Greetings from Philadelphia

[Heidi Maiers]

Garth,
Hard to say if it was human or mechanical error. If it is a cone 6 clay, it should never have gone beyond its maximum temperature of 2232 degrees (F) or it will literally melt to glass. If it is a cone 10 clay that was fired at that temp, it should still be OK. If a cone 10 clay is fired at cone 11, that will cause the same problems. The cone number signifies the absolute maximum temperature that is reached, not a degree more, and the kiln should automatically shut off at that point to keep the temperature from raising any more.

Was the base actually glass - like, or could it be that the piece was set on a dirty shelf that had residue of melted glazes off of other fired pieces and only the very bottom was glassy?
What did you use to smoothe your surface? If it was just a sponge and water, that wouldn't change the color - but any other substance might.

[Garth Herrick]

Heidi,

There was about a one inch radius semi-circular glass surface on the front face of the square base rising from the bottom center. It had almost a bottle gray green color. 2232F is the temperature, that was registred, and all at the store agreed this clay body should have been perfectly stable at cone 6

To smoothe the surfaces, I tried not to overuse water, because it would subtly erode the surface and expose too much grog, for my liking. I did a lot of tooling with wooden tools and used my fingers a lot. I suppose the natural oils in my skin had some effect. There was quite a motttled range of surface color, which I did not mind. I just did not expect the overall yellow cast that had no relationship with the store samples. Also my sculpture was far less porous and absorbent of water than the cone 6 sample, which also suggests a temperature discrepancy.

I asked the clay manufacturer how a specified shrinkage rate of 12 1/2 % could become a documented 18% in my case. They offered no clue, saying they never test their clays beyond their design limits, and did not want to jeopardize their relationship with the store that serviced me. The store sent them a sample from my firing for scientific analysis, but there was never any findings response.

Thanks for your input, Heidi!

Garth

[Heidi Maiers]

That is strange indeed. What I would want to find out then, is if the clay that you used was actually the clay that you thought you were using. As in a packaging error at the factory and the wrong clay was put in the box you bought. If it was actually a cone 5 clay, that would explain 2232 being too hot for it and also the fact that the color outcome was not even in the range listed for that clay - not to mention the excessive shrinkage, and the too high density. If you bought a 50 pound box, the two bags may or may not be the same, but to be safe, like you say, the next one I would definitely have fired in the 06 range. I have fired as low as cone 018 with great results on a cone 10 clay.

I agree with you about the grog issue - it is easy to overwork the surface with water and bring out too much grog. This is something I've been trying to work on myself since I use a high grog content clay so I can fire them without having to hollow them out.

At any rate, I'm sure you've learned a lot with your first ceramic piece and the next one will be much less frustrating and even more beautiful - if that is even possible.

[Linda Nelson]

Party On Garth! (let me be the first here to say that...)

Wow excellent work! I can't stop looking at the sculpture - it makes me want one of my son.

Welcome

Linda