If you have invested in a new ceramic kiln (new for you, 2nd hand is of course also possible), you must first get to know him (or her). Just like a new friend, you have to discover each other and slowly grow closer to together. Then something beautiful can blossom between you and your new kiln or soulmate!
Building strong relationships between people can take a long time. But with a new ceramic kiln it can be done much faster. The difference? With a ceramic kiln you can set and measure everything.
So first calibrate and compare is my motto, but how do you do that?
New versus old kiln: calibrate and compare
In my previous blog about the investment in a new small test kiln, I often got the reaction that glazes look very different in a small kiln in comparison to the other (larger). And indeed, that will often be the case.
Why is that? Because different kilns have different firing profiles. That is, although both kilns are fired at the same temperature according to the kiln controller, that does not mean that both kilns fire the glaze in the same way…
Let me explain this using Orton cones or pyrometric cones as they are called in non-brand English…
Pyrometric cones
Seger- (after the inventor), Orton- (after the contemporary manufacturer) or just cones for short, are small triangular rods with a certain number. It is a glaze pressed into a cone shape. These cones sinter and melt at a specific temperature and duration.
The difference with a glaze is that due to the specific shape, the cones bend, before they melt completely. This way you can see exactly during firing (or afterwards) which cone temperature has been reached.
These pyrometric cones undergo a chemical reaction (sintering) under the influence of time and temperature. Just like ceramic glazes, this reaction also needs time in addition to energy (heat).
Just like baking a cake (approx. 200 C), you not only set the (kitchen) oven temperature, but also a kitchen timer (approx. 50 minutes). So cones are temperature and kitchen timer in one!
Especially in the last 100 degrees (C) up to top temperature, the speed of firing is important. In that time frame, glazes begin to sinter and melt.
PTCR rings
“Positive Temperature Coefficient of Resistance” or PTCR rings for short are a great modern alternative to cones. They work on the same principle as cones, but instead of being able to see if they are bended or not, you measure the diameter after firing. On the basis of the supplied temperature-diameter-table you can then see exactly at which “ring temperature” you have fired.
With the help of the ring (or cone) temperature, you can then compare both kilns. And what are the results? That despite the fact that both kilns are set at the same temperature in the kiln controller, the “actual” glaze temperature can be very different.
This difference can be small, but also large. Between my two Rohde kilns there was a temperature difference of about 10 C. But between my twenty+ year old kiln and my new one the difference was more than 40 degrees C! If you don’t compensate for that in your kiln controller, glazes really come out very differently.
And I haven’t even mentioned the temperature difference that can occur in the kiln itself.
Same firing curve; controlled cooling
So if we have got the cone or ring temperature on both kilns at the same level after a few firings, we are done: calibrate and compare is complete, right? No, unfortunately not quite…
In addition to the firing rate up to top temperature, cooling is also important. Down to about 900 C, the glaze can be liquid enough to form small or large crystals. This is not the case in all glazes, but glazes with titanium or rutile, for example, will look very different during a slow or fast cooling period.
Small kilns will usually cool down faster than large ones. But this depends on the insulation value and the kiln load. A heavily packed kiln retains heat longer than a nearly empty one…
With some kiln controllers, in addition to the firing temperature, you can also set the cooling rate. For example, the Bentrup TC507 and TC707 have that capability. This way you can synchronize the firing curves of both kilns.
Your new ceramic kiln, your new friend
- Calibrate the top temperature with cones or PTCR rings
- Check any hot and cold spots in the kiln
- Use the same firing programs
- Synchronize the cooling rate of both kilns
In short, before you start using your new (test kiln): first calibrate and compare based on the old one. Make the firing profile as similar as possible, including the cooling rate. This way you will not get glaze surprises when firing your new kiln!
And when you’ve got everything set up and aligned, you can finally enjoy your new love… euh.. I mean kiln…
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