Robert Schapansky on tue 29 oct 96
Hi fellow clayarters;
I have a question that someone asked me and I wasn't too sure how to
answer; hopefully someone has a solution.
( Do you remember the bowls that I made at your place. I
brought them down to Judy's to fire and just left them with the rest of
the greenware. So yesterday I went to check if they had been fired and
they had. They also had this funny whiteish bloom on them. I didn't say
anything but Judy asked me later what kind of clay it had been because
they had this scum allover them. She thought maybe it was a low fire
clay because she had only see this before with a particular terracotta
that someone was using. I told her it was the M350 and she said she
couldn't think of any reason for it. I wondered if it had been that I
used a potscrubber to sand them but she didn't think that should do it.
Have you ever had this problem and what caused it and what can be done
about it. I understand that it could interfer with the glazes. At
present what we are going to do is give them a good washing, dry them
thoroughly and then do the glaze. Let me know if you have any answers.
Is it possible it did that because there where all kinds of different
clays being bisqued at the same time?)
The only thing I could think of was that since she made the things
at my place and I have very hard water; that this might be a deposit of
minerals. But my bisque never looks like that and I use the same clay
(Plainsman M350).
Thanks for any and all sugguestions that I might be able to pass on
to her.
Elizabeth Schapansky
Clay Vision
Chetwynd, B.C.
Wilsons on tue 5 jan 99
I have not seen anyone address this whitescum ?
This white scum gave me a real problem. I never did try Canola oil. I
did try sanding it off and that helped a little but it was still there!!!!
Then someone told me that it might be my own finger oil. I did not act on
this info for a least a year. Just last Nov. I had an order for some wine
chillers in terracotta. I threw them in the morning and went off to a
friends house where I saw some used latex gloves. I asked if I could take
them and I did. I went back to the studio and handled the winechillers
durning trimming with the gloves. After they dried I load them into the
kiln with the glovers. WOW was I happy when I unloaded the kiln. They
were all a beautiful terracotta color WITHOUT ANY WHITESCUM.
Thing have slowed down a little - nice to have our kids home for the
holidays but now they go back to college and I will be in the studio alone
again. Did anyone else notice that when their kids left for college that
there was a new space where being creative was easier. Or should i say it
takes a lot of creative energy to bring up childern. Well that is my
thought for the day - I trust all goes well in this New Year (:+O
Bill wilson
bilson@rmii.com
>----------------------------Original message----------------------------
>I've been meaning to ask about "white scum" on terracotta ever since I signed
>on to CLAYART. As usual, all I needed to do was wait. But I missed the
>original message. What is the cause? And is the canola oil simply rubbed into
>the bisqueware anytime after after firing?
>Thanks & Happy New Year!
>
>Dina Hofstetter
>aaaaster@aol.com
Paul & Lori Rozman - Lautermilch on wed 6 jan 99
This canola oil business is like taking aspirin for headaches: it takes care
of the symptoms (for now) but is not a cure. The "scum" is soluble salts in
the clay which migrate to the surface with the water as it dries. Same
principle as the Egyptian Paste. The slower the drying the more soluble
salts deposits on the outer surface. If you purchased a commercial clay and
got "scum" I would complain (or return the clay) to your distributor
(manufacturer) because they should know better. If you mixed the clay
yourself add a small amount of Barium Carbonate ( .5 % to 1 %) to your clay
Batch. I should add that this is a common problem at low-fire and especially
red earthenware because it contains more soluble salts and you can see it
better against the dark brown. If you want to believe that wearing gloves is
the solution then perhaps it is, but personally I like to touch my pots.
Good luck to you and all the best for the new year.
Paul Rozman
700 Russell Road, RR# 2
Ladysmith, BC, V0R 2E0
Canada
TL. (250) 245-1055
http://www3.bc.sympatico.ca/mtopottery
mtopottery@bc.sympatico.ca
-----Original Message-----
From: Wilsons
To: CLAYART@LSV.UKY.EDU
Date: January 5, 1999 10:09 AM
Subject: white scum
----------------------------Original message----------------------------
I have not seen anyone address this whitescum ?
This white scum gave me a real problem. I never did try Canola oil. I
did try sanding it off and that helped a little but it was still there!!!!
