Janet H Walker on tue 5 may 98
In the "things they don't tell you" dept. About using casting
slip. I keep looking at my books but there is lots on the practical
side that they don't say anything about. (I'm a subscriber to the
theory that much of expertise lies in knowing which tools to use for
the job.) I'm so much a beginner here...
I bought some prepared slip finally. Miller 500L (or something like
that) cone 10 stoneware casting slip, already mixed in a bag in a
2.5 gal box.
I weighed 100 ml of this and it weighed 185 g. This is somewhat too
heavy, no? Stored a long time and lost some of its water maybe?
I couldn't decide on a poor man's way to check the viscosity so I
would know how to at least keep it the same as I go along.
I was thinking you could measure the time it takes for some volume
of water going out the end of a turkey baster and then compare that
to the same volume of slip and compare the times then between slip
"out of the bag" and later on after using it for awhile or
reconstituting it. But I haven't any sense of what it might mean to
compare the time ratios or if they are even a valid measure of
anything.
It seemed to have settled very thoroughly in the bag and so I put
it in a bucket and used the jiffy mixer for awhile. How do I know
when I've done it long enough? Does this mix in air and is that
bad?
I filled my mold, crossed my fingers, waited 30 minutes and poured
out the remainder, putting it right back into the bucket with the
rest. Yes? Or has that changed enough in s.g. that it should go in
a separate backet and get adjusted before using it again?
Tomorrow morning I'm going to have to face the problem of what to
use to trim off the collar and to make the top edge of the cup be
the right shape. By then it should just fall out of the mold i
guess. How soon before I can fill up that mold again?
Egad. To modify Othello "She thinks too much and such gals
are dangerous."
Thanks for any pointers to decent folk wisdom on this topic.
Maybe I should just find the nearest painted frogs store in the
yellow pages...
Jan Walker
Cambridge MA USA
Berry Silverman on wed 6 may 98
We measure specific gravity with a hydrometer, but I can't recommend a
reading for you since every slip is different. But once you find the
right range, keep it there. You can thin with a small amount of water,
but too much water will cause stress cracks later. We use a very diluted
mixture of Darvan and water to thin when it really gets thick--very
little at a time.
Yes, the jiffy mixer is fine, but keep it under the surface or you will
whip in air. And, yes, it is fine to pour the remainder back into your
supply when you empty the mold. Just be sure to check the s.g. on a
regular basis to make sure it's not getting too thick.
To know when to empty your mold, just put a fettling knife vertically
into the well of slip in the center and draw it toward the edge of the
mold. Pull it through the slip that is set up along the edge, then pull
the knife straight up. The set-up slip will wipe the knife clean,
showing you exactly how thick the sides of your piece are. Don't worry
about the cut in the slip, because it is in the part that you will later
trim off.
We use a fettling knife also to trim off the extra collar, smoothing
with a slightly damp (not soaking) sponge. Don't try to do too much at
this point or you may damage the soft clay. Let it set up to leather
hard and then you can trim it as much as you want.
Berry Silverman, in Tucson, where we are gradually adjusting our casting
as the days get drier and the temp gets higher (in the low 90s right
now).
Bonnema on thu 7 may 98
Hi Janet,
A man named Herman Kleiner from NJ told me how to make an inexpensive
viscosity tester. We have been using this method for 15 years and are happy
with it. I bought a plastic paint flow or viscosity cup from Sears ( I
don't know if they still sell them though). It looks somewhat like a funnel
with a little hole in the bottom and a handle coming off the top to hold it
by. The hole in it as it comes for paint is too small, so I put it in the
freezer for a while to make the plastic hard and then carefully drilled out
the hole to 3/16 inch. We test our slip by mixing it up and then dipping
the cup in the slip and timing how long it takes to empty. For our slip, we
are happy if the flow is between 32 and 36 seconds. Also for our slip, a
cone 9/10 porcelain type clay, I keep the SG between 1.78 and 1.82. These
figures would all have to be arrived at through experimenting with the way
your slip pours and casts.
I would be glad to talk about slip casting in as much detail as I know if
you want to e-mail me.
Garret Bonnema
Bethel, Maine where it has been cloudy or raining for a while now.
--
Garret Bonnema
Bethel, Maine
bonnema@megalink.net
Chris Nielsen on fri 8 may 98
Berry,
One thing you did not mention is how soon a mold can be repoured.
They need to be dried out before pouring again......yes?
I've been thinking about using a clothes dryer set on Low......what do you
think?
Thanks,
-Chris
-----Original Message-----
From: Berry Silverman
To: Multiple recipients of list CLAYART
Date: Wednesday, May 06, 1998 5:00 AM
Subject: Re: casting about
----------------------------Original message----------------------------
We measure specific gravity with a hydrometer, but I can't recommend a
reading for you since every slip is different. But once you find the
right range, keep it there. You can thin with a small amount of water,
but too much water will cause stress cracks later. We use a very diluted
mixture of Darvan and water to thin when it really gets thick--very
little at a time.
Yes, the jiffy mixer is fine, but keep it under the surface or you will
whip in air. And, yes, it is fine to pour the remainder back into your
supply when you empty the mold. Just be sure to check the s.g. on a
regular basis to make sure it's not getting too thick.
To know when to empty your mold, just put a fettling knife vertically
into the well of slip in the center and draw it toward the edge of the
mold. Pull it through the slip that is set up along the edge, then pull
the knife straight up. The set-up slip will wipe the knife clean,
showing you exactly how thick the sides of your piece are. Don't worry
about the cut in the slip, because it is in the part that you will later
trim off.
We use a fettling knife also to trim off the extra collar, smoothing
with a slightly damp (not soaking) sponge. Don't try to do too much at
this point or you may damage the soft clay. Let it set up to leather
hard and then you can trim it as much as you want.
Berry Silverman, in Tucson, where we are gradually adjusting our casting
as the days get drier and the temp gets higher (in the low 90s right
now).
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