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Top elements are not required for slumping glass. Therefore, you can do slumping in any electric kiln.
Top elements are desirable in glass fusing, particularly when you get to large pieces (larger than a plate for example). They are not required for fusing jewelry and smaller pieces. In fact, it is possible to fuse any glass without top elements, but much more difficult to avoid cracking.
An electronic controller is essential in firing glass, to get the temperatures exact and control the heating and cooling rates.
We get many questions from people who want to fire glass and ceramics in the same kiln. Following are some of the issues.
Ceramics are fired at hotter temperatures than glass. So if you want to do both, you need a kiln capable of ceramic temperatures (2100-2300 degrees F). (Glass is typically fired at 1400-1600 degrees F.) Most glass kilns are not rated to ceramic temperatures.
Kiln functionality is optimized differently for glass fusing than for ceramics. For glass fusing you want to be able to turn the lid element on full blast during the critical part of the fusing process. So you don't want the lid element tied into the electronic controller. But for ceramic firing you probably would want the lid element tied into the controller, unless you didn't need its additional power to get to your ceramic firing temperature.
The complicated issue with top elements is that over time they will sag. Different manufacturers solve this in different ways.
Most Paragon glass kilns use a different type of element which does not sag, but these elements cannot go above 2000F. So in terms of ceramics, these kiln can only do low fire and china painting.
However, the new line of Janus Kilns from Paragon are made specifically for firing glass and ceramic together. They are Cone 10, plus have a lid element.
Since the "glass" kilns tend to be large and shallow, you don't have much element surface area on the sides of the kilns compared to the volume inside the kiln you are heating. So these are the toughest kilns to get to ceramic temperatures, especially without using the lid element.
Olympic will put a top element in any ceramic kiln for a small additional charge. They cut channels with a lip to keep the elements from sagging. It has a separate switch so you would just leave it off during ceramic firing. This would be the best option for someone who wants to fire ceramic and glass on a regular basis..