How to Calibrate Your Kiln Sitter for Accurate Firings

How the Kiln Sitter Works

If you’re anything like me, then your first kiln wasn’t or isn’t going to be the digital programmable kind. Many of us start out with a manual kiln that we got second hand and then one day, we may have a chance to upgrade.

Full disclosure, I have 4 different sized kilns in my home studio and none of them are digital or programmable. I operate digital kilns in the studio where I work, but at home, I’m still rockin’ the manual “Kiln Sitter” kilns.

My family of kilns

Manual kilns have switches for turning up the heat and they have a nifty contraption called a “Kiln Sitter” that uses a melting/bending cone to shut the kiln off at a specific temperature.

The kiln sitter uses a small, pyrometric “sitter cone” that’s propped onto 2 “cone supports” inside the kiln with a “sensing rod” laying over the centre. The cones are made by Orton who also makes small, rectangular bars for this purpose. I haven’t personally used the bars, but they are meant to be more accurate because they aren’t tapered.

sitter cone

The sensing rod goes from the inside of the kiln to the outside. On the outside, at the other end of the sensing rod there’s a “claw” holding a “trigger plate” that’s attached to a “weight”. These external parts create a latching mechanism. When the latch is up and being held by the claw, the kiln can be turned on.

kiln sitter latch

As the kiln approaches its target temperature, the cone in the kiln sitter will soften. As the cone softens, the weight of the sensing rod causes it to bend.

sitter cone

The sensing rod sinks down into the cone, causing the claw on the outside to rise. When the claw rises up it releases the latch weight, which falls down and shuts the kiln off.

It’s kind of genius, really.

Kiln Sitter Maintenance

Just like any piece of equipment, the kiln sitter requires a bit of maintenance. Not a lot, but if your witness cones are telling you that your kiln is shutting off early or late, then there’s a good chance your kiln sitter needs some attention.

Most kiln sitter parts never need to be replaced unless they’re broken or missing.

The main parts that generally need to be replaced after a lot of firings are the metal parts on the inside of the kiln. This includes the “sensing rod” and the “cone supports”.

Because these parts are metal and are subject to such high heat, they become oxidized and thin, or warped and worn out.

The porcelain “tube assembly” that the sensing rod passes through can get knocked by kiln shelves so if it becomes cracked or broken, it will need to be replaced. If you take good care of it, you may never have to replace it.

tube assembly

To replace the sensing rod and the tube assembly yourself, my advice is to take lots of notes and photos as you remove the old ones and reverse the steps to install the new ones.

Here is a great diagram from theceramicshop.com of all the kiln sitter parts in case you need to replace some of them.

Aside from replacing parts, the most common thing you’ll need to do to maintain your kiln sitter is to calibrate it so it shuts off at the right time. And that’s what I’m going to show you how to do in this post.

How to Know if Your Kiln Sitter Needs Calibrating

The kiln sitter is designed so that when the sitter cone bends to a 90° angle, the kiln shuts off. And when the kiln shuts off, the witness cone of the same number should be fully bent as well.

On the Orton small cone box, it says:

If your sitter cone is bent to more or less than a 90° angle, then your sitter probably needs calibrating. Make sure you’re always placing the sitter cone so the sensing rod sits in the centre of the cone for accurate results. If the rod is sitting on the thinner side of the cone, the kiln will shut off early. If the rod is sitting on the thicker side, the kiln will shut off late.

If you’re using the rectangular bars, then there isn’t a thin or thick side. Just make sure the rod is placed over the centre.

When you put witness cones in your cone 6 firing, (which you absolutely should be doing) the tip of cone 6 should be bent over and pointing down towards the kiln shelf at the same time that the cone 6 sitter cone bends enough to release the trigger and weight to shut the kiln off.

This system works great as long as your sitter is calibrated properly and your sensing rod hasn’t worn out.

If your witness cones (on the shelf at the level of your kiln sitter) don’t look perfect when your kiln shuts off, your sitter needs to be adjusted. This image shows how I like my cones to look.

