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absorption and shrinkage rates for cone 6 bodies i used

updated wed 12 may 04


Maurice Weitman on tue 11 may 04


(Since this message contains a table, it might be best if viewed with
a non-proportional font, say Courier or Monaco. But I'll let it
slide if you want to view it as is. I've got bigger fish to fry.
Don't you read the papers people? Sheesh! I mean the world is going

As part of my preparation for my working with cone 6 oxidation
firing, I gathered six clay bodies that were readily available to me
in the San Francisco Bay Area that would offer some variety of color
and texture.

I may test other bodies in the future.

The bodies are:

eastbay a porcelaneous (happy now?) body used by S.F. area potters
formulated by East Bay Clay in Richmond, California

The balance are all made by IMCO, up the road a piece in Sacramento

imco porc their Cone 6 Porcelain (their name)
imco 1-c their 1-C Stoneware, a sandstone body
imco nw Navajo Wheel, a dark brown body
imco nw35 (a.k.a. 412) Navajo Wheel with 35-mesh grog
imco 50/50 "Quarry Tile and Sculpture" mix with 80-mesh grog

I began by slicing ~ 1/2" thick tiles from bags of each clay and
drawing a ten-centimeter ruler on their faces.

After bisque (cone 04) and glaze (cone 6 at 4:30) firings (with no
glaze), I weighed each "tile" while still warm on a digital scale
accurate to 1/10 gram, and recorded their weights in the "wt. before"

I then boiled the tiles for two hours.

After patting dry quite thoroughly, I weighed the tiles again and
recorded their weights in "wt. after"

The lengths of the now shrunken inscribed 10 cm. "rulers" were
recorded in "length"

Shrinkage % is (10 - length) divided by 10

Absorbancy % is (weight after - weight before) divided by weight before.

The "1" and "2" after each name represent the two samples of each
tile. There was only one sample for the 50/50 body, so sue me.

body name length shrink wt. wt. absorb %
before after
eastbay 1 8.8 12.0% 176.7 178.6 1.1%
eastbay 2 8.8 12.0% 167.6 169.3 1.0%
imco porc 1 8.0 20.0% 119.2 119.4 0.2%
imco porc 2 8.1 19.0% 119.4 119.4 0.0%
imco 1-c 1 8.3 17.0% 132.7 133.4 0.5%
imco 1-c 2 8.2 18.0% 119.6 120.2 0.5%
imco nw 1 8.7 13.0% 130.9 131.9 0.8%
imco nw 2 8.6 14.0% 139.6 140.6 0.7%
imco nw35 1 9.0 10.0% 136.9 146.4 6.9%
imco nw35 2 9.0 10.0% 123.5 132.6 7.4%
imco 50/50 8.3 17.0% 128.3 130.8 1.9%

Not shockingly, IMCO has different numbers published for their bodies:

East Bay Clay published no numbers. "We don't need no steenkin'
numbers." My kinda guy.

Subjectively, the IMCO porcelain body was very wet in the bags. Too
wet to my liking for throwing. This may account for some of its high
shrinkage rate.

I threw test pots with the other bodies six months ago and don't
remember their consistencies.

Before the end of the month, I will throw some test pots and make
test tiles with the new version of East Bay Clay's porcelaneous body.
Then I expect to glaze and fire those and the other test pots I made
using the same MC6G glazes I used in my first firing I recently
posted, and will also use MC6G Waterfall Brown and perhaps one or two
other glazes that will work well on darker bodies.

Thank you for your attention to (or is it in) this matter.

And I mean that sincerely.

Maurice in Fairfax, California, where I just cut my hair for the
first time since October when I decided I'd forego such stylin' until
I could throw a pot in my new studio in what at that time was called
a garage but was really just a storage heap and workshop. That pot,
a very nice pitcher, I might add, was thrown about three hours ago
and will have to wait to be trimmed until after I return from my trip
to Oregon and Idaho. I feel nekkid and a few pounds lighter, I tell
you, but my mother would approve. Maybe. If I shaved my beard,
maybe. But it's my first beard and I haven't been clean-shaven since
1974 and my mother died in 1986 anyhow, so who am I trying to please,
anyhow? Can't we all get along?