Shrinkage rate?

Ceramics
#1

So … I am making this torturous (miniature) kitchen sink. I’d like it to be a certain size (give or take some). I made a form to start the hand-building and IIRC, I planned for about 11% (?) shrinkage.

Measuring the form and the two sinks that have been fired gives me an idea of the shrinkage rate. One shrank 8.4%, the other was 10.6%.

They were created:
(a) on the same form
(b) using clay from the same batch (B-mix)
© fired in the same kiln AFAIK, although one was fired twice and one was fired three times.
(d) both air dried more than one week before the bisque firing so they were both quite dry

The one that was fired twice shrank 10.6%.
The one that was fired 3x shrank 8.4%.
While two data points is not a statistically relevant sample, I was surprised that the one fired 2x shrank at a 25% greater rate than the other one.

I’m not complaining, mind you, because either of those sizes is fine for what I’m doing. But the engineer nerd in me is curious about what could create such a significant difference in two parts that were created in an identical manner.

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#2

For extreme consistency you want to make sure items are fired not only in the same kiln, but the same shelf/zone. There can be up to a cone difference between the top and bottom unless you have A) a multi zone adjustable kiln and B) have tweaked to do those adjustments.

Tangentially related side note: this is why you usually try to fire lids on the pot or immediately adjacent.

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#3

Others with more experience may well correct me, but I believe that the amount and effectiveness in wedging prior to forming can affect the amount of trapped air in the clay, which can affect the amount of shrinkage.

Wedging on the canvas work surface can also remove some moisture from the clay, and small amounts of clay (as I suspect you were using) have a very large surface area to volume ratio, so if one piece was wedged more than the other, than the moisture content of one could be lower than the other, even with the same starting clay batch.

This would result in a bigger effect on a golf ball sized piece of clay than on a softball sized piece.

Did you happen to measure for shrinkage before they were fired?

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#4

No, I didn’t. But for clarification, are you asking about comparing the size of the “bone dry” greenware?

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#5

Yes. As the parts dry, even before firing, there will be shrinkage due to moisture content.

#6

True. The bone dry one I have that is not fired shrank about 6% between wet and bone dry.