Any time I get a new kiln or am performing any maintenance on a kiln, or if my firings aren’t completely perfect, the first thing I’ll do is re-calibrate my sitter. It only takes a couple minutes.

What You Need to Calibrate Your Kiln Sitter

For the calibration process, you need a little round device called a “firing gauge”. You should be able to get one at any ceramic supplier.

You also need a teeny tiny screwdriver for the tiny set screw that holds the trigger plate in place on the latching mechanism.

My kilns need a tiny flathead (-) screwdriver but yours may be a Phillips (+). Have a look at the set screw to find out.

And the last thing you need is a pencil to mark where your trigger plate started.

The 3 things you need to calibrate your kiln sitter:

  1. Firing gauge
  2. Tiny screwdriver
  3. Pencil

How to Calibrate your Kiln Sitter

1. Slide the firing gauge over your sensing rod and cone supports. The sensing rod goes through the hole in the center and the cone supports go through the grooves in the side. This places the sensing rod at the same angle that it would be if the small sitter cone was bent perfectly, at a 90° angle. When the rod is at this angle, the latch should be just ready to fall.

     

An example of 2 different firing gauges. The one on the left is newer and the one on the right is older.

2. With the firing gauge in place, swing the latch weight upwards and see where the top of the trigger plate comes in relation to the bottom of the claw. Is there a gap between the bottom of the claw and the top of the trigger plate? Does the plate move snugly past the claw? Or does the claw block the plate from moving past?

What you want is for the trigger plate to come into contact with the claw, with no gap in between. It shouldn’t be so high that it can’t move past the claw, nor should it be so low that it moves by without touching the claw.

They should brush against each other. If that’s the case, then your kiln is calibrated. If the trigger plate is too high or too low, then it needs to be adjusted.

3. I always mark the trigger plate with a pencil before I make any adjustments so I know what level it started at. Draw a line across the trigger plate, where it meets the latch weight that’s holding it.

4. Use your tiny screwdriver to loosen the screw just enough that the trigger plate will slide up and down. Adjust the plate so that it’s just touching the bottom of the claw and then tighten the screw.

And that’s it! That’s all it takes to calibrate your kiln sitter.

What if Your Calibrated Kiln Isn’t Shutting Off at the Right Time?

If you’ve followed the steps above for calibrating your sitter and your kiln is still not firing accurately, it could be because you have a worn-out sensing rod.

When the sensing rod is heated and cooled it will oxidize and the outer layer flakes off, just like any piece of rusting metal. You may notice little green/black flecks on your kiln shelves after a firing.

As this happens over and over, eventually the sensing rod will become much thinner than it started. It may even wear down to a point. This can affect the timing of when your kiln shuts off.

If your sensing rod is worn down to a point, then it’s not the same size as when it started out so the rod won’t sit at the same angle.

       

Here are the sensing rods in 3 of my kilns. The one on the left is fairly new. The one on the right has been used the most. You can see how it’s starting to disintegrate and wear thin. It still has a fair bit of life in it. I’ve seen much worse.

Since the rod rests on the cone on the inside of the kiln, a thinner point will have it sitting a bit lower. Since it’s on a fulcrum like a teeter-totter, this means the claw on the outside of the kiln will sit a bit higher. One end goes down, the other end goes up.

If the claw starts out higher, it doesn’t have to move as much before it releases the latch to shut the kiln off. This results in an under-fired kiln load.

Here’s a graphic to show what I mean:

If your sensing rod is worn out like this, I recommend ordering a new sensing rod for your kiln. And in the meantime, you can follow the steps below to adjust your sitter so it shuts off later.

How to Adjust Your Sitter So it Shuts Off Earlier or Later

If your calibrated kiln sitter is consistently over- or under-firing then you can override the calibration process by sliding the plate further up or down, depending on whether you want the kiln to shut off earlier or later.

If you want the kiln to shut off earlier, slide the trigger plate down.

If you want the kiln to stay on longer, slide the trigger plate up.

You’ll have to experiment with how much you slide it up or down. This is where marking the plate with a pencil first will guide you. If you accidentally move the plate too much, you can bring it back to where it started and then make sure to slide it only a little bit.

