Marni Turkel on fri 18 jul 97
I read with interest comments about Neph Sy that were recently posted. I am
currently in the midst of trying to figure out why I am having major
variation in the amount of soda ash I need in my casting slip. I use a soda
ash-sodium silicate mix to deflocculate a cone 6 slip (equal parts Neph Sy,
kaolin, kentucky ball, and silica) and recently have been having "yucky"
slip problems suggesting that I have way too much soda ash. A while back I
started adding in the slurry from the sponging water to cut down on slurry
that had to be disposed of. It seemed like a good idea at the time, but now
I wonder if it contains a concentration of soda ash. I am also considering
city water (snow melt) from my home as oppose to the well water from the
pottery which I have been using for clay but not glazes. Another
possibility that had been suggested was the soda from the Neph Sy; I was
recently told that it could vary a great deal. My supplier says that no one
else has had problems with the current batch of soda ash, but that is
always another possibility.
I switched to this mix a few years ago when I started having problems with
a commercial casting slip I bought in dry form. I would put in the water,
the normal amount of soda ash, and the clay, and it was over-deflocculated
(checking weight and flow time) before I added a drop of sodium silicate.
The clay company, of course, said it must be something I was doing. I was
not adding slurry from the cleaning process, and I was working in the old
studio at my home and using good quality city water.
My question is: could the Neph Sye vary so much that I need to cut in half
the amount of soda ash I usually use? Or is the variation so small that it
would be negligible and probably have introduced the problem somewhere
else?
A second question while I'm at it: In the process of exploring solutions, I
tried straight Darvan, but was not happy with the casting results. Has
anyone switched from soda ash-sodium silicate to Darvan? and if so, do you
use the same specific gravity and flow time?
Marni Turkel
Stony Point Ceramic Design
Santa Rosa, California
Tony Hansen on mon 21 jul 97
I would not use sodium silicate/soda ash any more.
In comparison to the conventional soda ash-sodium silicate system,
Darvan produces
slips with longer casting range, higher solids content, improved
viscosity
stability, fewer "soda" or "hard spots", and gives increased
mold life. However, although slips are supposed to reclaim better
without
the need for constant adjustments with more deflocculant my experience
is that
additions of Darvan are needed. It is a lot harder to over deflocculate
a
slip with Darvan than sodium silicate, thus it is easier to achieve that
elusive ideal state of 'controlled flocculation' where the slip is thin
enough to work well but thick enough not to settle out.
You can find information on the principles of deflocculation and some
trouble shooting tips at
http://digitalfire.com/magic/slip.htm
--
T o n y H a n s e n, I M C thansen@digitalfire.com
=========================================================
INSIGHT5/Magic of Fire II demos at http://digitalfire.com
Marni Turkel on tue 22 jul 97
Tony,
Thank you for your suggestions.
Last month when I started having trouble with my soda ash/sodium silicate
deflocculated slip I tried mixing a batch of fresh clay using Darvan as the
only deflocculant. I mixed it to the same thickness as normal (specific
gravity of 1.75 with a flow time in a Lehman 500 ml viscometer of 1.5
minutes) and the casting came out gritty and wavy indicating slip that was
to thin. I tested it carefully between each addition of Darvan, and don't
think I over-deflocculated.
Can you tell me if I should be going for heavier slip, or a slower flow
time? It normally casts well at this weight and time with soda ash/sodium
silicate. Also, do you know if Darvan might still work well if I am getting
elevated sodium levels from my water or the Neph Sy?
Thanks,
Marni Turkel
Stony Point Ceramic Design
Santa Rosa, California
Tony Hansen on thu 24 jul 97
> I tried mixing a batch of fresh clay using Darvan as the
> only deflocculant. I mixed it to the same thickness as normal
> (specific gravity of 1.75 with a flow time in a Lehman 500 ml
> viscometer of 1.5 minutes) and the casting came out gritty and
> wavy indicating slip that was to thin. I tested it carefully
> between each addition of Darvan, and don't think I over-deflocculated.
> Can you tell me if I should be going for heavier slip, or a slower flow
time?
1.8 is a normal target. If your slip is thin and you suspect over
deflocculation, why not add some more dry. The slip should gel up
after 15 minutes or so. Use less Darvan than you think you need,
try casting and if it gels after a minute or two in the mold,
add some more darvan and try again. Keep doing this until it says
fluid long enough for casting. This state of 'controlled
flocculation' gives the best casts and keeps your slip from
settling out.
--
T o n y H a n s e n, I M C thansen@digitalfire.com
=========================================================
INSIGHT5/Magic of Fire II demos at http://digitalfire.com
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