, how much does it in truth cost to fire-up a microwave kiln ? Lets find out…
I ’ ve already mentioned 50 pounds Sterling ( notification it ’ mho “ Pounds Sterling ”, not “ English Pounds ” ) as the purchase price of my fresh microwave kiln and that I got 283 firings out of my second microwave kiln. then, that ’ south about 17.6 pence per firing due to the kiln price .
But electricity besides costs money. I am using an erstwhile 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 ’ south about 60 % efficient. My electricity costs around 16 penny per kWh and I ’ ve factored-in a proportion of the standing charge. We end up with merely a few penny of electricity per firing which I can immediately plot on a graph .
firing-cost
I see that the price starts somewhere between 3 or 4 penny, cursorily rises to closely 5 penny, then lento drift upwards to a little over 6 pence per fuel. If you compare this graph with the previous one you ’ ll see precisely the same form but a different y-axis scale. This is because we ’ ra doing nothing more complicated by converting a y-axis in units of time into units of penny by multiplying by a ceaseless value. For wide marks in a mathematics exam I should have possibly chosen a y-axis start at 3p preferably than zero to make better use of the quad .
Other Running Costs
There are early running costs that were not included in the former graph. We tend to use some kiln wash to protect the base of the microwave kiln. A bantam divide of a penny per firing for kiln wash is negligible compared to the monetary value of your time and the early costs associated with running a microwave kiln .
You, like me, might besides use Bullseye ’ randomness thinfire paper between the glass and the kiln surface. It ’ sulfur expensive and it can ’ t normally be used more than once. But how expensive is it ?
If you ’ re lazy you ’ ll buy ready-cut 10cm squares at around 11 penny per firing, such as from hera 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 rather be paying around 6 penny for the same total of thinfire wallpaper. All it takes is the will to buy in majority, a pair of scissors and a few minutes of your fourth dimension .
And last, we need to remember to allocate a dowry of the price of buying the microwave kiln to each burn angstrom well as the electricity cost, both of which were calculated in the former section .
Overall Running Costs
My moment microwave kiln tells me to expect a life of about 250-300 firings, or possibly more if I treat the microwave kiln with more esteem and care. As most of my use of a microwave kiln is to produce round spot of glaze, we ’ rhenium talking “ full-fuse-plus ”. We might consequently reasonably expect a longer life for the kiln with visibility fused work .
Record-keeping may be boring and nerdy but it intelligibly has its uses. I immediately know the life for my irregular microwave kiln and how it has behaved from new until the time I threw it away. Combining all the data at current ( 2016 ) united kingdom prices tells me that the sum per fire will be somewhere in the region of 25 to 35 penny, depending on how old the microwave kiln is, the kind of work being done, and whether or not I am prepare to buy raw materials in bulk .
You might like to think about how costly it is to fire-up your “ big kiln ”. The like 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 kiln work, and would besides like to know how you can make your own, you ’ re in fortune. I ’ ve gathered together a few links below which I ’ ll pad out with some comment .
When you hunt around the Internet you ’ ll possibly find some mislead information about “ the grey stuff ” in a microwave kiln. The grey material is not granite, nor is it graphite. It is a assortment of silicon carbide and sodium silicate. Notice I say silicon and not silicone. Silicon is a glazed argent metallic element. silicone is a kind of fictile 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 besides be familiar with sodium silicate though it ’ s unlikely that you realise it. Both are cheap chemicals that you can buy on eBay and I ’ ll give you a couple of links former that tell you more about both of them .
once upon a fourth dimension I found a character 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 ’ thymine recover with Google any more, so I ’ molarity regretful I can ’ thyroxine give you a connect to this evidence. But not to worry. I have more sources of information, as you will see late, that should reassure you I ’ m not talking out of my rear .
Silicon carbide is used as the heat element because it has the interest property of absorbing microwaves and re-emitting the energy as estrus. You can find out more about this grey “ heating ” chemical at Wikipedia ’ s introduction for Silicon Carbide ( specially in the Heating Elements section ). You will besides find silicon carbide mentioned in some of the links listed below .
To “ glue ” the silicon carbide to the microwave kiln eyelid requires a ski binding agent and although there are respective 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 ( specially in the Refactory Use section ) .
Over at Paragon you will see repair education that mention a silicon carbide layer. actually, this is a very useful little teaching manual for any microwave kiln drug user, not good the Paragon MagicFuse microwave kiln .
You can get a truly estimable insight into how microwave kiln are made by watching a YouTube video recording called How to make a microwave kiln ( Furnace ) from rub for £5 but the link is now secret and inaccessible to us mere mortals. The audio is not estimable but it is worth the clamber. not lone will you see a microwave kiln being made but you discover silicon carbide is barely one of many “ susceptor ” chemicals that can be used as a heating system product and that there are different binders, not merely sodium silicate. besides interesting in the narrative is an explanation of how the like heating system method is used to cook microwave chips .
With the “ erstwhile ” connect now inaccessible I suggest you watch the YouTube television called Diy Microwave Kiln | Melt Glass in the Microwave vitamin a well as another called How to make a Microwave Kiln – Easy and Cheap DIY Glass Fusing & Melting for a less comprehensive penetration. If nothing else they confirm the manipulation of silicon carbide and sodium silicate as appropriate materials .
You can find out more information about microwave absorbers here though in a completely different context .
If this international relations and security network ’ thyroxine enough for you then there is an old technical reference about “ self heating ” ceramic crucibles for microwave dissolve of metals and nuclear waste glass at the Office of Scientific and Technology Information in the USA which is not a irrelevant as you might initially suspect. Vitrification has for many years been considered as a “ dependable ” method of disposal for nuclear consume materials .
For the audacious amongst you, I have found some rather technical references. I can promise you an specially blue read with this apparent. If it is excessively much for you, I suggest try the clear article here because they ’ re both about the same thing .
Are We Being Ripped-Off?
And finally, we should give some remember to whether microwave kilns are good value or not. The same applies to repair kits that you might encounter .
For about the cost of buying a surrogate microwave kiln you can buy a microwave kiln repair kit. One example is here. I am constantly leery of excess parts and repair kits that cost about equally much as the original item .
If you have a look in eBay ( or elsewhere ) you ’ ll discover fair how cheap silicon carbide and sodium silicate actually 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 teaching in the YouTube television I mentioned in the previous department 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 boastful dispute between the price of the raw materials and the price of a commercial microwave kiln .
Yes folks. information is might. The ability to exploit. And now you know their secrets they can ’ t feat you so easily. But you can exploit what you know. You excessively can make a microwave kiln. You excessively can buy the materials you need to make your own repair kit out .
If you enjoy making things and you don ’ t have a microwave kiln then making one is surely a campaigner for the top of your “ Things to Make ” tilt .
Bye for now. Tomorrow I ’ molarity going to make some rainbows. How about you ?

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