If your kiln’s over- or under-firing just a little bit, like less than a cone, I would only move the trigger plate by a hair or two.

If it’s a full cone over or under, you may need to move it a millimetre or two. I would start with less and increase a bit after a test firing if needed.

That’s also what the pencil marks are for. So you can move a little at a time and see how much that adjustment affected when the kiln shuts off. You’ll usually have a better idea of how much to move it the second time.

If you’re present for the end of the firing and watching your witness cones through the peep, you can always shut your kiln off yourself if the sitter doesn’t fall on time or turn your kiln back on if it shuts off before your cones have melted.

Important Safety Announcement (Soapbox Time):

When making these extra adjustments, you’re guessing how far to slide the trigger plate up or down. It’s very easy to move it too much, which could mean your kiln won’t shut off at the right time.

I very strongly recommend being present for the end of your firing to watch your cones fall and make sure your sitter doesn’t shut the kiln off too early or late.

I actually VERY highly recommend being present for the end of EVERY firing. The kiln sitter is NOT a fail-safe method for shutting your kiln off.

I’ve had a sitter cone get stuck to the cone supports and the kiln just kept on firing. Luckily I was there watching my cones melt so I shut the kiln off myself. I had to force the claw up to allow the latch to fall.

When I opened the kiln the next day, I saw that the cone had split in half and one half had stuck to the cone support with the sensing rod stuck right to it. Who knows how long the kiln would have fired for?

I’ve also had my sensing rod stick to the top of the tube assembly. I didn’t realize this was happening for several firings. When I pushed the rod up from the inside of the kiln, it didn’t fall back down. It just stayed stuck up there.

The tube had filled with metal flakes from being fired so many times and shedding so many layers. It seemed like the metal flakes had a magnetic effect on the rod that was causing it to stick wherever I pushed it.

So even if the cone was melting, the sensing rod wasn’t actually resting on the cone. It was stuck to the top so the kiln would never have shut off if I wasn’t there watching.

I sucked the metal flakes out of the tube with a vacuum and the rod was able to fall freely again, no longer sticking to the top. Now I test before each firing, pushing the sensing rod up to make sure it falls back down again.

Better safe than sorry

Since it’s so easy for something to go wrong (and I’ve been badly burned in a fire!) I never walk away from a kiln and trust it to shut off on its own. How many times have you heard of a pottery studio burning down? Once is too many. I’m always there to make sure the sitter falls and the kiln shuts off. I may go out for a few hours but I’m always there at the end.

Even if your kiln has a timer to shut it off, I don’t trust mechanical devices like this to work every single time. Call me paranoid or overly cautious, I don’t mind.

I admit that it’s highly unlikely that you’re kiln’s going to burst into flames. I’m not trying to scare anyone. But there’s still the risk of your entire kiln load being over-fired, causing your clay to bloat and your glazes to run. I’m just not willing to risk ruining my kiln load of pots that I’ve worked so hard on.

I want my cones to be perfect when my kiln shuts off and the only way to ensure that is to be there watching the cones melt at the end of the firing.

Sometimes it would be late and my kiln would be taking forever and I’d have to go to bed. I would set an alarm for every half hour and go out to check the kiln. It’s annoying, just ask my boyfriend. But it’s important!

The life of a potter, right?

Tips for monitoring those late night firings

Eventually, I got a baby monitor so I can hear the sitter fall from inside the house. It works great!

And to take it a step further, I now have a set-up where I use a baby monitor app that connects two mobile devices over wi-fi. I have a tablet set up in the kiln room that uses its camera to act as a webcam watching the kiln sitter.

The tablet sends the video in real-time to my smartphone so I can actually see (from bed!) when the kiln shuts off. Super fancy!

But for your safety and the safety of everyone around you, please don’t leave your kilns unattended. Especially after adjusting your kiln sitter.

Ok, safety rant over.

I hope you found this post useful and you can now get your kiln firing to the right temperature by calibrating your kiln sitter. If it helped you, please share it with someone who might need to read it.

Do you have any kiln sitter tips to share? Please leave a comment below.

 

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