Thursday, September 13, 2012

Electric Kiln Ideas

Downdraft ventilation protects connections from corrosion

I have a top-loading Cone Art electric kiln that is nearly 20 years old. It is equipped with a Perfect Fire programmable controller.

In my former studio, I used a downdraft ventilation system. The kiln was fired zillions of times to cone 9 in that studio. The elements had to be changed a couple of times when firing times got unacceptably long. However, during that period, the inside of the control box remained pretty clean and I never experienced failure of an electrical connection caused by corrosion.

In my current studio, I got lazy. Since the kiln is in a small room with windows that permit a cross-draft, instead of hooking up a downdraft vent system, I've been venting the room by opening the windows and putting a fan in one of the windows. After less than 30 firings, two elements failed due to corrosion at the connectors. Rust seems to collect on the floor, falling from the inner surface of the stainless steel kiln sheath. There is corrosive dust coating everything inside the control box.

Here is what I think must be going on.... When you have a downdraft ventilation system, you are sucking gases at a slow rate from inside the kiln and expelling them outside the building. Anyone who has used a vent like this with metal ducts soon learns that kiln fumes are highly corrosive to metal. Where do these corrosive fumes go if you are not sucking them out through your duct-work? Some fumes come out the top. Some penetrate right through the porous brick and corrode the inside surface of the metal sheath of the kiln. And some enter the control box and corrode the connections.

When you use a downdraft vent system, the gases that are sucked out of the kiln have to be replaced by fresh room air sucked into the kiln. Assuming your peep holes are closed most of the time, the only other way for make-up air to enter the kiln is from the top and through numerous holes at the back of the control box. At the back of the control box, there are two holes for each element and another hole for the thermocouple. When your downdraft vent fan is running, there is a small flow of clean room air that passes through the control box and into the kiln. This inward direction of air flow prevents corrosive fumes from entering the control box.

After I clean the corrosive dust out of the control box and clean up my wires and connectors, and rewire the kiln (see next section), I will install a downdraft vent.

Simplify the wiring

I haven't worked on many different electric kilns. My guess is that there probably are a lot of electric kilns like mine that have goofy wiring. The goofiness probably resulted from the kiln originally being designed to work with switches and later retrofitted with a programmable controller.

The programmable controller on my kiln operates 4 different relays. When the controller sends a small, 12-volt current through one of the relays, a 2-pole switch in the relay closes a circuit that passes a 240-volt current of up to 25 amps. This amp rating is enough to power two 10-amp kiln elements.  Each of my 4 relays could simply be wired directly to two kiln elements. That would be simple and logical and make it easy to track down the cause of any elements failing to work properly.

What I actually have is a goofy setup. There are 3 needless switches. The power coming from relay #3 powers elements 5 and 6 directly. Power from relay #4 powers element 7 directly but sends power to element 8 through the bottom switch. Relay #1 sends power to elements 1 and 2 through the top switch. Relay #2 sends power to elements 3 and 4 through the middle switch.

My middle switch got so corroded that it no longer passes current. My first thought was that I need to replace it. I found that I could buy similar infinite switches on ebay for around $10 or from a kiln supplier for $35 - $45, but as I studied the wiring and how these switches work, ... I realized that current simply passes through them. The programmable controller regulates the kiln. The switches regulate nothing.

These switches (infinite switches) are kept at their "high" setting at all times, meaning that current simply passes through the switch and is never interrupted by the switch. The switches have nothing at all to do with regulating power to the kiln elements. There is no reason why power should not pass directly from the relays to the elements, bypassing the switches.

Here is what I plan to do.... I will rewire the kiln such that each of the 4 relays will power 2 elements directly. I will remove the switches. If two elements powered by the same relay get stuck on or off, I will immediately know which relay is not working properly.

Smoking in an electric kiln

Last winter I enjoyed making small pieces, decorating with terra sig, and firing and smoking pots in a wood stove. I'm now thinking of doing similar but larger pieces in an electric kiln.

I know that there are a lot of people who have tried enclosing their pots with combustibles within aluminum foil "saggars". Others have used ceramic saggars. Others have pulled pots with tongs and placed them into closed containers with combustibles. I think there is an assumption behind all these methods that smoke is bad for electric kiln elements. 

However, I've seen a video showing Michael Wisner using combustibles right on the shelf next to a pot in an electric kiln. In other words, the whole kiln can be the saggar. How bad is that for the elements?

I'm thinking that if  you can fire a kiln full of corrosive fumes to cone 9 more than 100 times before you need to replace the elements, electric kiln elements must be pretty darn tough. You can do your smoking at a fairly low temperature - say 500 degrees F.  It's hard for me to imagine how low temperature smoke could be more harmful than those high temperature corrosive fumes in a cone 9 firing.

Anyway, I've had experience coiling my own elements and replacing kiln elements ... and it's not all that expensive or difficult.

What I'm think of doing is this.... I will fire earthenware pieces decorated with polished terra sig to cone 05. Then, I will do a a smoke firing in the kiln to 500 degrees for just a few minutes with combustibles right in the kiln. I think I will be able to do this many, many times on one set of elements. I'll let you know....