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slip carving - help!

updated mon 1 oct 01


Cindy Strnad on sat 29 sep 01

Dear Patti,

I think the shrinkage of your two kinds of slip
must differ. The first slip works well with your
black clay body, but the second slip, having been
made primarily of a different clay than that of
the first slip, does not match the shrinkage of
the clay body.

Since the red slip fit well, and the white clay
slip did not, I wonder if you may have used a clay
such as EPK (which has less shrinkage) for the
white slip. If so, you might try using a fairly
pure ball clay instead. It will not be as white as
an EPK slip, but the shrinkage may be closer to
that of your red clay slip.

Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730

Craig Martell on sat 29 sep 01

Hi Patti:

The most likely explanation that I come up with is that the pure white slip
has a much different shrinkage rate than the pot you applied it
to. Usually when slips pop off in the way you describe they are shrinking
less than the piece they are applied to.

The cure is to make the white slip shrink more by adding some ball clay,
and or a plasticizer such as bentonite, vee gum, or macaloid. You can make
the additions of ball clay by 5% increments, up to perhaps 20% and apply
each test to the clay you are going to use. If you get to 20% added ball
clay and the slip still shells you can then add the other
plasticizers. Usually not more than 3% of the others. Choose the sample
that works. Neat thing is, you don't have to fire these babies to get the
results. Just get them to the bone dry state and see which slip is doing
the job. You can even force dry them for the worst case scenario to see
which slip will handle extremes.

regards, Craig Martell in Oregon

Wade Blocker on sat 29 sep 01

When you make your slip always add some CMC to make it stick to the
clay. Unless you have a very heavy coat of slip which might contribute to
the falling off, you should have no problems in the future. Three coats of
slip brushed on are just about the limit of slip you want to have on an
object. Mia in ABQ

Patti Kratzke on sat 29 sep 01

Since I don't have a wheel, I'm forced to explore other forms of clay
entertainment (it's been good for me). So I made some slip out of red
clay, and deflocculant (sodium silicate), and added some dry white slip
to bring the specific gravity up to around 1.8 or so (I'm told the range
is 1.79 to 1.84). I then used it to cover a black clay coil pot, and
carved a design into it, revealing the black base through the red slip.
Then I let it dry, and it's hanging together fairly well so far (not
fired yet). So I did a black clay slab mug, and went through the same
process pretty much, only this time I used a slip that I made purely out
of the dry white slip, with SS again as deflocculant. The SG was again
around 1.8 or so, and I think I used a proportionately similar amount of
deflocculant in each, but I can hardly claim scientific certainty on
that. Anyway, I carved a design into the slip-covered mug, same as the
coil pot, and let it dry. That was last Sunday. Today I picked it up,
and the slip is cracking off of the mug, mostly in in the non-flat areas,
like around the bottom and the rim, etc. What might have caused this?
The specific gravity of both slips is within the range I was told it was
supposed to be, although I can't say they are the same - the red slip
might have had a slightly higher SG, but still not a big difference. Is
it the coil vs slab? The addition of clay in the first slip? I think I
let the red one dry a little slower - could that have made a difference?
I think I might have "burnished" the red one a little more in the carving
process - would that be it? All of the above? None of the above?
Anyone have any experience with this? Thanks!