Jim Murphy on fri 18 oct 02
Dear Clayarters,
Oh to have a casting slip body (for plaster molds - not pressure casting)
that when fired to Cone 6 would possess the strength, vitrification,
non-warping curves and openings, and the character/dependability displayed
daily to each of us in the form of the "porcelain throne".
Seriously, I do not not wish to make my own "throne" (I wish to make some
cast statuary art to be fired at Cone 6). I wonder what Feldspars and firing
temperature do sanitaryware manufacturers use to end up with those durable
non-warping beauties we all know and love ? (LOL)
I've searched the internet for clues. I visited WBB's (a clay distributor)
webite at
http://www.wbb.co.uk/wbbhtm/applicat/sanitaryware.htm
WBB gives the following recipe and chemical analysis for a typical
sanitaryware body:
Ball Clay 25%
Kaolin 27%
Feldspar 22%
Quartz 26%
Chemical analysis of typical sanitaryware body:
SiO2 TiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O Loss
65.0 0.4 22.5 0.5 0.3 0.2 3.0 1.7 6.0
WBB's website doesn't indicate the intended firing cone or temperature (Cone
10 ?). Anybody have any educated guesses for the intended firing temp of the
above recipe and which feldspars might be used ? I notice the relatively
high Na (1.7%) and low silica (65.0%) and I think the recipe must be using
Sodium Feldspar.
I've read there may be solubility issues with Soda spars in casting slip.
Also, in general, bodies with Soda Feldspars (Kona F4, Nepheline Syenite,
etc.) may have a narrower firing range and are more likely to warp compared
to using Potash Feldspar (Custer, etc.) and yet the sanitaryware
manufacturers have no problem. (Hmmmm - maybe Soda Feldspar is the ticket ?)
I've read that Potash Feldspars are less viscous during melting albeit at
higher firing temperatures. However, adding 2-4% Talc to an existing recipe
using Potash Feldspar can bring the firing temp down 2-3 Cones. (Double
Hmmmm - perhaps go with Custer Feldspar & Talc ?)
Let's say using the above WBB sanitaryware recipe, which I want to make into
a Cone 6 casting slip, I would use 25% FC-340 Ball Clay (relatively large
particle size ball clay blend) and 27% Kaolin (say 10% Velvacast, 10%
Kaopaque 20, 7% No. 6 Tile kaolin). I have Darvan 811 as a deflocculant.
I've got 200 and 325 mesh silica. My understanding is that by using smaller
particle-sized silica (325 mesh) in a body recipe, less silica overall is
required because the smaller particles will be more easily fused by the
fluxes.
My ceramic supplier has the following feldspars in stock: Custer Feldspar
(Potash), Kona F4 (Soda), Minspar 4 (Soda), Nepheline Syenite (Soda), and
Talc (Nytal 100HR).
So, in my present state of mind of not really understanding feldspars/fluxes
in casting bodies, I'm seeking help/guidance in selecting the Feldspar(s) to
use in a casting slip with fired properties similar to sanitaryware
(especially non-warping and strength) that will mature/vitrify at Cone 6.
Shall I use Custer Feldspar & Talc, a 50/50 combination of Custer and Soda
Feldspar, straight Soda Feldspar, Soda Feldspar & Talc, etc.? Which Soda
spar to use ? (I'm getting dizzy thinkin' of all the possibilities!)
If anyone has any guidance to offer, please feel free to contact me on or
off-list.
TIA,
Jim Murphy
Marni Turkel on sat 19 oct 02
Jim, It was interesting to read some of the things you mention about
properties contributed by different materials.
I can't tell you about sanitaryware, but I have some practical
experience formulating c5 casting slip. My experience adding talc to
a body to flux it down was completely unsatisfactory. It seemed to be
very sensitive and I had bloating and warping problems that would be
from as little difference in temperature as the visual cone at 3:00
or 6:00. It seemed as if it kicked in as a flux all of a sudden. One
part of the kiln would be fine another part was not. I also didn't
like the color it imparted to the clay. It seem to take the warmth
out of it.
I have non clay as 50% of the mix: 12.5% Silica (200 mesh), 12.5%
Custer, and 25% Neph. Sye (those percentages are of the total batch
of clay). I make 400 lbs at a time and try to use increments of 50
lbs, so I don't have a lot of weighing out with the attendant
possibility of error. I ended up adding a few percent of Frit 3124 to
the recipe to flux it down after I got it as close as I could to to
what I wanted using whole bags. I also added an equal amount of
Velvacast to keep the 50-50 ratio just in case it mattered. Figured I
could use the variation in particle size. At some point in the
future, I may go back and try 25% Custer and 12.5% Neph Sye with some
extra frit to see if that affects the casting qualities since I have
heard repeatedly that Neph Sye can cause problems.
I haven't tried FC-340, though it has been highly recommended by one
person I respect and a "proceed with caution" by another. If you go
with ball clay, I would recommend staying away from Kentucky OM4 and
Tennessee #1. I recently switched to Foundry Hill Creme and so far am
happy with that.
Earlier this year Ione kaolin became unavailable and I had to find a
substitution for it. I was surprised at the effect the kaolin had on
the vitrification of the clay. I settled on half and half McNamee and
Snocal kaolins--price and availability. Using only Snocal for the
kaolin, my old formula was tight. An unglaze cup with water in it
hadn't sweated after 7 days. There were cracking problems in the mold
with Snocal and it seemed brittle, so I didn't want to use it alone.
