Bill Walker on sat 13 sep 97
In general, casting bodies use maybe 10-15% casting clays,
like K-T Cast, Velvacast, etc. while throwing bodies are likely
to use "jigger clays" like Tile#6. (These are American clays --
I do not know what clays you ahve in Australia) The difference is
that casting clays are coarser, while "jigger clays" are finer and
often contain bentonite or montmorrillionite to improve plasticity.
Throwing bodies often include separate additions of these
plasticizers. These plasticizers interfere with the deflocculation.
Also, throwing bodies often include grog, which gives you a rough
cast surface.
However, most throwing bodies can be made into casting bodies. You
just need to used more deflocculant to overcome the bentonite. You
might have difficulty using dried out clay body as a starting point
since the clay particles are get stuck together during drying and
never really get unstuck when you make the slip, no matter how long
you let it slake. A rule of thumb is to never use more that 50% scrap
in a casting slip. (Other people with other thumbs will tell you 30%
or 40% or any other arbitrary amount.) If you can get some of the same
body that is still wet, you could use half and half to prepare your
slip. Try small batches until you figure out how much deflocculant
you need. Too much sodium silicate can really screw things up.
Bill Walker
Alfred NY USA
walkerw@bigvax.alfred.edu
>From: Ian Hodgson
>Subject: Throwing and casting bodies
>----------------------------Original message----------------------------
> I'm trying to find out the difference between a throwing body and one
> for making casting slip, specifically porcelain. A friend gave me some
> good quality porcelain, which is dried out and dirty. I thought I
> could use it to make casting slip, but have been told that if it is a
> throwing body it will not be any good for casting. Apart from the
> deflocculant needed for preparing casting slip, what other material
> would I need to add to make it OK? I have been unable to find the
> information spelt out in any reference book.>
>
> Ian Hodgson
> Canberra
> ihodgson@agso.gov.au
Bill Amsterlaw on sun 14 sep 97
Hi Clayart:
Bill Walker made the statement that you can only add a certain percentage of
reclaimed scraps in a casting slip because once the clay dries, the clay
particles become more or less permanently stuck together. I wonder if Bill
Walker or anyone else could go into this in more detail.
Are you talking about trying to re-wet leather-hard scraps or scraps that are
bone-dry? Hasn't all commercially available clay been dried at some point in
its processing? Are you saying that if you take dry clay from a bag, wet it,
let it go bone dry, then rewet it ... that the rewet clay has physical
properties that differ from the clay you can make from the powder as it comes
from the bag?
If the clay particles can become more or less permanently "stuck together",
what does this do to the physical properties of the clay? As the clay gets
worse for casting, does it get better for throwing?
Bill Walker wrote:
>>
You might have difficulty using dried out clay body as a starting point since
the clay particles ... get stuck together during drying and never really get
unstuck when you make the slip, no matter how long you let it slake. A rule
of thumb is to never use more that 50% scrap in a casting slip.
<<
- Bill Amsterlaw (wamster@msn.com)
Plattsburgh, NY
Marni Turkel on mon 15 sep 97
Bill Walker asked for more information on recycling casting clays. I will
also be interested to hear what people have to say. I mix about 100 gallons
of slip per week and have been making batches of 100% scrap for several
years now. I have not detected any appreciable difference in the castings
and can't trace any technical difficulties to the practice. The only
problem I have found is a build up of soluble contaminants that
dissappeared when I started using water without the contaminants. When
using a sodium silcate-soda ash deflocculant, I found I only needed to add
sodium silicate but no soda ash to give good working properties, if the
scrap is dry. I add as much sodium silicate as for a fresh batch. With
Darvan as the deflocculant I need to add Darvan, but not as much as for a
fresh batch. I have read various admonitions about using limited amounts of
scrap (30% is what comes to mind) and would like to know what problems too
much scrap would cause if not in the actual casting process.