Then someone told me that it might be my own finger oil. I did not act on
this info for a least a year. Just last Nov. I had an order for some wine
chillers in terracotta. I threw them in the morning and went off to a
friends house where I saw some used latex gloves. I asked if I could take
them and I did. I went back to the studio and handled the winechillers
durning trimming with the gloves. After they dried I load them into the
kiln with the glovers. WOW was I happy when I unloaded the kiln. They
were all a beautiful terracotta color WITHOUT ANY WHITESCUM.
Thing have slowed down a little - nice to have our kids home for the
holidays but now they go back to college and I will be in the studio alone
again. Did anyone else notice that when their kids left for college that
there was a new space where being creative was easier. Or should i say it
takes a lot of creative energy to bring up childern. Well that is my
thought for the day - I trust all goes well in this New Year (:+O
Bill wilson
bilson@rmii.com
>----------------------------Original message----------------------------
>I've been meaning to ask about "white scum" on terracotta ever since I
signed
>on to CLAYART. As usual, all I needed to do was wait. But I missed the
>original message. What is the cause? And is the canola oil simply rubbed
into
>the bisqueware anytime after after firing?
>Thanks & Happy New Year!
>
>Dina Hofstetter
>aaaaster@aol.com
Louis Katz on wed 6 jan 99
Hi Bill,
Sounds like plain old scumming. The real culprit is calcium sulphate that comes
from your water, your clay or plaster used in your studio.
Here is some text from articles I have written on water, and calcium you can get
the entire articles at the URLs at the bottom of this message.
"Gypsum is a major culprit in scumming. Dissolved in water, the gypsum is
deposited on the surface of the ware at the water dries. Barium carbonate, also
slightly soluble in water, is able to eliminate gypsum scumming very effectively
The dissolved barium carbonate reacts with calicum sulphate becoming calcium
carbonate and barium sulfate. Barium sulfate and calcium carbonate are much mor
relatively insoluble in water, preventing the calcium from migrating with the
water to the surface of the clay. After the barium sulfate comes out of solution
more barium carbonate dissolves. Despite its desirable qualities, barium
carbonate is poisonous."
"Ingredients that we use in our clay and glazes come from nature and are variabl
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 adhesio
between the dry glaze coat
and the ware. This poor adhesion allows the melting glaze to
bead up like water does
falling through the air. "
"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 tak
the water off the top of the slop
bucket before you begin to dry it out. D
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 th
clay, and keeps the effects of salts in
your water to a minimum.
3.Use distilled water in spritz bottles, and kee
their use to a minimum.
4.Avoid the use of Gertsley s borate in slips.
Keep Gertsley out of your scrap clay.
http://www2.tamucc.edu/lkatz/lkatz/articles/calcia.html
http://www2.tamucc.edu/lkatz/lkatz/articles/waterwater.html
--
Louis Katz
lkatz@falcon.tamucc.edu
NCECA Director At Large
Texas A&M-CC Division of Visual and Performing Arts Webmaster (512) 994-5987
Ric Swenson on wed 6 jan 99
In my experience, the white scum on fired terracotta clay is caused by
salts....add about 1% Barium Carbonate to your terracotta body when
mixing it...or having it made.... and the white scum will not appear.
Observe cautions for Barium Carbonate when mixing. Use a dust mask,
do not eat or smoke when the dust is around, wash hands, etc. From
testing....we found it does not leach out as a heavy metal after
fired.
Many years ago....They used BaO3 as a rat poison, so respect its
potential to be hazardous.......just as you would with any material you
use that is dusty. Right?
Regards,
Ric
Wilsons wrote:
>
> ----------------------------Original message----------------------------
> I have not seen anyone address this whitescum ?
> This white scum gave me a real problem. I never did try Canola oil. I
> did try sanding it off and that helped a little but it was still there!!!!
> Then someone told me that it might be my own finger oil. I did not act on
> this info for a least a year. Just last Nov. I had an order for some wine
> chillers in terracotta. I threw them in the morning and went off to a
> friends house where I saw some used latex gloves. I asked if I could take
> them and I did. I went back to the studio and handled the winechillers
> durning trimming with the gloves. After they dried I load them into the
> kiln with the glovers. WOW was I happy when I unloaded the kiln. They
> were all a beautiful terracotta color WITHOUT ANY WHITESCUM.
