Pam Cresswell on wed 4 oct 06
I am working on a project where I need to be somewhat accurate about the
finished size of pots, in order to have fixed sized objects fit inside them.
So, first thought was to make a shrinkage rule from a slab of my clay. I
rolled it out, careful to roll in several directions so it would shrink
evenly. In drying to bone dry, it barely changed, what was an inch wet was
just barely under an inch. However a thrown pot from the same clay shrinks
quite a bit. More than 12 percent, probably 20 or more. I used a "lid
master" caliper set to the 12 % setting, thinking it would be about the
right size after firing, but the pot shrunk too small before I even bisque
fired it, let alone glaze :-)
My solution is obvious; my shrinkage rule will have to be a thrown pot, or
several thrown pots in graduated sizes. I am just very surprised that a slab
would behave so differently from a thrown pot. Doesn't every book say to
make a shrinkage rule from a slab????
Hmmmm
Pam, in back to cool KC (yesterday was hot!)
Michael Wendt on thu 5 oct 06
Pam wrote:
"I am working on a project where I need
to be somewhat accurate about the
finished size of pots, in order to have
fixed sized objects fit inside them."
Pam,
you have hit on a key difference
between thrown and slab work.
Depending on the rate you throw
and the amount of water you use,
the shrinkage of thrown pieces
can vary because they are absorbing
water while slab work is often losing
moisture to the process.
For this reason, I propose
you make a series of pots near the
sizes you plan and measure them
when wet, bone dry, bisqued and
fired to cone.
Then construct a table of sizes
on a spread sheet that gives you
what you need.
Incidentally, for making pots of
a particular size, you need a scaling
factor ( a number larger than 1).
You get it by dividing the wet
diameter ( or height ) by the fired
size.
example:
wet size 10"
fired size 8.5"
10/ 8.5 = 1.176
Notice that the shrinkage rate
for the clay is 15% but the scaling
factor is 1.176 which appears
to imply that the shrinkage is
larger (it's not)
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
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