Marni Turkel
Stony Point Ceramic Design
Santa Rosa, California
Marni Turkel
Stony Point Ceramic Design
Santa Rosa, California
Jonathan Kaplan on wed 17 sep 97
The basic difference between casting bodies and throwing bodies is the
ratio between plastic materials(clays) to non plastic materials (fillers,
fluxes, etc.). And of course the chemistry necessary to suspend a high
concentration of solids in as small a quantity of water as possible.
Casting compostions are generally a 50/50 mixture of plastics and non
plastics, although other ratios do work, most do adhere to the 50/50 ratio.
Mix up a formula with this ratio as a pastic body and throwing it. Not
very easy.
Plastic bodies for pressing, jiggering, throwing, and handbuilding are
usually in ratios between 70-90 parts plastic ingredients, and 30-10 parts
non plastic. The water of plasticity is of course, different for these
bodies, generally in the 20% range or so.
I have used many formulas in a great deal of the above ratios and depending
on what you require in a plastic body, for me, the 70/30 ratio or the 80/20
ratio provide excellent compositions for pressing, jiggering, throwing, and
handbuilding.
We recycle all our casting scrap into new batches. We have small blungers
that all the spares and related trimmings go into. It is blunged into a
slurry, adjusted for specific gravity and viscosity and added into the new
batch each week or so. The reclaimed batch usually requires the addition of
an extra small amount of defflocculant to bring the mixture into specs.
Our casting bodies are dry blended for us in Denver at Mile Hi. It is easy
for us to correct problems in the slip as we have prepared dry blend for
all our casting bodies.
Jonathan
Jonathan Kaplan, president jonathan@csn.net
Ceramic Design Group Ltd./Production Services
PO Box 775112
Steamboat Springs CO 80477
Plant Location (please use this address for all UPS shipments)
30800 Moffat Ave Unit 13
Steamboat Springs CO 80487
(970) 879-9139*voice and fax
http://www.craftweb.com/org/jkaplan/cdg.shtml
http://digitalfire.com/education/clay/kaplan1.htm
Eleanora Eden on fri 19 sep 97
Hi Jonathan and all,
Certainly throwing with casting slip isn't as easy or fun as other bodies.
Neither is throwing with hi-fire porcelain, which as I recall is something
like trying to throw with vanilla pudding. Give me low-fire casting slip
any day compared to that stuff. Anyway, that's not the point. The point
is that it can be done and it can solve alot of problems to do it. I do it
all the time...not my throwing body of choice but it gets me where I need
to go without alot of diddling around.
Eleanora
.............
Eleanora Eden 802 869-2003
Paradise Hill
Bellows Falls, VT 05101 eden@sover.net
Jonathan Kaplan on sat 20 sep 97
>----------------------------Original message----------------------------
>Hi Jonathan and all,
>
>Certainly throwing with casting slip isn't as easy or fun as other bodies.
>Neither is throwing with hi-fire porcelain, which as I recall is something
>like trying to throw with vanilla pudding. Give me low-fire casting slip
>any day compared to that stuff. Anyway, that's not the point. The point
>is that it can be done and it can solve alot of problems to do it. I do it
>all the time...not my throwing body of choice but it gets me where I need
>to go without alot of diddling around.
>
>Eleanora
>
>............
>Eleanora Eden 802 869-2003
>Paradise Hill
>Bellows Falls, VT 05101 eden@sover.net
Eleanora is correct .Sure it can be thrown, but, it is indeed like throwing
with vanilla pudding. Its just alot easier to work with bodies that may
exhibit the properties that you wish. However, I would encourage breaking
as many "rules" as possible in working with clay to get the results you
want.
Take care
Jonathan
Jonathan Kaplan, president jonathan@csn.net
Ceramic Design Group Ltd./Production Services
PO Box 775112
Steamboat Springs CO 80477
Plant Location (please use this address for all UPS shipments)
30800 Moffat Ave Unit 13
Steamboat Springs CO 80487
(970) 879-9139*voice and fax
http://www.craftweb.com/org/jkaplan/cdg.shtml
http://digitalfire.com/education/clay/kaplan1.htm
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