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We recently had a customer bring some “funny” looking fired Orton Pyrometic cones back to Stone Leaf Pottery, confused about what had happened during his firing.
Never having seen cones in this shape after pulling them out of the kiln, we immediately sent an email to the Orton Ceramic Foundation to find out what the heck was happening.
We feel cones are a vital part of the firing of each kiln, and consistently use cone packs, even in our digitally controlled kilns. Using cones allows the potter to monitor the thermal process on each shelf, area, or zone of the kiln.
In response to our “whaaaaaat?” email, we got this explanation from ceramic engineer Thomas McInnerney at the Orton Ceramic Foundation.
What you are seeing is bloating. This can be caused by any or all of the following conditions within the kiln:
1- Flame impingement (having the cones in direct line with a flame or the gases coming from a flame)
2- A fast initial firing rate (the first 500 degrees of the firing is when byproducts of combustion are burned out of the cones, if the temperature rises too fast, the reactions do not have time to complete, trapping them inside the cones to come out at higher temperatures)
3- A highly reducing atmosphere within the kiln (Due to overloading the kiln, out of balance gas to air ratio at the burner, or increasing the temperature too rapidly).
The regular cone series (cone 010 – cone 3) are red in color due to the inclusion of red iron oxide. These cones are more prone to bloating when exposed to firings that go into reduction, like Raku. So, to counter this, we produce an iron-free series that does not contain the Red iron oxide. The iron free SSK-06 should perform better in their Raku firing.
If iron free cones would be more suitable to your firing needs, please ask us to identify those boxes on our shelf for you. We’ll be stocking a limited selection of temperatures from now on.
We learn something new here every day at Stone Leaf Pottery. At Stone Leaf, we teach classes and sell supplies; using what we sell every day increases our education level about every product in our store. When something goes wrong in your studio, we’ll work together to solve the problem. We’ve been serving the Colorado Clay Community for 15 years, and have decades of experience in clay. Our strong relationships with our manufacturers help both us and you create quality pottery every day.
If you’ve had an “odd” experience with any product we sell, shoot us an email. We’d love to figure out the problem and write a post about it.