Wednesday, March 17, 2010

What's going on?

I made several of these hand-pinched rice mixed with paperclay bowls sometime ago. The inside was coated with Southern Ice porcelain paperclay since I wanted to use the Celadon glaze.

After Cone 10 reduction, I found all my bowls had cracks in them. On the bottom of some of them, it looked like the Celadon glaze had pooled and formed a thick layer.

I was puzzled as to what happened as I had successfully used this method before. I decided to break some of them open to see what was going on inside. The bottom picture shows the culprit and the cause for the cracks.

I concluded that the Southern Ice porcelain paperclay was applied too thickly and in the high fire it contracted more than my regular paperclay and separated from the "shell."

This time around, instead of layering the Southern Ice porcelain paperclay layer by layer using the brush (this is the way I normally do it; it is a slow process but you have control over the thickness of the porcelain application), I decided to do something similar to slip casting, ie pouring my Southern Ice paperclay slip into my bone dry rice-and-paperclay bowls, wait for a while and then emptying the excess out. This resulted in too thick of an application of the Southern Ice porcelain paperclay. My experience with this porcelain paperclay is that it shrinks more than the regular paperclay and when this inner layer is too thick, it is has enough cohesion to pull/shrink away from the bottom of the bowl.

There are several ways to get around this:
1. Go back to layering the inner porcelain layer by hand with the brush method.
2. Use a thinner porcelain paperclay slip.
3. Wet down the bone dry bowl a bit so that it is not so "thirsty" and won't "suck up" the porcelain slip too quickly.
4. Do not wait too long to pour out the excess, if you are using the slip cast method.

I'm back using the #1 method as I can control how many layers of the porcelain slip I apply. I have not experimented with the other methods.

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