Janet Kaiser on thu 28 oct 99
Shana,
It sounds to me that the THRIXOTROPY is too high, but I am afraid I cannot
help any further... The reason is, I am afraid I cannot think in lb.. and
oz. nor am I familiar with Georgia Kaolin. I know the US is holding out on
metrication, but if you personally change to using grammes/grams you will
probably find it is easier in the long run. Glaze and slip recipes are a
whole lot easier and more accurate in gm -- honestly!!
But let me get off my soap box... :-)
For future reference here is my FAULTS IN CASTING chart:
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FAULT 1. description; 2. cause; 3. remedy
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PINHOLING:
1. small holes just beneath surface on mould side of article
2. fluidity too low
3. increase water addition
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WREATHING:
1. small uneven ridges on slip side of article
2. thixotropy too low
3. decrease alkali content
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BRITTLENESS:
1. difficult to fettle or cut without breaking
2. thixotropy too low
3. decrease alkali content
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CRACKING:
1. small cracks where handles join the body
2. thixotropy too low
3. decrease alkali content
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SOFTNESS AND FLABINESS:
1. soft casts difficult to handle without distortion
2. thixotropy too high
3. increase alkali addition
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SLOW CASTING TIME:
1. Cast takes too long to form
2. fluidity too high or thixotropy too low
3. decrease water or decrease alkali addition
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BAD DRAINING:
1. Slip failing to drain from narrow sections
2. Fluidity too low or thixotropy too high
3. Increase water or increase alkali addition
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Remember a slip which is too thick to pour can be made fluid by the addition
of a deflocculant, which weakens the bonds between particles (e.g. sodium
silicate, soda ash). The slip itself can be made less viscous by using only
a small amount of plastic clay. The % or defloc. will vary with the type of
slip from 0.5 to 1.5% of the dry weight. SOME CLAYS WILL NOT DEFLOCCULATE!
As a batch of casting-slip is used and re-used, plaster from the moulds will
reverse the fluidity. Eventually the slip must be discarded.
Hope this helps?
Janet Kaiser
The Chapel of Art, Criccieth, GB-Wales, UK
HOME OF THE INTERNATIONAL POTTERS' PATH
http://www.the-coa.org.uk
postbox@the-coa.org.uk
-----Original Message-----
From: Shana Colleen La Fore
To: CLAYART@LSV.UKY.EDU
Date: 27 October 1999 20:06
Subject: PROBLEMS W/ BONECHINA SLIPCASTING
----------------------------Original message----------------------------
Hi,
I was wondering if there is anyone who might know about slipcasting with
transluscent porcelain?
I have been working on a cone 6 bonechina casting slip and have had little
luck. The latest formula I tried follows:
Bone Ash...3lb 1oz
Georgia Kaolin ...2lb 8oz
Silica/flint...1lb 13oz
Feldspar(potash)...1lb 3oz
I mixed 8lb 9oz dry materials with 4lbs of water and 35 grams of Sodium
Silicate
After mixing, leaving overnight, remixing, then straining, I poured the
slip into a plaster mould. After 2 min I poured it out. It built up a wall
over 1/4 inch thick. SO, I broke it apart and left it open to dry (so I
could pop it out easier & throw it away). After letting it dry for two days
the slip was still wet. It was the consistency of a handbuilding clay.
It is to a point where I am researching areas that have exhuasted even my
professor's expertise. I am not sure what to do next.
If anyone has suggestion or useful references please let me know.
Thanks,
Shana LaFore
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