Linda Arbuckle on wed 5 aug 98
I recommend the use of about 1/2 % barium carb in terracotta to avoid
scumming. The barium carb reacts with soluble materials in the clay,
like sulfates and becomes an insoluble form, barium sulfate. As I
recall, before CAT scans and MRI's, one often had insoluble barium
sulfate introduced into the body one direction or another to do x-rays.
Not fun, but not soluble/poisonous.
Scumming can form an obstacle between the glaze and clay surface and
cause problems, e.g. crawling.
Linda
--
Linda Arbuckle
Graduate Coordinator, Assoc. Prof.
Univ of FL
School of Art and Art History
P.O. Box 115801, Gainesville, FL 32611-5801
(352) 392-0201 x 219
e-mail: arbuck@ufl.edu
John Hesselberth on thu 6 aug 98
As I
>recall, before CAT scans and MRI's, one often had insoluble barium
>sulfate introduced into the body one direction or another to do x-rays.
>Not fun, but not soluble/poisonous.
Linda, they still use barium sulfate for small intestine Xrays. Had some
just the other day. You have to drink a pint of barium sulfate suspended
in water. It is just about the consistency of a nice glaze suspension.
Tastes really good too. With all the concern about barium in our glazes
it give one pause. It is important to remember that barium sulfate is
about the only form of barium that is nontoxic because it is virtually
insoluble in water. It passes through the body with essentially no
effect--well none we really want to discuss on ClayArt anyway.
John Hesselberth
Frog Pond Pottery
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com
visit my web site at http://www.frogpondpottery.com
"Why should we be in such desperate haste to succeed and in such
desperate enterprises? If a man does not keep pace with his companions,
perhaps it is because he hears a different drummer. Let him step to the
music he hears, however measured or far away." Henry David Thoreau,
Walden, 1854
Tony Hansen on sat 8 aug 98
>----------------------------Original >message----------------------------
>I recommend the use of about 1/2 % barium carb in terracotta to avoid
>scumming. The barium carb reacts with soluble materials in the clay,
>like sulfates and becomes an insoluble form, barium sulfate. As I
>recall, before CAT scans and MRI's, one often had insoluble barium
>sulfate introduced into the body one direction or another to do x-rays.
>Not fun, but not soluble/poisonous.
Have been putting it in thousands of tons since 1972 and the maxiumum
I've found is necessary is 0.5%.
However barium is only slightly soluble and only a small part reacts
with
the salts, the rest is still carbonate. However it reacts in the kiln
and according to tests we've had done no trace of barium leachate
can be detected in fired ware.
--
T o n y H a n s e n thansen@digitalfire.com
Don't fight the dragon alone http://digitalfire.com
Calculation/Database Software for Ceramic Industry
John K. Dellow on sun 9 aug 98
Linda , does the change from barium carb to barium sulfide take place in
the souring or during the firing ?.
Do you bother to test the bodies you make up to see if they really do
scum. Is it possible the scumming you see in the Art School environment are
really due to handling of terra-cotta by persons who have been working in
white ware. Leaves similar white marks on the fired terra-cotta pot.
Would it not be better to make up your terra-cotta bodies with raw
materials that do not cause scumming ?.
John dellow the flower pot man
Earl Brunner on mon 10 aug 98
In a message dated 8/9/98 8:50:07 AM Pacific Daylight Time,
dellowjk@kewl.com.au writes:
<< Would it not be better to make up your terra-cotta bodies with raw
materials that do not cause scumming ?. >>
I don't know about Linda, but I would attribute the scumming on bisque here in
the southern Nevada desert to be from the hard water more than the materials
in the clay.
Earl Brunner
Linda Arbuckle on tue 11 aug 98
John,
I have, due to a "middle-aged" moment, forgotten to put the Ba in the
body. Yes, it did scum and reinforced my thoughts that one needs to use
BaCO3 in terracotta. No, it's not about whiteware contamination. I made
the clay at school and used it at home in my own studio. Classic
scumming.
Since the scumming happens in the drying (soluble salts wick to the
surface), I assume that binding up the sulfates takes place at least in
part while the clay is still in a wet state. If it only happened later,
the stuff would still migrate to the surface.
While I have not tested this theory, I assume that terracotta clays
themselves, being a secondary (transported) clay mixed with other
minerals (iron gives the red color, fluxes - K, Na, etc. - help the clay
mature at lowfire temperatures), are the source of some of the soluble
things that cause scumming. If you use lowfire whiteware (often ball
clay and talc), you don't have scumming problems. So, you can't
eliminate materials that may cause scumming and still make terracotta
clay.
Linda
--
Linda Arbuckle
Graduate Coordinator, Assoc. Prof.
Univ of FL
School of Art and Art History
P.O. Box 115801, Gainesville, FL 32611-5801
(352) 392-0201 x 219
e-mail: arbuck@ufl.edu
John K. Dellow on wed 12 aug 98
> If you use lowfire whiteware (often ball
> clay and talc), you don't have scumming problems. So, you can't
> eliminate materials that may cause scumming and still make terracotta
> clay.
Linda i have been throwing for 22yrs and mainly in terra-cotta or
earthenware . If i must be labeled I'm an industrial thrower, no degree in
ceramics. I was warned not to use barium in the 70`s by an 80 yr. old
potter. The father of the potter i was working for. I have never knowingly
used a body containing barium & never had an scumming problems. If you are
throwing as much as 1 ton a day then even 1/2% barium is too much. It is
absorbed by the skin.
Erol brummer thinks it might be your water. There is one commercial
terra-cotta body available in Australia which has barium carb added & this
is because of the water & i won't use it. My current clay supplier is of the
same mind as me regarding barium.
John Dellow
| |
|
