boutique@mail.island.net on sat 25 oct 97
We are a production ceramics shop, specializing in slipcasting and
pressing. We have been doing some work with terra cotta and find
when we wash and bisque the pieces that they will often have a white
scum on them.
We have been told that the addition of Barium in the clay or slip
will eliminate this. We don't usually work with Barium and therefore
would appreciate the voices of experience. Does the addition of
Barium solve this problem? I know that real caution must be used
when dealing with Barium. Does anyone know of an alternate additive?
Failing that, has anyone worked with Barium and be able to recomend a
percentage wt./volume to initiate some tests with?
TIA
Dave Quist
Fine Art Productions
Louis Katz on sun 26 oct 97
=3C/NOFILL=3E
------------------
Barium Carbonate is used to reduce or eliminate scum. It works on
Calcium Sulphate scumming and probably someother compounds as well. It
works because it is slightly soluble in water.
The chemical reaction is as follows Barium Carbonate plus Calcium
Sulphate yeilds Barium Sulphate plus Calcium Carbonate. The beauty is
that while Barium Carbonate and Calcium Sulphate are both reasonably
soluble, the products of the reaction are not.
Being non soluble the Calcium Carbonate cannot migrate to the surface.
People have had luck using very small percentages of Barium Carbonate
to eliminate scumming. I have read on people using 1/10 of 1 percent
or a tenth of a pound ber hundered pounds.
I would suggest that if you are going to use barium carbonate that you
use as little as you can, and that you disperse it throught the clay.
I would make the following suggestions to try and eliminate the
problem without the barium. I don't use barium where I teach except
when I mix casting slip, and then in very small percentages.
Below is a copy of an answer I wrote for the NCECA Newsletter on
Water.
Louis Katz
Down Below the Pipik of Texas
lkatz=40falcon.tamucc.edu
http://www2.tamucc.edu/lkatz/lkatz/index.html
Ask the Doctor
Water, Water, Everywhere,
All the Same I think?
Dear Doctor,
Every fall I have trouble with all of my glazes crawling. My bisque is
free of dust and my fingers are clean. I have even tried wearing
rubber gloves. Could this crawling be caused by a diet rich in cottage
cheese?
Sincerely,
Sore Knees
Dear Ms. Knees,
Ingredients that we use in our clay and glazes come from nature and
are variable in composition. Unfortunately this can also be true of
our water. Soluble salts from our water (and clay) can bring about
crawling in at least two ways. As greenware dries these soluble
compounds are deposited on the surface of the clay. The resultant scum
that appears is most frequently whitish. It can cause bad adhesion
between the dry glaze coat and the ware. This poor adhesion allows
the melting glaze to bead up like water falling through the air.
Salts in glaze suspensions can also cause crawling from flocculation.
Flocculated glazes require more water to have the same degree of
fluidity in application as unflocculated glazes. More water usually
means more shrinkage that can cause cracking in the unfired glaze
coat. More shrinkage also causes a weaker adhesion between the bisqued
clay body and the dry glaze coat. Alone or together the cracks and the
poor adhesion allow the glaze to pull itself in beads or just crack
and fall from the pot.
If you live in places where the tap water is hard, and are having
problems with scumming, crawling, or can=D5t get terra sigs or casting
slips to deflocculate properly you might want to consider other
sources of water. The graduate studios at Arizona State University use
rain water collected from the roof downspouts to breed mosquitos and
to mix clay. Clay mixed with the rain water is much superior to clay
mixed with Tempe tap water. Here at Texas A=26M University -Corpus
Campus we use the condensate water from a small room size dehumidifier
for slips, sigellattas, and troublesome glazes. This is the same
distilled but dirty water extracted from the air by air conditioners.
It is nearly free of soluble impurities. At large universities this
water is often available from the physical plant or a tap in each
building. Ask your heating and cooling person.
Our dehumidifier is also use to dry and heat the air for a closet
sized hot box, used for drying slipcasting molds. Since we use the
water taken from the molds for the casting slip this arrangement
begins to approach a closed system.
Barium Carbonate is often added to clay to reduce scumming. It removes
it because it is slightly soluble in water and reacts with calcium
sulfate to produce two insoluble compounds. Being insoluble prevents
the new compounds from being scumming agents. Since Barium Carbonate
is poisonous (used to be used as rat poison) many people have stopped
adding it to their clay and slip recipes and decided to live with
scum. I like scum,particularly in cone 6 soda kilns but I have come up
with this list of suggestions to help keep scumming to a minimum
without the use of barium Carbonate:
1. Keep your clay barrels, slip buckets, and slop bins covered to
reduce evaporation. Every bit of water you dry into your clay leaves
its soluble salts.
2. If you recycle, use an excess of water but take the water off the
top of the slop bucket before you begin to dry it out. Do not transfer
excess water from one slop bucket into another, just send it down the
drain. This actually has the potential to carry some soluble material
away from the clay, and keeps the effects of salts in your water to a
minimum.
3. Use distilled water in spritz bottles, and keep their use to a
minimum.
4. Avoid the use of Gertsley=D5s borate in slips.
5. Be kind to younger siblings, especially those newly elected to the
NCECA board.
Copyright 1996 Louis Katz
Steve Mills on sun 26 oct 97
This is a common problem here in the UK with Staffordshire Red Clays, it's
caused by salts in the clay migrating to the surface during firing,
particularly where pots have been touched or handled during production. The
addition of a VERY small amount of barium solves the problem. Something in
the order of 2 %. It is commonly added to these clays during processing. The
addition of Barium Carbonate promotes what I believe is called a "Radical
Exchange" with the chloride(s) in the clay so that it/they become
carbonate(s) and visi versa (Chemists on the list please correct if
necessary).
Steve.
Steve Mills
Bath
UK
John Baymore on tue 28 oct 97
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If you are going to use Barium Carbonate to solve the scumming problem:
It is important to remember here that any barium carbonate added in excess
of that which converts the soulbles remains in the claybody as relatively
soluble, and somewhat toxic BC, not insoluble, Barium Sulphate. While it
is certainly a small trace amount, the constant exposure if this is a FT
job (as an artist meaning 1000 hours a day =3Cg=3E) might be an issue over =
the
long haul considered in combination with other barium carb. exposures.
The goal in adding BC to clays is to find exactly the amount needed, and no
more. That way there is no actual BC left in the clay body (and clay
dust). All the BC is converted to Barium Sulphate. This can only be
established by testing.
If you don't have enough there will still be scumming. If you have enough
the scumming will be gone. But If you have too much the scumming will be
gone too. How to tell? Do a series of tests. Back off on the BC
content until the scumming just barely becomes apparent and you have found
the threshold amount. Don't go over this amount by too much.
The =22standard=22 amounts you hear are really only general approximations =
on
the magnitude of scale of the amount necessary.....the point being you only
need a little. Some clays need only 1/10 of a percent, cause they have
little soluble salts that cause problems. Others have lots more. If you
clay =22needs=22 only 1/10 of a percent, and you add 2 percent, you will =
have 1
9/10 percent by weight barium carbonate still in your clay body.
Also remember that =22run of mine=22 can vary the soluble content in the =
clays.
You probably should test each new large bulk batch of clays ordered for
the necessary threshold value.
Best,
...........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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