Joan Berkwitz on fri 9 may 03
I make my own pouring slip, it's half talc and half clays, with sodium =
silicate/soda ash. Also screen and mill it for 18 hrs. Very good, used =
it for years. However, I'm trying to add mason stains to achieve colored =
slip for a colored casting. I add the stains after milling and do not =
mill again. After several days, mixture becomes thixotropic and will not =
pour with a reasonable addition of water. Does the sodium silicate ratio =
need adjusting, or ??? Please help. Joanie
John Rodgers on sat 10 may 03
In a word - "Yes".
To do this well and easily, you need a viscosimeter. It's just a little
plastic funnel with a certain size hole in it so you can time the rate
that the slip runs through the hole. If the flow is slow - ie, takes a
long time to drain, add more silicate. If the flow is to fast - the
tiniest bit of epsoms salt will help slow it down.
This will take some time and experimenting to get right so it's
important that you kjep a record of every flow test that you do. That
way you develop a historical record for reference in the future that
will make the adjusting easier.
BEFORE YOU START -- be absolutely sure your slip density(specific
gravity?) is correct. If heavy,, add water to adjust, if light, you
will need to add more dry components and mix well. After the density is
right on, then and only then do you adjust the viscosity with the sodium
Be sure the silicate is cut 50/50 with water before you add it to the
slip. Otherwise problems can be induced.
Make your adjustments by adding tiny amounts of silicate at a time. It
is really easy to over do it!!
If you take out a 1 liter measure of slip, and then adjust that one
liter using measured quantities of silicate/water mix, you can then
calculate approximately how much silicate/water to add to the whole
batch. Just be careful and do it slow. Don't dump in the whole
calculated batch of silicate at one time, Add it a little at a time,
mixing and testing as you go. But the adjusting and testing of the 1
liter will give you an idea of how far you have to go to get it right.
I surmise that you know about what the silicate does, but just to
re-state .... it's addition ionizes the mix and the like charges on the
particles repel each other. This increases the slipperyness of the slip
and increases the flow rate WITHOUT the addition of water. More water
tends to cause the particles to settle out from the water and increases
cracking problems after casting. Epsom salts will cause the particles to
clump together, countering the effects of the silicate. Epsom salts is
hard to work with, and it is best to avoid having to use epsom salts at
One last thing. If you plot on a graph the change in viscosity vs. the
amount of silicate added, you would find that the data would result in a
"U" shaped curve, with the greatest viscosity/least silicate on the
upper end of the "U" on the right side, and the greatest
viscosity/greatest amount of silicate on the other upper left side of
the "U" . The least viscosity will be at the bottom of the "U". The goal
is to be near the bottom of the "U" without any epsom salts added. The
perfect viscosity for casting will not be right at the bottom but
slightly to one side. It has been a long time since I have considered
this and I have forgotten which side it will be on but it will not be
dead center. As I recall, it will be on the side with the lesser
silicate, not more. But your testing should clearly demonstrate which,
and that is the important part for you and justification for recording
Even with the proper density and viscosity adjustment you may find there
is still a very slight tendency towards being thixotropic. This very
small amount is simply something you live with and mixing thoroughly
just before casting should be sufficient to do the job through a casting
Now, a question!! Why do you not ball mill after adding colorants?
The one commercial slipmaking company of which I have personal knowledge
does NOT ball mill their white slip as the ingredients are so fine when
they are received, but they ball mill ALL of their slips (white) after
colorants are added. The owner has reported that long experience has
demonstrated this as the only way to avoid having a streaked or mottled
appearance in the clay after casting and firing.
Best of success,
Joan Berkwitz wrote:
>I make my own pouring slip, it's half talc and half clays, with sodium silicate/soda ash. Also screen and mill it for 18 hrs. Very good, used it for years. However, I'm trying to add mason stains to achieve colored slip for a colored casting. I add the stains after milling and do not mill again. After several days, mixture becomes thixotropic and will not pour with a reasonable addition of water. Does the sodium silicate ratio need adjusting, or ??? Please help. Joanie
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