This post will help you understand the nitty-gritty details of using a microwave kiln. For starters, make sure to check out our Microwave Kiln Before You Buy post! It goes over all the basics of fusing glass in a microwave kiln. If you're looking for more detailed tips to get more out of your microwave kiln, read below!
Should you use the turntable in the microwave with a microwave kiln?
The turntable of your microwave rotates your food so that heat is absorbed more evenly than if it was sitting in the middle, and it does the same thing for your kiln. If you have kiln bricks, put them under the bottom of the kiln to keep it off the turntable while still allowing the kiln to rotate. If you don’t have kiln bricks, you can flip your microwave’s turntable upside-down, creating an insulating pocket of air that will protect the bottom of the microwave (remember to take out the plastic rotating piece underneath).
However, this means the kiln will not rotate, which can lead to more inconsistent heating – and the microwave kiln already has problems with that! If you’re not using the turntable, it’s important you check the lid of the kiln for any hot spots forming, and rotate the lid around the base to move the heat around.
Three options for raising your microwave kiln up. I use small kiln bricks because they absorb less heat, but the big kiln bricks are more stable, and no kiln bricks is OK too.
Microwave watts and the microwave kiln
The power of a microwave is measured in watts. Home microwaves range from 600 to 1200 watts, and the standard is around 800-900W. The stronger your microwave is, the faster it will heat up and the hotter it will get at max power. We are using a 900W microwave in the studio, so if you have a weaker or stronger microwave, it will take more or less time for you to get the same results. The most important thing is testing your microwave kiln in your microwave to get a feel for how hot it gets how quickly. This is especially important if you have a 1000W microwave or above – glass that doesn’t get hot enough can always be refired, but glass that gets too hot can’t be taken back!
This is what I mean when I say "glass that gets too hot can’t be taken back"!
Are all microwave kilns made equal?
Short answer: Yes. Long answer: Mostly yes, but not exactly.
The construction of microwave kilns are the same – the lid consists of a layer of radiant heating material that absorbs microwaves and turns them into heat, surrounded by a thick insulating material to keep the heat inside. This sits on top of a base made of the same insulating material with a small working platform where your glass rests to be fired. I have read that the exact composition of the metallic heating material varies from manufacturer to manufacturer, but when you're buying online, sellers can source the same kiln from the same factory and sell it under many different brands.
For this article, we bought two small microwave kilns under two different brand names, and a large microwave kiln from a third brand. When they got here, the only noticeable difference is that one of the small kilns weighed 2.5oz more than the other one. Most of the extra weight was in the lid, so I was hoping that kiln had more heating material and would heat more evenly or get hotter, but the two small kilns are functionally interchangeable. If I hadn’t burnt a hole in one of them (see more on that below!), I wouldn’t be able to tell them apart at all.
The two small kilns I used, with the Amazon titles and UPCs for reference.
So feel free to buy the cheapest microwave kiln you can find online! Chances are good it came from the same place as one that cost $10-$20 more; it should work exactly the same and last just as long. Some of the pricier microwave kiln listings you see online come with safety gloves and a small amount of fusible glass to play around with, but the kiln itself should still be the same make and model as the solo listings.
Making sure you don’t wreck your microwave kiln, or "What happened to your microwave kiln?"
You may have noticed that one of the microwave kilns in my pictures has a large scar around the working platform. The kilns we bought came with a small amount of dust from the lid caked onto the base of the kiln, around the working platform. One of the kilns fused slightly at that point, melting some of the dust permanently onto the base of the kiln and scorching the lid slightly. The other kiln…
The melted base and lid. I circled where the scar is on the lid, but the base is impossible to miss! It all works fine still at least.
The very first time this microwave kiln was fired, the dust went supernova where it touched the lid and ate a huge hole into the base! There wasn't a bunch of dust on this particular base, no more than on the other kiln, but the difference was huge. Inspect your kiln before firing and try to sweep off any black dust on the base first!
After this happened, the lid and base were actually stuck together slightly, and I was scared that I had ruined the microwave kiln with the very first fire! But after peeling the lid off, I’m pleased to report that the melted base has had no effect on how the microwave kiln works and the hole hasn't gotten any bigger. If the scorched parts on the lid and base aren’t lined up, it can rest a little unevenly, but scraping the melted parts back down to flat will make that go away. Hopefully this doesn’t happen to you! But I’m here to tell you it’s not the end of the world if it does.
Temperature differences in the microwave kiln / How to tack fuse (contour fuse) in a microwave kiln
The hottest temperature glass reaches in a kiln is called the "working temperature", and it determines how much the glass melts and fuses together. Lower temperature fusing, called tack fusing or contour fusing, leaves glass looking 3D and "tacked" on, while a hotter full fuse can get your glass totally flat and smooth. We took an in-depth look at what our glass does at temperatures from 1375°F to 1500°F (745°C to 815°C) using an electronically controlled kiln to hit precise working temperature, which you can read here.
This is an image from our Tack Fuse Tip Sheet - check it out here!
Those basics still apply with the microwave kiln, but reaching working temperature with it is a much different beast. The only way to know how hot your kiln gets inside is trial, error, and experience. The amount of glass you're firing, how thick it is, how long you let the microwave run, how many times you stop and look inside the kiln, all of these things influence how your glass fuses, and that's before we even talk about stuff like microwave power or how many times your kiln has been fired.
Starting with the same murrine, the squares on the left were tack fused and the squares on the right were full fused.
For example, these squares all started out as the same murrine, but were fired to different temperatures for very different results. The squares were fired with these schedules in a 900W microwave at full power. Each "+" separating the times means I stopped there and opened the kiln to take a look, which lowers the heat inside.
- Tack fused (less heat): 2 minutes + 1:30 minutes + 30 seconds + 30 seconds + 30 seconds = 5 minutes total
- Full fused (more heat): 3 minutes + 2 minutes = 5 minutes total
The microwave was active for the same amount of time with both batches, but the results speak for themselves. The more you stop and check on your glass, the finer control you'll have over it, but it also cools the kiln down each time, so the total processing time may end up being the same or longer than something you fire hotter. Moving the kiln lid around a lot also increases your risk of disturbing your glass and knocking it out of place! If I was going to do this again, I would use two 45-second blasts at the end instead of three 30-second ones. This would fire them slightly hotter, smoothing the uneven top edge out more, and come with one less opportunity to bump my glass.
Can you anneal glass in a microwave kiln?
Annealing is simply the process of letting your glass heat up and cool down together, slowly and evenly. Annealing prevents breakage from heat shock and keeps stress from being frozen into your glass, a problem that can result in cracking later.
Annealing cannot be be done in a microwave kiln, but that being said, the small size of projects that fit into a microwave are much less likely to contain enough stress to cause breakage. Even when firing in a large microwave kiln, it shouldn't be an issue.
That's all the tips and tricks for the microwave kiln I have to share right now, if you have any questions about anything covered here (or not covered!), please ask in the comments below! I'm still working on an in-depth look at large microwave kilns, and project inspiration posts are coming soon. Thanks for reading!