Getting hot and cold by the minute

I’m sure you’ll have seen diagrams that describe what a kiln schedule is trying to achieve. Nice straight lines going up and down. Nice straight lines when a target temperature is being held. It all looks so tidy and logical.

But is the reality of a kiln schedule really so simple and straightforward? What exactly does AFAP (as fast as possible) really mean?

I decided to investigate how my kiln behaves more closely.

Recording the internal temperature was easy because there’s a thermocouple inside the kiln and the kiln controller uses it to display the air temperature within the kiln. The top of my kiln gets hotter than the sides so I also recorded the temperature on the top of the kiln on the outside using a multimeter that could measure temperature.

The purpose of the kiln schedule you see below was to look at how fast the kiln could heat up and how fast it could cool down. I was also curious about whether or not the kiln controller overshot or undershot target temperatures and whether or not it could perform controlled cooling effectively.

Step  Ramp(°C/h)  Target(°C)  Hold
1     AFAP        820         0:20
2     AFAP        516         0:20
3     120         371         0:05
4     END

Much as you’ll see in diagrams of kiln schedules, you may think that this is what will happen:

  1. the kiln heats up from room temperature at a constant rate up to 820°C
  2. the temperature holds steady at exactly 820°C for 20 minutes
  3. the kiln cools down at a constant rate until 516°C is reached
  4. the temperature holds steady at exactly 516°C for 20 minutes
  5. the kiln controller allows the kiln to slowly cool at 2°C per minute down to 371°C
  6. the temperature holds for 5 minutes at exactly 371°C, and then finally
  7. the kiln cools down at a constant rate back down to room temperature.

Surely, the only questions to be answered are how fast is AFAP when heating up and how fast is AFAP when cooling down. Oh, and what about the external temperature? Let’s find out…

I sat in front of my kiln taking internal and external temperature readings every minute for a couple of hours, then every two minutes and then less frequently as the kiln got cooler. What you see below is the graph I produced from the information I recorded. Compare it with the kiln schedule. It’s not so simple as you might expect unless you know your physics.


The blue line is the internal temperature curve. The purple line is the external temperature curve. All very pretty, but what have I learned?

To begin with, I see the kiln heats up very quickly to begin with, but slows down as the temperature rises. It’s getting harder for the kiln to get hotter because heat is being dissipated into the kiln insulation materials (eg kiln bricks). I also see that the cooling curve coming down from the process temperature all the way to room temperature is a similar but reversed curve that is very much shallower – those kiln bricks certainly know how to hold in the heat. But, more to the point, it is clear that AFAP means one thing for heating and another for cooling – their rates are different.

Though it’s not so easy to see from the graph, I can see from my notes that the kiln temperature slightly overshoots the 820°C target temperate by a couple of degrees and later on it undershoots the 516°C target temperature by a degree or so. Not a problem really because the kiln controller’s doing a far better job than I could manually and it’s a perfectly normal quirk of a kiln being controlled by a kiln controller. It’s not a problem because the thermocouple is measuring the air temperature and not the temperature of the glass the kiln.

Something else that you can’t see very well from the graph, but I can from my notes, is that the controlled cooling of step 3 (at 2°C per minute) is similar to the natural cooling rate of the kiln. This has an interesting consequence. When my kiln is at more than about 440°C the kiln controller is able to control the cooling at exactly 2°C per minute because the natural rate of cooling for the kiln is more than 2°C – the kiln controller just provides an occasional pulse of heat when needed. But, when the kiln is below about 440°C, the natural rate of cooling for the kiln is less than 2°C per minute so the kiln controller is not able to make the kiln cool faster. So, for this particular kiln schedule and my particular kiln, it’s barely necessary to bother with step 3 because the kiln’s naturally doing exactly the same thing as the kiln controller wants to do. So, for example, asking the kiln controller for a 3°C per minute really would be pointless. I will have to bear this in mind for the future.

Another notable aspect of the kiln temperature curve is that the rate of cooling gets slower and slower as it cools down. It is similar to the heating curve, where the rate of heating gets slower and slower as it heats up. This is all to be expected because heat losses depend on the difference in temperature inside and outside – exactly the same as with your home in winter months when it’s colder outside (or in summer when it’s warmer outside).

And finally, a few comments about the temperature outside the kiln. In addition to noticing that the outside of the kiln reaches no more than 90°C (so is unlikely to set fire to anything) it is also interesting to see that this maximum temperature is not reached until about 40 minutes later. With worse insulation this would be hotter and quicker.

So, how does your kiln behave?


About chatterglass

Maker of stained glass frippery.
This entry was posted in Experiment, kiln controller, kiln schedule, temperature curve and tagged , , , , , . Bookmark the permalink.

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