Wednesday, August 17, 2005

Dry Trimmings: Throwing Log #3

I trimmed my vase last night.

The piece was beyond leather hard. It was cold to the touch, so it still had some moisture in it , but it was too dry. The walls at the base were absurdly thick, and there was an asymetric bulge about half way up the side. The piece was pulled out of circular when I removed it from the wheel

I probably should have tossed it, but I need the practice.

I centered it on the wheel. You put the piece, inverted, onto the wheel head, decide which part of the wall is most circular, and then try to line the piece up with the axis of rotation so that there is no perceptable wobble when the piece rotates. I brace my arms agaist the splash pan, hold out a finger until it barely touches the piece, and slowly rotate the wheel. The piece rotates, and you find the place that is pushed out the most off center (POMOC). Opposite of that point, is the place where the the piece is pushed in most off center (PIMOC). Observing the distance between the tip of my finger and PIMOC, I stop the wheel at POMOC, and push the piece towards the center of rotation half the observed distance.

Sounds complicated, but it only takes a few seconds. I can suceessfully pop a partially trimmed piece off of the wheel, check the current thickness of the walls, then recenter and continue.

I ended up using a steel rib to shave off most of the excess clay. The rib needs to be bowed to keep it from chattering on the piece, and my fingers are really sore this morning. I'm thinking of wrapping a bit of wire around the rib for next time.

The clay peeled off the pot in little chunky shavings. I'm working with Stardard Clay's 266, so the shavings look like milk chocolate that has been run through a rotary grater (from my trufflin' days). I saved them all in a ziplock bag, in case I need to make slip.

The piece is a pleasing shape. Its on the drying rack.
Over the weekend, we went down to Daley's Ceramics in Vineland, and bought some clay. We picked up a hundred pounds of raku clay, fifty pounds of 266, a box of pyrometric cones for our kiln sitter (cone 6), and a new trimming tool.

Joe is starting to mine his own clay, and says that it fires to an ivory yellow at cone six. Julie and I plan to pick some up, to give it a try. Joe wants to know how it does in a raku kiln.


Blogger Maureen McHugh said...

What does cone 6-10 mean? What does a cone do? Does it heat up?

I love the names of the materials.

3:30 PM  
Blogger SquidgePa said...

Pyrometric cones are little wedges of clay. They are made to slump at specific temperatures, after absorbing sufficient heat. The come in a range of formulations. The cones are numbered, to specify their temperature rating. An 'Oh' in front of a number can be thought of as a negative sign in the chart.

Large cones can be used in a kiln to measure how hot a kiln gets. The cones are placed in front of a peep plug, and are periodically checked. Typically, three cones would be used; one cone for the temperature that you want to reach, and two 'guard' cones, one one cone higher, and one one cone lower. The temperature is judged by how the cones slump in relation to each other.

Our electric kiln is controlled by a kiln sitter. A small cone is placed in a mechanism that reaches through the wall of the kiln. When the cone slumps, a rod drops in its center, and electrical power is cut to the kiln.

I fire to cone O5 or O6 for bisque firing. This corresponds to a range of 1800-1900 degrees F.

Cone 6-10 has a temperature range of 2165 - 2380 degrees F. Saying that a clay has a certain cone specifies the temperature that the clay will vitrify at.

Vitrification is a quality of properly fired clay. Take a piece of pottery in your house, and flip it over. Find an unglazed part of the clay, and lightly touch you tongue to it. Your tongue easily pulls free. If you happen to have a piece of clay that has only been bisqued, or a raku piece available, do the same; your tongue will stick, since the clay tries to absorb the water from your tongue. The bisque or raku piece is not vitrified; the other clay piece is.

5:23 PM  

Post a Comment

<< Home