Microwave kiln: do you have one?
May 10, 2023 10:07 AM Subscribe
I have an old microwave, I would like to use it as a kiln. There's a lot to take in and a lot to compare. Maybe you've already done some of this work and will spill the tea.
This is for use in a shared space, so I would like to think safety, ease of use. I would need to get everything that is needed to go with the kiln, tell me if I am missing anything below. Can I use the same kiln for glass, clay and metal (only claymetal?). Can I use it for anything else? How do you use yours?
I need a micro, a kiln, paper for the bottom of the kiln, kiln gloves to remove it, a heatproof plate to place it on. Anything else? What microwave kiln should I buy?
Materialwise I would like to provide the basics and then they can get fancy with their owns stuff. I need some worked clay, some simple glaze, like white or clear (?), maybe some safe coloured glazes (or underglazes, what's the diff?). It says low temp glazes are good, can you give me some keywords to acquire these?
Should we make people work outside? Is it feasible to say "don't leave the room until your pot is cooled and opened"? How can I do this safely in a multiuser environment. Many TIA!
This is for use in a shared space, so I would like to think safety, ease of use. I would need to get everything that is needed to go with the kiln, tell me if I am missing anything below. Can I use the same kiln for glass, clay and metal (only claymetal?). Can I use it for anything else? How do you use yours?
I need a micro, a kiln, paper for the bottom of the kiln, kiln gloves to remove it, a heatproof plate to place it on. Anything else? What microwave kiln should I buy?
Materialwise I would like to provide the basics and then they can get fancy with their owns stuff. I need some worked clay, some simple glaze, like white or clear (?), maybe some safe coloured glazes (or underglazes, what's the diff?). It says low temp glazes are good, can you give me some keywords to acquire these?
Should we make people work outside? Is it feasible to say "don't leave the room until your pot is cooled and opened"? How can I do this safely in a multiuser environment. Many TIA!
Microwaves cause water molecules in food to vibrate, producing heat that cooks the food. I'm not sure how this would translate to heating clay to the required firing temperature which wouldn't have any moisture left well before that point.
posted by Mitheral at 2:01 PM on May 10, 2023
posted by Mitheral at 2:01 PM on May 10, 2023
FYI to the other posters: microwave kilns are absolutely a thing, for fused glass and for ceramics, usually small pieces with low-fire glazes.
I am not going to recommend a kiln as I've never used one personally, but I would get big, cheap microwaves at goodwill or similar because they will get beat up and encrusted with clay and paint and whatever. A microwave is a microwave is a microwave.
posted by cilantro at 2:12 PM on May 10, 2023 [3 favorites]
I am not going to recommend a kiln as I've never used one personally, but I would get big, cheap microwaves at goodwill or similar because they will get beat up and encrusted with clay and paint and whatever. A microwave is a microwave is a microwave.
posted by cilantro at 2:12 PM on May 10, 2023 [3 favorites]
My husband used a small kiln for fused glass and I am not sure it was good for the microwave. The door button seemed a bit wonky afterwards. I'd get a larger sized one with lots of clearance above it because it will get extremely hot. Things didn't always work perfectly so I'd definitely run a few tests before spending a lot on supplies.
posted by oneear at 2:54 PM on May 10, 2023 [1 favorite]
posted by oneear at 2:54 PM on May 10, 2023 [1 favorite]
I see full size microwaves at ReStore reasonably often, if you need a more powerful one.
posted by theora55 at 7:44 PM on May 10, 2023
posted by theora55 at 7:44 PM on May 10, 2023
Response by poster: As per the first sentence, I have a microwave specfifically fit for purpose, my question is all about the kiln and firing processes. I would like to kindly suggest to the first two respondents that if you doubt a specific thing exists maybe a tiny google before responding wouldn’t hurt.
posted by Iteki at 9:51 PM on May 10, 2023 [4 favorites]
posted by Iteki at 9:51 PM on May 10, 2023 [4 favorites]
> It says low temp glazes are good, can you give me some keywords to acquire these?
I'd contact your town's old-school pottery supply shop, which should have a wide range of materials and a knowledgeable sales clerk to walk you through it. It's what they do.
posted by sebastienbailard at 10:36 PM on May 10, 2023
I'd contact your town's old-school pottery supply shop, which should have a wide range of materials and a knowledgeable sales clerk to walk you through it. It's what they do.
posted by sebastienbailard at 10:36 PM on May 10, 2023
Best answer: I have one. Key issue: the inside of your microwave must be metal, not plastic.
