Questions from a beginner – can you help?

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    Hi, I’m Rachel. I’m a total newbie at fusing, but a long time admirer of fused glass. And I’m using newbie tools – the microwave kiln. The Fuseworks one, to be specific.

    I have some concerns. Up until yesterday I had never broken a piece. Before yesterday I had also never used a layer of clear glass over top of my pieces. Yesterday I made two pieces with clear glass over top and they turned out great! Then I started looking at a piece I had made previously. It hadn’t turned out so well; I didn’t leave it in long enough and the edges were still sharp. It also didn’t have the clear layer over top. I had read that it’s okay to fuse a piece twice. Maybe this was a mistake. But I decided to put this old piece back in with a layer of clear over the top.

    Midway through firing I heard a sound. I opened up to check on my piece and it had broken. The glass had moved off the kiln paper and onto the kiln surface, which I know can permanently damage the kiln. I quickly grabbed a pair of tweezers and pulled the glass off the kiln; it appeared to come off cleanly so I thought no harm done. But when I removed the kiln paper I noticed that it was browned and had left a small brownish stain on the kiln itself. I figured this must have happened because it was a previously fired piece, as I had never had a problem like this before.

    Just now I tried to do another piece, not using any previously fired glass, and the same thing happened, so I’m thinking the problem isn’t with refiring a piece. Now the stain on my kiln is quite a dark brown. Have I ruined my kiln?

    Thanks so much in advance for any help you can give!

    Stephen Richard

    You have left quite a bit out in your description – thickness, lay up, size, schedule.  What temperature was the kiln at when you heard the break?

    So I speculate.  You heated up too fast for the the thickness of the glass.  The first heating has a lot of separate pieces.  The second firing is of a thicker single piece – go slower.  Look at your manufacturer’s website and you will find a number of suggested firing schedules.

    The brown mark is the binders of the fibre paper burning out.  As you pass 400C – whatever that is in F – you will find that the stain has gone.

    Kilns are robust.  You will find it difficut to damage them, unless you drop them.

    If you want you could go to the glass tips blog below and find a number of pieces of information that may help.

    Stephen Richard

    blogs at: and


    I have a microwave kiln also, but mine is the larger microkiln.  I’m wondering if you have some metal on the bottom-inside of the kiln, maybe imbedded in the base?  I had the same thing happen to me when I put a brass filigree between glass layers (duh, I know).  Within 10-20 seconds of starting the microwave I had a mini lightening bolt in the mic kiln which scorched the base, and melted a dime size area of the glass.  It made a chinking sound which prompted me to stop the microwave and check the glass.

    Hope this helps.



    It is not unusual for microwave kilns to change color so I wouldn’t worry about that too much.

    What I think happened is that the glass heated too quickly and the top piece moved off and broke. When you fuse two pieces together you must increase the heat very gradually until they are fully fused. This is not easy to control in a microwave kiln and even though your other pieces were successful each new piece can vary.  Whether a piece has been fused before makes no difference.

    The big problem with microwave kilns (I have one, too) is that you cannot control the temperature which is essential for successful fusing.  I use mine to test glass color or to make glass dots, but my serious fusing is done in larger kilns.

    If you are serious about making fused glass jewelry, you might want to consider a small table top kiln which costs under $500.





    I have not used a microwave kiln, so my offerings are based on my understanding of glass as a material (composition, physics, etc.), my fusing knowledge and experience, and my experience with microwave ovens.

    As others said, you really have no way of knowing what temp your kiln is at at any point in the process – a real problem for troubleshooting, understanding how glass behaves, or having much control over the process.  That said, we do exercise some control over the temperature and rate of heating in a microwave every time we use the defrost function, or key in less than full power.

    Glass, even small pieces, do have a couple of really touchy temperature points.  One is around 350F – if you heat too rapidly up to and through this temp, you can get breakage from thermal shock.  The second is the strain point – at about 850F for COE 90 glass – what the Fuseworks products are and their kilns are designed to use.  Opening your kiln around this point is a good way to get broken glass and, when cooling the glass, to have bad stresses put into it if it doesn’t break.  This is why all the basic fusing books tell you, when cooling down and annealing your glass, not to open the kiln from about 1000 – 1100 F and below, until room temp.  You can cheat on this a bit, but it really is not a good idea to open the kiln until it is at most, about 200F.

    You do need to make sure your kiln is very clean.  You could try heating at about 50% power or on defrost for a portion of the time, the quickly shifting to full power for the time necessary to fuse the piece.  If even the cost of a new small kiln is too much, keep your eyes on ebay (where I got my first one) for a real kiln, and have fun!


    Fused Glass, Wood, Beads, PMC, Ceramics

    Usable and Wearable Art

    Art for the Sake of Art


    When I first got my microwave kiln at home I wanted to add some decals to some cabs I had fired in my kiln, they all went ka-boom.  After I stopped being mad I looked back at what went wrong and I agree with Piranga, I heated the glass to fast, so I went back and tried a couple of things, I put the piece of glass in the microwave ( no kiln) for 30 seconds on high and then when it was warm to the touch I put it in the kiln and used the defrost setting for 30 seconds and then the full power, no exposions. I also heated the piece with my heat tool until it was very warm and then put it in the kiln, again no explosions.

    Try experimenting with the pieces that broke if you still have them.

    One of my student has gotten really good at doing textured pieces in the microwwave, so anything can be done, it just take time and experimentation.

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