Wednesday, December 5, 2012

Clay comparison

            The red stoneware that I've been using is a mix that is no longer available called Dark Chocolate stoneware. It was a gift from another studio, where it had lain in a closet for a number of years, during which time the company that used to make it went out of business. So it is time for me to search out another nice cone 6 red stoneware.
            I've been trying out a box of clay called Hawaiian Red. It's fairly different from the Dark Chocolate stoneware, and I'm not sure whether I like it or not. It has a lot of grog, so it was clearly intended to be used for handbuilding. I went ahead and started throwing pots with it anyway; grog usually doesn't bother me. But this stuff feels like sandpaper to throw. If I decide I like other aspects of this clay enough, I'd be willing to overcome my annoyance at how it feels to throw, but I'm undecided as of yet.
            I was a little surprised at how different the Hawaiian Red looks when raw compared to the Dark Chocolate. They look fairly similar after being fired to cone 6. The orange bowls in this picture are the new clay, and the brown bowl is the old clay.

            It even looks gritty after I've trimmed it, which is neat, but it also means I have to sharpen my trimming tools more often when working with it. We'll see what I think after I've tested some glazes on this stuff.


  1. What does cone 6 mean?

  2. In essence, it is a way of labeling a certain amount of "melted-ness" of ceramic materials. Pyrometric cones are specially formulated from glassy materials to melt at a certain point based on time and temperature - thus they melt similarly to the way that other ceramic materials do in the kiln.

    The cones themselves are constructed like little pyramids, and they bend over when they are "done." Thus we can use them to measure whether a kiln has fired appropriately for the clay and glazes we are using. The thermocouples in the kiln do measure temperature, but since both time and temperature combine to create what is called "heat work," the temperature doesn't necessarily tell you all you need to know about the firing.

    Cones are numbered from cone 022 (coolest) to cone 15 (hottest). (Actually they do go hotter, but no studio potter would use the upper limits beyond cone 13 or so.) Cone 6 and Cone 10 (most often used) are around 2232 F and 2350 F respectively. So anytime I say something is fired to cone 6, I'm saying I fired it to around 2232 F, but with pyrometric cones to measure the heat work. Saying "cone 6" is more accurate than the temperature for the purpose of knowing how the clay and glazes will react.

    Perhaps I will post this as an actual post with some pictures so you can see what I mean.