After months of diminishing returns with repair of my ancient kiln, I got a new one. I re-fired the few pieces that hadn’t been destroyed in previous kiln-tastrophies. The ones that survived and aren’t spoken for will be on Etsy on Friday. Keep Calm though, it’s only, like, 3 items. Had a little overfire that caused some clay to blister.
Anyway, the sneak peak pic of the firing lead my friend Doug to ask some pretty cool questions. Soooo…
LEARNING TIME!!!!
Does the mineral content of clay have an effect on the firing process?
Yup! You can actually change the temperature at which clays mature or vitrify (become solid enough to fuse or “glassify” to hold water without it soaking into the clay). For example “True” porcelains are “high fire” (about 2400ºF) - but modern potters and clay manufacturers have manipulated to composition in different ways to create “midrange” (about 2000ºF) porcelains. This saves a lot of electricity and/or gas as well as dramatically increasing the range of glaze and underglaze colors one can use.
Also, does the thickness?
Yup2! When clay is thick it takes a lot longer to dry. As a result a lot of potters will fire the ware much more slowly to give any trapped moisture more time to dry out. Additionally, when clay is thin it can warp as it is fired closer to maturity. This is why a lot of potters hate making plates when they start out!
Does the shape?
Sometimes! Sort of! It depends on how much you’re “pushing” the material to do things it doesn’t want to do. For example, it’s challenging to make pieces where not all of the weight is directed directly downward. Like- I’ve made robots that went into the kiln, and came out missing an arm. Stuff like that leads to potters making clay supports that fire with the work, and even dedicate entire firings to one piece. After all- if an arm falls off your sculpture and lands on your cups, it’s gonna be a sad sad mess.
How about the type glaze and the overall finish?
OMG totally! Glazes mature- or become glass- at all different temperatures based on their composition. So, you can make work out of high fire porcelain- and then glaze it with lower firing glazes- but that means you can never fire it above the temperature the glaze is designed for. Sometimes you fire in “reduction” (an atmospheric firing where you limit the flow of gasses in a kiln after it reaches temperature- not a thing you do in an electric kiln…). Reduction can result in a lot of really interesting finishes and can actually completely change the color of a glaze. Also, there are a lot of “alternative firing techniques” that can create amazing finishes and dramatically change glazes. Do an image search for “raku” or “saggar fired” ceramics. It’s amazing the things you can change just by manipulating heat and/ or the way you expose clay and glazes to different types of organic materials.
- The pic I posted is Raku fired stoneware with a plain old low fire commercial clear over flat red underglaze. That’s the same red I use on my robots.
THIS CONCLUDES TODAYS EPISODE OF SEE MO MO-SPLAIN!
Feel free to hit me with any other questions you have. This was fun!