Shana Colleen La Fore on wed 27 oct 99
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
John Rodgers on thu 28 oct 99
Shana,
I have worked with translucent porcelain slip for many years, and most
recently
with a special casting slip, non-porcelaneous.
>From your description of your clay, you probably need to do some more
work on your slip. Just on the surface of it, it sounds like your water
and silicate are off.
I probably will be repeating what you already know,
but bear with me, maybe what follows might help.
Your clay seems to lack what I would describe as "Crispness", a quality
that
will
allow water to pass from the clay through the mold, and allow a clay
buildup
which upon removal from the mold, and after sitting a few minutes will
result in the clay form fracturing
if
bent. When removed from the mold it should dry fairly rapidly.
Two things probably need to be adjusted and they must ALWAYS be adjusted
in the same
order.
1) Specific gravity.
To determine this, I prefer to use an erlenmeyer(sp) flask of known
volume or
a graduated cylinder...probably because I am a bit of a scientific
nit-picker...and a good gram scale. But you can do it with two plastic
cups.
Measure the slip into one and water into the other, up to the same level
in
both cups.
Weigh the water, and weigh the slip. You will be looking for a ratio
between
the weights of the water and the weight of the slip.
Now divide the weight of the water into the weight of the slip. If the
ratio of
water to slip is anywhere near correct the results of your division
should be
somewhere near 1.6. Whatever your weight of water, it is taken as 1. and
the
slip is measured against it, so your slip would weigh 1.6 times the
weight of
water by itself or a specific gravity of 1.6. It is a reflection of the
amount
of clay in your slip as compared to the water.
For porcelain/chinaclay slip a SpG of 1.6 to 1.65 is pretty good.
If the number is low, you will have to add some more solids to your slip
mix to
get the number up a bit. If it is to high, you will need to add a bit of
water
to reduce the number a bit. And be careful with adding water, you can
over do
it really quick, especially with porcelain/chinaclay slips. Once you
have the
specific gravity correct, you will need to adjust the pourability of the
slip
by adjusting the flocculated/deflocculated condition.
2) Flocculation/Deflocculation.
Slip needs to be adjusted so it has a certain viscosity, or pourability.
It
should move through a given size orifice or hole with a certain
velocity. To
test this condition, you really need a viscosimeter funnel and a
stop-watch.
The funnels are available through many supply houses...Georgies, Laguna
Clay,
Duncan, and many others. They are very inexpensive, and a must if you
are going
to be making your own slip regularly.
To run the test, simply place a quantity of slip in the funnel up to the
appropriate mark, note the time and release the funnel opening so the
slip
flows through. The time it takes to drain is a measure of the viscosity
of the
slip. Ideally it would be somewhere between 45 and 75 sec.
If the time is very high, you need to add a tiny bit of sodium silicate
to the
slip. The silicate will ionize in the water in the slip and the
resultant like
electrical charges will cause the clay particles to repell each other,
resulting in a very slippery, very fast pouring slip, without adding any
water
and messing up the specific gravity.
Now you can get to much silicate, and that is very bad. The clay will go
to
goo, and once done, it's tough to ever adjust. Some folks simply throw
it out
and start over, others will struggle through the adjustment process and
finally
get back to some reasonable slipperyness, but beleive me, it is a real
task and
involves adding epsoms salt solution to the slip to neutralize the
silicate.
Lets not go through all that. Get it right to start....and do that by
being
very patient and add very small quantities of silicate until you have it
right....flow rate of 45 to 75 sec.
Imagine a "U" shaped curve. The upper end of the left side of the curve
is
"Over-Flocculated" - representing very gooey,sticky clay,with lots of
silicate, and the upper end of the right side of the curve is
"Under-flocculated"- representing stiff, unmaneageable clay with no
silicate,
but with the specific gravity properly adjusted to 1.65.
. You will begin to add small amounts of silicate. As you do so, the
point of
underflocculation will begin to adjust downward, and the slip will begin
to
"loosen up" or become more liquid like, as you reach the bottom center
of the
curve. That is the point where the clay will have its best
performance....lowest viscosity and highest velocity..... 45/75 seconds.
Either
side, and the viscosity will increase and the flow rate will slow.
Once at the bottom of the curve, should you continue to add silicate,
the slip
will become sticky and unmanagable...and at the extreme upper end it
will be
absolute goo and totally unuseable. The mess can be reversed to some
degree
with careful addition of epsom salts solution, but it is really hard to
ever
get the clay to perform properly....and in my experience, it never
would.
When you go through these procedures, keep a journal. It is a most
valuable
tool. you will be testing many times, and every test needs to be
recorded. A
history will begin to tell you about the performance of your slip, and
once you
have it adjusted correctly, mix it well and test it for specific gravity
and
viscosity every time, just before you do your casting. The condition of
the
slip will change over time as you remove water and silicate out of the
clay
during casting operations. Reclaimed clay will have considerable effect,
and
you will find that you will need to adjust both the specific gravity and
the
viscosity from time to time.
I think if you go through this process it will give you some insight
into the condition of your slip. The numbers will set you free.
I don't know what you have cast, but it sounds pretty thick for
porcelain. I have poured peices that thick in other clay, and the pieces
would be left in the molds 1,2,3, even 4 days. It depended on what it
was, and the climatic conditions. The best I ever worked with was clay
in New Mexico. The dryness there was a dream. In Alaska summer was good,
but the coolness in big buildings in winter slowed the drying process
considerably. And the absolute worse was/is in the southeast USA where
the humidity can be 98/100% much of the time it seems...things dry slow.
Especially thrown works.
One trick that may help...don't push your molds. Make sure they are
absolutely dry before you cast them again.
And finally, set some fans in your casting, drying room. Don't let the
fans blow directly on the molds when cast or drying, nor on the pieces,
but set the fans so the air circulates really good. You will be
surprised how it helps. You want uniform drying and circulating air does
best for this. If the fans blow directly on the work, cracking and
warping can occur.
I hope this helps you out. If you want to ask me any questions, feel
free.
Good luck.
John Rodgers
Shana Colleen La Fore wrote:
> ----------------------------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
Bill Campbell on thu 28 oct 99
Shana-- Your problem appears to be that you don`t have your slip set right.
The specic gravity [SG] and your defloculation are critical. It looks like
you are too high on the curve. As you go down the curve [add more sodium
silicate] the slip will be less viscous and it will cast thinner. It will
become less plastic and a little more brittle. There are ways to check and
control your slip. I will only give you a start here because you could fill
books with how to doctor your slip.
1. always start by setting the SG first. SG is the weight of the slit times
the weight of water. you can do that by weighing out a specic amount of water
and replacing that amountwith slip and weighing again. [don`t forget to
deduct the weight if the containers] The number that you are looking for is
somewhere between 1.7 and 1.9 I would guess that you are looking to keep it
below 1.78.
2. Test your defloculation. Use a flow cup for testing paint or buy a flow
tester from one of the suppliers. They are cheap . By putting a specfic
amount of slip into that container you can time the flow through the hole in
the container. That flow rate can be changed by adding defloculant.
3.Cast the slip at different flow rates and observe the changes [be careful
to cast for the same amount of time for each test. You can them if you have
the same mold. What you are watching for is ideal clay. Not too soft or not
too brittle. Watch tehe inside of the casting to see if there is any
curtianing or erosion.
Slip is not as simple as you might think,but it is worth the effort to learn.
Bill Campbell
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