I primarily used my kiln for sintering silver, the Artclay brand, but occasionally glass work. You'll want a thousand watt microwave primarily because those just have more space, as the work area is very small even on the larger kilns. Otherwise less power just takes a bit more time.
The whole process to heat up is just a few minutes. For glass, mine takes just 2 or 3 minutes, and you can start melting some metal at roughly double that. The inside lining of the lid, the susceptor, is what the microwave actually heats up. The outer white bit is just for insulation, and it stays very hot inside while being relatively less hot on the outside. The kiln paper can be reused but when I started I ruined a bunch of that and so get extra.
To get started you will need to calibrate your setup - I used glass rods that I knew melt at a specific temperature. I would poke the rod out through the vent, and could monitor how melty it was getting while the microwave was running. It took about six of them to establish how long to nuke the unit to reach that temperature and then estimated the time to get to my target of 900C. It's always a bit of an estimate though - some materials are more reactive to the kiln, and the thickness of the work also impacts the process. Microwave kilns are fast - it's easy to overshoot your target temp.
Something that is 900C on the inside should be handled carefully. The best part is that when you are starting take your time, you don't have to handle it hot at all - just open the microwave and wait. My kiln would take about 20 minutes to cool off and only be very hot on the inside, and for glass I would wait about 40 minutes for everything to cool completely.
Eventually you will want a reliable method to move the kiln by the bottom of the crucible assembly, and I had a separate method to handle the larger upper section. The issue is not with how hot the outside of the kiln gets, which looks deceptively cool but really doesn't get any hotter than something coming out of your oven. The difference is the inside - careless handling can expose the hot core up in the lid or you can accidentally dump out the contents of kiln.
My method is to carry the kiln out on the microwave's tempered glass plate. This way the kiln always stays together and you aren't reaching into a box to move the hot bits around. I always wore regular high temperature gloves/mitt around the kiln, which weren't expensive or fancy. At the studio we had welding gloves, at home just high temp cooking gloves. The studio uses large ceramic tiles under and around the microwave, and unglazed tile or brick would also be effective and safe.
At home I had a large baking sheet that was upside down in front of the microwave for the actual cool down, and I would just pull out the the glass plate, put it down and then lift the kiln onto the cookie sheet. Simple, safe and reliable. Once the kiln is on the sheet getting the lid off safely and moving the work is straight forward. My kiln has a vent in the top that I was careful to avoid touching with the gloves.
I regularly used my kiln in my kitchen microwave, I never had an issue with smoke or smells, but I also didn't put anything particularly toxic in the kiln. At least nothing worse than microwave popcorn.
Mine is a fancy one from Japan, but I've used a few different ones and if I needed to replace it I would just get the most basic one. It's a simple tool that either survived shipping and works the first time or not.
Most glass work I did was in the lower end of the 745°C to 815°C range, which is where 90 COE glass will fuse together. The issue with glass is that a piece might completely fail if it gets overheated by relatively small amounts - even 20 degrees over can make it totally black or turn it into a puddle. Plus some glass can't be mixed with another COE as they won't fuse, while other glass need to be cooled slowly, which is difficult in a microwave kiln.
I tried doing some glass frit work, which is usually around 450 to 550 °C. This was actually harder for me to manage - the kiln often overshot the temp and ruined that type of work. Frit work is very popular as it can be small pieces and it's low temp, but it was just beyond my particular ability at that time.
For metal I stuck to Artclay silver, and never tried to go above 900c, which is the equivalent to a cone 10. It was very consistent for that material. Even in my real kiln glass work is more temperamental. At least glass is cheap, but it's is heavy so I would track down a local supply shop for an inexpensive source. They will have the specific information for the working temperature range of each product, as glass can vary widely.
posted by zenon at 10:44 PM on May 10, 2023 [3 favorites]
I primarily used my kiln for sintering silver, the Artclay brand, but occasionally glass work. You'll want a thousand watt microwave primarily because those just have more space, as the work area is very small even on the larger kilns. Otherwise less power just takes a bit more time.
The whole process to heat up is just a few minutes. For glass, mine takes just 2 or 3 minutes, and you can start melting some metal at roughly double that. The inside lining of the lid, the susceptor, is what the microwave actually heats up. The outer white bit is just for insulation, and it stays very hot inside while being relatively less hot on the outside. The kiln paper can be reused but when I started I ruined a bunch of that and so get extra.
