I've been doing this kiln forming thing for almost 15 years now, so you'd think that I'd have it ingrained in my brain when I need heat from the top and when I need heat from the side. Recently I was attempting a new project that was 3" x 3" thick by 6" long. It was dammed with clay dams and then heavily bricked to keep the dams in place. I concentrated hard on making sure the firing schedule was pristine and the annealing was generous and smart. i never even considered the heat work issues. I was blissfully ignorant!
The resulting piece came out of the kiln over fired on the top and under fired near the shelf. It had small champagne bubbles near the surface that I didn't like. I could see that the bottom layers just weren't fully fused and lacked clarity, but all the air was squeezed out perfectly. I fused the piece in my "go-to" kiln which is a top fire. I didn't think the heat work would matter, but it did! I made some calls and did some research to figure it all out! Color me embarrassed! It was proven to me that we can't rely on just instinct; we really do need to think about what we need... and why.
So here's the skinny on that.....
You need heat from the top when you are doing thinner work. This means anything below 9mm or less than 1/2". This is basically shallow fusing, tack fusing, slumping, draping, combing/raking, and boiling. The heat needs to penetrate through the glass from the top down through the shelf or mold. It is important that the heat is uniform across the entire area of glass and that it is not too close or too far from the elements. Most fiber kilns are top firing, but there are exceptions. (I new all of this, but just wasn't thinking about it because, Uh, well, I really love my fiber kiln, and I put almost everything I can in it, but alas...it's not perfect for every job.)
You need heat from the sides when you are working thicker than 9mm or over 1/2" as a general rule. This means deep thick fusing, casting, annealing, basically doing thick or deep work of any kind. Traditional ceramic kilns are side firing only. They can be used for glass, but are better suited for casting and thicker work. (I knew this too! I have a brick kiln and should have realized this was what I needed. Fusing habits need to be tempered with thinking....note to self!)
Why You'd Want Both:
When a kiln has both top and side fire elements, the energy flow is usually divided between the two heat sources. For example, Evenheat's GTS 2541-13 distributes the heat approximately 70% to the top and 30% on the sides. This allows for very even heating allowing a wide range of activities to be successfully executed. A front loading kiln, like Paragon's GL-24ADTSD, has the same type of divided heat. Some manufacturers even allow you to control which elements you use for which activities. A popular example of this is the Olympic 186GFE, which has a toggle switch that allows you to change from top to side elements in the middle of a firing. That's convenient! Another good option is a hybrid kiln, like Jen-Ken's JK22-cerama-glass, that fires glass and ceramics. One of these could be the perfect fit for just about every project. They're deeper than traditional glass kilns, and also allow you to switch "on the fly" from top to side fire, and then back again. (I've actually got a hybrid kiln, that I don't use half as much as I should. By now you've probably noticed that I love my kilns and have a few of them. Like potato chips, one is just not enough!)
Just like every artist is different, every kiln is different too. Each kiln design, either top fire, side fire, or a combo firing is configured to perform certain processes with ease and other processes are a just a bit of a stretch. You kiln will cheerfully attempt every job, but just might not be the best "soldier" for the job. After my recent project had a few technical errors, I called my friendly kiln engineer and asked for some heat work advice. Here are my conclusions, and the final recommendations.
- If you are fusing and slumping as a general rule.............................top firing is great!
- If you are casting into molds, using billet, or stacking thick..........side firing is great!
- If you are working thicker, using bricks & dams, doing pot melts, thicker bars, boils, and deep drops and deep molds...............................................................combo firing is great!
Drum Roll! The Final Recommendations:
- If you are getting your first small kiln (under 14" and less than 6" deep) don't worry about this too much. You're going to get pretty even heating throughout the entire kiln chamber just based on the smaller volume of space that needs to be heated.
- If you are going bigger, you really need to to think about the type of work you plan on doing with the kiln. If you want to "do it all" make sure you have a kiln that gives you options. Get something with top and side firing capabilities.
- If you are a crazy control freak that wants to shift back and forth between top and side elements during a firing, contact us at KilnFrog and we'll get you a custom switch to do that!
Your Kiln Gal - Gail
(Epilogue..... I'm re-doing this project and firing it in my hybrid kiln! Stay tuned!)