Marni Turkel on fri 31 may 02
Jeff,
I'm working with a c5 vitreous white casting slip, no iron except
possibly as an impurity. At present, I don't use any sodium silicate
to deflocculate, mostly Darvan 811 and very little bit of soda ash. I
do sometimes use sodium silicate, just don't happen to right now. 6
months or so back, the moulds started getting a lot of brown
staining, casting times were increasing to get usual thickness and
pieces were really time consuming to clean. I also started having
some glaze flaws on pieces where the build up in the molds was the
worst. We cleaned the molds with vinegar which dropped the casting
time back to normal and added extra barium carbonate and everything
was back to normal. At that time I was using a little sodium silicate
in the batch (3 tablespoons in 450lbs of clay), but not enough that I
would think that was my problem.
When I use sodium silicate, I always try to balance it with soda ash.
If there is too much sodium silicate, the pieces are brittle and tear
when you try to cut them. If there is too much soda ash, the casts
are "flabby". Also, it took a while to figure out that soda ash
persists in dried scrap but sodium silicate doesn't and that can
affect casting quality if you are recycling large percentages of
scrap in a batch. I don't know what adding barium to
over-deflocculated clay would do, but I'd be sure to try it on a
small batch first.
I've never worked on iron bearing clays, so I can't help you there.
Before I had to quite throwing I had both red and white clays in
production and that was enough of a hassle, so that every time I
think how much I'd like to have some red ware again, I think through
the process and decide I just can't be that clean and careful.
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
Longtin, Jeff on fri 31 may 02
Marni,
Your comment suggests barium affects the workability of the clay, not so
much the deflocculation. That's very interesting!
I thought it was just the opposite? I thought barium was used, in
conjunction with soda ash, to deflocculate iron rich bodies. I wonder,
instead, if you add barium to soften the workability problems caused by the
excessive soda ash/sodium silicate you need to use to counter the iron? (We
are talking about iron red bodies here, correct?)
It certainly raises some interesting questions?
Sometimes when I've added too much sodium silicate to the clay body and I
find the edge/seams brittle and harder to clean I just add more
undeflocculated clay to dilute the silicate. I never thought to add barium?
Is your body iron rich/iron red?
Finding a workable iron red cone 6 slip-casting body would be great, do you
know of one?
Jeff Longtin
-----Original Message-----
From: Marni Turkel [mailto:marni@MARNITURKEL.COM]
Sent: Friday, May 31, 2002 2:21 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: White Fuzz on Plaster Slabs
Jeff,
You mentioned soluble salts in barium coming "to the clay surface as
the pot is drying and cause a certain amount of scumming". My
understanding of adding barium to slip clay is different. I do notice
an effect as a deflocculant, but the reason I add it to my slip clay
is to counteract soluble salts. If I notice the greenware pieces and
the molds are getting discolored (I assume from migrating salts) and
the seams and cut edges of the ware become harder to clean, I adjust
the barium up until the problem stops. I don't know if it is from
seasonal changes in the well water or batches of dirty clay.
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
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