>
> Thing have slowed down a little - nice to have our kids home for the
> holidays but now they go back to college and I will be in the studio alone
> again. Did anyone else notice that when their kids left for college that
> there was a new space where being creative was easier. Or should i say it
> takes a lot of creative energy to bring up childern. Well that is my
> thought for the day - I trust all goes well in this New Year (:+O
>
> Bill wilson
> bilson@rmii.com
>
> >----------------------------Original message----------------------------
> >I've been meaning to ask about "white scum" on terracotta ever since I signed
> >on to CLAYART. As usual, all I needed to do was wait. But I missed the
> >original message. What is the cause? And is the canola oil simply rubbed into
> >the bisqueware anytime after after firing?
> >Thanks & Happy New Year!
> >
> >Dina Hofstetter
> >aaaaster@aol.com
John K. Dellow on wed 6 jan 99
> Bill i think you are confusing finger prints from stone ware with scumming. It
> always a problem haveing the two in the same studio . White scumming is from
> solable salts in the raw clay or the water used to either make up the clay bod
> or the water used in throwing etc. I posted the canola oil fix a while ago.
> Bricklayers down under have used any white oil " even motor oil " to remove
> scumming on bricks.
John Dellow the flower pot man
http://welcome.to/jkdellow
Veena Raghavan on thu 7 jan 99
I would love to find a solution to throwing with red earthenware and not
getting scum, but I do not want to use Barium. Is there any other solution
at the throwing/making stage other than barium to counteract the salts?
Thanks in advance.
Veena
Veena Raghavan
75124.2520@compuserve.com
John K. Dellow on fri 8 jan 99
Veena , if you are mixing you own clay body , use deminarised water (
expensive) or collect rain water if possible.
John Dellow the flower pot man
paul on wed 13 jan 99
Veena,
I would like to just point out one thing for you. Theoretically, Barium
Carbonate (poisonous) combines with salts in the red clay to form Barium
Sulfate (non-poisonous). I point this out to let you know that using Barium
Carbonate in this situation may not be as harmful as you would first
believe. Also, one usually uses one half of one percent in the clay body
mixture.
best regards - Paul Wilmoth
earthenware, and wood/salt stoneware
pwearthenware@email.msn.com
-----Original Message-----
From: Veena Raghavan <75124.2520@compuserve.com>
To: CLAYART@LSV.UKY.EDU
Date: Thursday, January 07, 1999 9:37 AM
Subject: Re: white scum
>----------------------------Original message----------------------------
>I would love to find a solution to throwing with red earthenware and not
>getting scum, but I do not want to use Barium. Is there any other solution
>at the throwing/making stage other than barium to counteract the salts?
>Thanks in advance.
>Veena
>Veena Raghavan
>75124.2520@compuserve.com
Gavin Stairs on thu 14 jan 99
At 09:51 AM 13/01/99 EST, Paul wrote:
> ... Theoretically, Barium
>Carbonate (poisonous) combines with salts in the red clay to form Barium
>Sulfate (non-poisonous). I point this out to let you know that using Barium
>Carbonate in this situation may not be as harmful as you would first
>believe. Also, one usually uses one half of one percent in the clay body
>mixture.
True, true. But the most dangerous time in handling such materials is
while they are dry powders, before all the above chemistry has taken place.
When you have a bag of Barium Carbonate in your hand, it doesn't much
matter what it will turn into when you have it in the clay.
Likewise, after firing, it is neither of the above: it is some part of a
complex aluminosilicate, and one doesn't really know how much will come out
until one has tested.
All of which is merely to state the obvious: when handling poisonous
materials, take care. Take care for yourself to begin, and take care of
your unsuspecting potential victims in the end. 1% of Barium Carbonate in
a body is probably not significant, but a half a pound of fine powder in
your hand is.
Gavin
===============================================
Gavin Stairs
Stairs Small Systems (S3)
921 College Street, # 1-A
Toronto, Ontario, Canada M6H 1A1
(416)530-0419
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