Using only McNamee, a cup would be discolored in about 10 minutes. I
ended up splitting the two and added a little more frit to flux it.
So that's what I know.
Marni
--
Marni Turkel
Stony Point Ceramic Design
2080 Llano Rd 1B
Santa Rosa, CA 95407
Phone: 707-579-5567
Fax: 707-579-1116
http://www.marniturkel.com
Jim Murphy on sun 20 oct 02
on 10/19/02 6:47 PM, Marni Turkel at marni@MARNITURKEL.COM wrote:
> My experience adding talc to
> a body to flux it down was completely unsatisfactory.
Marni,
I've heard Nytal 100HR Talc is good.
> I have non clay as 50% of the mix: 12.5% Silica (200 mesh), 12.5%
> Custer, and 25% Neph. Sye (those percentages are of the total batch
> of clay).
Last year, as a beginner, I experimented with Cone 10 porcelain casting slip
recipes, based on the standard 25% Ball Clay, 25% Kaolin, 25% Feldspar, 25%
Silica recipe. I used 25% Custer Feldspar. Fairly easy.
Now, moving to Cone 6 casting slip bodies, I figured I could use the same
Ball Clay (FC-340) and Kaolin (Tile 6, Velvacast, Kaopaque 20) materials for
their casting properties - keeping their recipe components near 25%, and
decrease silica recipe percentage (by using 325-mesh silica instead of
200-mesh) while increasing Feldspar to develop a body with high greenware
strength and high resistance to warpage during firing. (I don't need
translucency.) I had doubts that sticking with Custer as the lone spar would
work to vitrify at Cone 6 which is why I raised this subject.
I see you're using a total 37.5% feldspar combination of Custer and
Nepheline Syenite in your Cone 5 body. Very interesting.
I've been advised to checkout some Cone 6 body recipes in Richard Zakin's
book "Electric Kiln Ceramics". I'm anxious to see which Feldspars are used.
Should receive the book later this week.
Apparently, Soda spars ARE "OK" to use in casting slips - everybody has
tried them but me !
Thanks Marni,
Jim Murphy
Jim Murphy on thu 24 oct 02
on 10/24/02 6:54 PM, Ron Roy at ronroy@TOTAL.NET wrote:
> The slip below is for cone 10 - sub in Neph Sy for the feldspar and you
> have a cone 6 slip
No "news" to you Ron, but, for the other previously unaware Clayarters the
following info is from Digitalfire's website: "Nepheline Syenite contributes
high alumina without associated free silica in its raw form. Since nepheline
syenite has more alumina than most feldspars, substituting it into recipes
means that on one hand a lower melting temperature is achieved while on the
other a more viscous melt results because of the extra alumina."
So, Nepheline Syenite is a good thing! I'll go ahead and get some from my
supplier. I like your suggestion for subbing Neph Sy and subbing spar back
in if needed. G200 is not readily available for me, however, Custer Feldspar
is.
> It is OK to use some talc but be aware - at cone 6 MgO is very active so
> the range of your slip will be shorter.
Well, since I'm experimenting, I'll make several small batches of casting
slip (with Neph Sy alone, Neph Sy + Custer, Custer + Talc, etc.), pour them
into plaster molds of "test bars" I've made, then fire them (the test bars)
altogether and evaluate from there.
> Neph Sy is good for slips - the sodium that leaches out helps
> deflocculation so you don't need as much water (less shrinkage) - if you
> are casting you will need to deflocc more - use the right kind of Darvan.
I noticed, from the sanitaryware recipe on the referenced WBB website, a
specific gravity of 1.8-1.83 g/ml is typically used (less water). Perhaps
the high amount of Ball Clay (25%) in the recipe helps with this as well.
I'll stick to 25% Ball Clay in my test recipes, set the SG at 1.8, and use
Darvan 811 as deflocculant.
Thanks Ron,
Jim
Ron Roy on tue 29 oct 02
Well who am I to argue with Digitalfire - been there done that before -
don't like doing it but - somebody has to.
When I compare G200 and Neph Sy with the fluxes in unity I see that Neph Sy
has virtually the same amount of alumina that G200 has. That is not what is
unique about Neph Sy.
What is significant is the high amount of sodium compared to potassium -
remember now - sodium is very slouble in water - so it tends to deflocc
glazes and bodies so watch out for that.
The other siniifcant property is the low SiO2 - in fact some potters like
it because it has no free silica and so is safer to breath.
So it has it's uses - melts well at cone 6 - starts the deflocking of slips
and terra sigs but - you have to counter that defloccing in glazes and
throwing and handbuilding clays.
We use Epsom salts to reflocck bodies but the answer for glazes is to sub
in potash spar and adjust the silica down.
I am tempted to say - again - this is not complicated - especially of you
bother to look at your materials as a molecular formula. And I can add -
line blends do not give you that info so they are not a good way to
understand them.
RR
>No "news" to you Ron, but, for the other previously unaware Clayarters the
>following info is from Digitalfire's website: "Nepheline Syenite contributes
>high alumina without associated free silica in its raw form. Since nepheline
>syenite has more alumina than most feldspars, substituting it into recipes
>means that on one hand a lower melting temperature is achieved while on the
>other a more viscous melt results because of the extra alumina."
>
>So, Nepheline Syenite is a good thing! I'll go ahead and get some from my
>supplier. I like your suggestion for subbing Neph Sy and subbing spar back
>in if needed. G200 is not readily available for me, however, Custer Feldspar
>is.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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