I’ll admit it – I’m a bit of a clay body elitist, and for a while didn’t have much interest in anything under cone 10.
My foray into the middle echelons of firing temp began with an impulse buy: Minnesota Clay Company’s Polar Porcelain in black. It was a late night online shopping spree, I was already on the hook for shipping, so I thought, hey, why not. I’ll use it…some day. Oh yeah – it also fires to cone 6-8. Interesting.
Fast forward to my inaugural firing of the decidedly non-white, non-vitreous Dave’s Porcelain. I immediately knew I needed a new clay body, and wasn’t sure where to start, so I turned to an expert for advice. I chatted with Monterey-based ceramicist Peggy Alonas who very generously sent me test tiles of Southern Ice, Pier, and Coleman porcelain.
I fell for the Southern Ice hard and fast. So pristinely white it’s almost blue, so glassy and smooth…I could go on about it ad nauseam. For a moment I thought I’d found my clay match made in heaven. Much to my dismay, I soon learned that the City of Industry Laguna location recently stopped carrying Southern Ice. While I did find it through a few online retailers, it was prohibitively expensive to ship. I was back to square one.
Next I called Laguna to chat with a sales rep. I asked for the whitest, most vitreous porcelain they had. He recommended one of their newer, and very popular clays, WC-437 Frost 6 Porcelain. I was skeptical of this cone 6 clay that calls itself porcelain, but willing to give it a try. The fact that I could fire the Frost 6 and black Polar Porcelain in the same kiln load provided the extra incentive I needed to give it a go. Not to mention the fact that my kiln probably wouldn’t have the longest life if I kept firing to cone 10.
I spent the next three months honing new designs and prepping my greenware. And then the moment of truth…
- I’ve met my match! Frost 6 is definitely the clay for me. It’s similar in color to the Pier Porcelain, but almost as vitreous as Southern Ice.
- Shrinkage, shmrinkage. The shrinkage rates of the black porcelain and Frost 6 were satisfyingly similar. In the future I will make the black porcelain greenware polygons about 1mm smaller to try to achieve a perfect fit. For now, I can file them down fairly easily.
The big issue
Warping. I’m seeing some pretty significant warpage – particularly on the black porcelain pieces. The thinner and more elongate the piece, the worse the warp.
I need to get to the bottom of the warpage issue. At this point, I can only use about 50% of the fired polygons (not even taking into account the many greenware casualties). It’s possible that my firing schedule could have been too aggressive, or that there were temperature irregularities inside the kiln. I plan on researching the issue further, and in the future I will place more cones throughout the kiln during firing to get a better picture of what’s going on.
I would love to be able to salvage the warped pieces, especially since they are among the most time consuming designs. Again, more research is needed, but I do wonder if I could re-fire them with a different schedule, or possibly use kiln bricks to weigh them down. More on that to follow…