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I've heard the tales of terror - relays stuck, kiln super heated, shelves melted and eventually all melted right through the bottom of the kiln. "Never leave the kiln on if you are not there." hmm.
Kiln manufacturers say the elements will give up and cook in half well before that.
I've searched high/low on the 'Net and can't find a picture of one of these "China Syndrome"(movie) types of fiery conflagrations. Only pictures of melted pottery or glass - some badly warped shelves.

I don't have the luxury of staying home while my kilns are running. In fact, I consider totally cool that I can pull stuff out while the coffee is brewing, put something else in, jump in the car and go to the day job and pull items out after dinner or sooner(depending on process). So I rigged a system to shut off the kilns if they go too high. It works - might seem mildly jerry-rigged to some, but it is IMO very sound, and makes matters no worse. With no changes to the kiln or it's electronics. Costs about $50-60 per kiln.

I'll share if there is interest.

In over 20 years of working with kilns, I have NEVER seen or heard of a documented instance of a kiln "burning to the ground." It's virtually impossible.

There are far better things to worry about than a rogue kiln.

In my 40 years of playing with glass.....
I've seen glass melted half way through a kiln brick floor
The stainless steel mold melted.
I was told the elements were still fine and working.

Always put a kiln on a fireproof surface.
A thick fireproof surface.
Well away from combustibles.


Another time I was doing a long firing with elevated temperatures being carefully held.
I had actually physically opened up and checked the mechanical relay (with the power off of course) prior to starting the firing.
Everything was fine.
It wasn't.

The rapid cycling of the contactor caused it to overheat and the contactor contacts arc welded themselves together in the on position.
I had a mercury relay set to be pretty much wire and go in its own new electrical box. The kiln dropped temperature but I was able to save the firing by installing the new relay.
I no longer use mechanical relays.
If your relays are cycling on and off more than once a minute because of the type of firing you are doing, more than likely you are exceeding the capabilities of that mechanical relay

Kevin Midgley wrote:In my 40 years of playing with glass.....
I've seen glass melted half way through a kiln brick floor
The stainless steel mold melted.
I was told the elements were still fine and working.

Always put a kiln on a fireproof surface.
A thick fireproof surface.
Well away from combustibles.


Another time I was doing a long firing with elevated temperatures being carefully held.
I had actually physically opened up and checked the mechanical relay (with the power off of course) prior to starting the firing.
Everything was fine.
It wasn't.

The rapid cycling of the contactor caused it to overheat and the contactor contacts arc welded themselves together in the on position.
I had a mercury relay set to be pretty much wire and go in its own new electrical box. The kiln dropped temperature but I was able to save the firing by installing the new relay.
I no longer use mechanical relays.
If your relays are cycling on and off more than once a minute because of the type of firing you are doing, more than likely you are exceeding the capabilities of that mechanical relay

Kevin - since you are believer in the possibility, I'll tell my "jerry rig" solution. Cheap digital pyrometer(DP) that comes with a solid state relay that hooks up to the output of said DP. Load side of relay, which is normally OPEN is hooked to hot of the circuit and ground (each of my kilns is on separate GFI breaker protected circuit). Each of my kilns sits on 6"" of Insoblock (1900f) insulation board, but that is really just to raise them up. Probe of DP is between kiln bottom and Insoboard. I have dialed each one of the DPs in so if it ever goes over the temperature that results from casting, the relay closes (creating a nice short ), the GFI/breaker shuts off the juice to the entire circuit in a few hundredths of a second. If it doesn't work - nothing is made worse, there is no electrical danger. Requires zero changes to the kiln and costs $50 - ish and maybe an hour of install and dial-in time done while actually taking kiln toward your upper target temps.

P.S. There was no way I could meet all the requirements of my premises insurer so this was my next best gambit. I sleep better at night and I "cook" whenever I want.

Yeah, I detest the sound of the relays. But from what I read, statistically speaking Sold State relays(SSR) generally fail closed circuit too. Mercurys fail, statistically, Open if a normally open(NO) type. The mercury relays generally need more coil voltage than what a digital kiln controller is putting out to a mechanical relay, but that issue can be easily bridged with an appropriate SSR(attached to kiln controller) controlling the load for the separate transformer that will/can power the mercury relay. I have spare relays here but next one to have issue will probably get the mercury upgrade.

suggestion I would offer is to place a shallow metal baking pan beneath the box containing the vertically positioned mercury relay. Should one ever fail the mercury clean up is simpler.

Kevin Midgley wrote:suggestion I would offer is to place a shallow metal baking pan beneath the box containing the vertically positioned mercury relay. Should one ever fail the mercury clean up is simpler.

Very good suggestion Kevin. Don't know if they leak upon death, but I will consider that. I've dealt with mercury before -yeah, that crap just smears around and refuses to be picked up. Ah, I see from the 'Net that mercury will bind to sulfur readily, forming granules that are much easier to deal with.

Back to relays - Mrs. Kaiser of Kaiser-Lee says she hasn't had a relay die in 17 years in one of her books. Mine have died about every 2-3 years or about every 300-500 "cooks". Mercifully in the open position so far Some people may have issues because their kiln is in a slightly cold location(under 60 f.) and they walk away after they hear the first click. Which might be the one where it gets stuck on. Don't know - haven't experienced it myself. I made arrangements to not get burned.

You might want to read this post on relays from the archives
http://www.warmglass.com/phpBB3/viewtop ... ay#p353553

Tony Smith wrote:You might want to read this post on relays from the archives
http://www.warmglass.com/phpBB3/viewtop ... ay#p353553

Thanks Tony! Now I'm thinking I'll go to SSRs and my backup system. *Was slightly surprised nobody kicked me for my idea of having a backup to just shut off the darn power at the source.