To get started you will need to calibrate your setup - I used glass rods that I knew melt at a specific temperature. I would poke the rod out through the vent, and could monitor how melty it was getting while the microwave was running. It took about six of them to establish how long to nuke the unit to reach that temperature and then estimated the time to get to my target of 900C. It's always a bit of an estimate though - some materials are more reactive to the kiln, and the thickness of the work also impacts the process. Microwave kilns are fast - it's easy to overshoot your target temp.
Something that is 900C on the inside should be handled carefully. The best part is that when you are starting take your time, you don't have to handle it hot at all - just open the microwave and wait. My kiln would take about 20 minutes to cool off and only be very hot on the inside, and for glass I would wait about 40 minutes for everything to cool completely.
Eventually you will want a reliable method to move the kiln by the bottom of the crucible assembly, and I had a separate method to handle the larger upper section. The issue is not with how hot the outside of the kiln gets, which looks deceptively cool but really doesn't get any hotter than something coming out of your oven. The difference is the inside - careless handling can expose the hot core up in the lid or you can accidentally dump out the contents of kiln.
My method is to carry the kiln out on the microwave's tempered glass plate. This way the kiln always stays together and you aren't reaching into a box to move the hot bits around. I always wore regular high temperature gloves/mitt around the kiln, which weren't expensive or fancy. At the studio we had welding gloves, at home just high temp cooking gloves. The studio uses large ceramic tiles under and around the microwave, and unglazed tile or brick would also be effective and safe.
At home I had a large baking sheet that was upside down in front of the microwave for the actual cool down, and I would just pull out the the glass plate, put it down and then lift the kiln onto the cookie sheet. Simple, safe and reliable. Once the kiln is on the sheet getting the lid off safely and moving the work is straight forward. My kiln has a vent in the top that I was careful to avoid touching with the gloves.
I regularly used my kiln in my kitchen microwave, I never had an issue with smoke or smells, but I also didn't put anything particularly toxic in the kiln. At least nothing worse than microwave popcorn.
Mine is a fancy one from Japan, but I've used a few different ones and if I needed to replace it I would just get the most basic one. It's a simple tool that either survived shipping and works the first time or not.
Most glass work I did was in the lower end of the 745°C to 815°C range, which is where 90 COE glass will fuse together. The issue with glass is that a piece might completely fail if it gets overheated by relatively small amounts - even 20 degrees over can make it totally black or turn it into a puddle. Plus some glass can't be mixed with another COE as they won't fuse, while other glass need to be cooled slowly, which is difficult in a microwave kiln.
I tried doing some glass frit work, which is usually around 450 to 550 °C. This was actually harder for me to manage - the kiln often overshot the temp and ruined that type of work. Frit work is very popular as it can be small pieces and it's low temp, but it was just beyond my particular ability at that time.
For metal I stuck to Artclay silver, and never tried to go above 900c, which is the equivalent to a cone 10. It was very consistent for that material. Even in my real kiln glass work is more temperamental. At least glass is cheap, but it's is heavy so I would track down a local supply shop for an inexpensive source. They will have the specific information for the working temperature range of each product, as glass can vary widely.
posted by zenon at 10:44 PM on May 10, 2023 [3 favorites]
> Should we make people work outside?
According to this decent-looking overview: Microwave Kilns: Great Tools for Testing Low-Fire Lusters, Enamels, and China Paints - published 2010/ republished November 2022 - Ceramic Arts Network
According to this decent-looking overview: Microwave Kilns: Great Tools for Testing Low-Fire Lusters, Enamels, and China Paints - published 2010/ republished November 2022 - Ceramic Arts Network
Due to fumes, the microwave needs to be vented properly or fired outside.posted by sebastienbailard at 11:13 PM on May 10, 2023 [1 favorite]
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Have you fired other kilns? There's a reason the walls and roof are made with firebrick, a special material that is made of the highest-firing clay material that will not fail at extremely high temperatures. Interior shelves are also ceramic. Anything else will deform from heat or vaporize. On the other hand, there are many, many small kilns, sometimes called tabletop kilns, jewelry kilns, glass-working kilns (the type that can soften/slump glass over special molds, not a glassblowing kiln!), test kilns (often used to fire small samples of clays and glazes on smallish tiles). Sizes range from shoebox to maybe 2 cubic feet of inside space. These can fire and cool in a half-day, usually. And, they can also be used for slumping glass. Can you do some research at a crafts workshop or art school? Or even a ceramics/glass supply retailer? I'm sure they will have valuable suggestions. A library ought to be able to supply you with a few books on glassworking and beginning ceramics so you will understand the processes and tools a little better.
posted by citygirl at 1:41 PM on May 10, 2023 [1 favorite]