vivek patel on sun 24 sep 00
hi,
recently i had a problem with the saltglazed stoneware
sewage pipes that i manufacture. i think there is
something wrong with the fireclay that i used . this
is the problem.
when the pipes were extruded and dried in the sun
scores of handling marks were visible after drying and
when i looked at the surface with the magnifying glass
i could see small white specks very very tiny about
the size of pin holes on large parts of the surface.
Now when the kiln was fired and the sample was ripe
for salt to be added into the kiln, salt was added but
even after repeated salting attempts the sample could
not catch salt and ultimately we had to leave the
firing after wasting so much of coal.
the fired result was also horrifying. the pipes
instead of being a bright brown shiny colour were
white and at the most a light creamy or fawn colour
and all the handling marks and all sorts of marks were
visible.
could anyone please advise me as to what may have gone
wrong. the temperature for salting also had been
reached and the kiln was fired exactly according to
our previous experience of about 20 years.
recently i read in a book by singer and singer that if
the clay contains more calcium and manganese salts a
sort of white scum of salt develops or comes to the
surface of the body thereby reducing the ability or
receptiveness of the body to salt. Must that be the
problem? i dont know i am confused.
the book also recommended quick drying of the ware so
that these salts do not have enough time to come to
the surface.
please help
any help will be appreciated
thanks
vivek patel
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philrogers pottery on sun 24 sep 00
Vivek,
Without seeing the material I think it would be very difficult for anyone to
give you a definitive answer to your problem. Firstly, in addition to my
answer yesterday about your thermocouple: It is entirely dependant on the
material that your thermocouple sheath is made from. Many of our American
friends are used to sheaths made from metal. In G.B. we have porcelain
sheaths. That is why we are able to coat them in Alumina wash and expect a
reasonable life. My current sheath has lasted 30 firings with no apparent
damage.
Your latest problem is much harder to solve without sight of the offending
article. It could be as you said, that the clay does include salts that make
saltglazing difficult. I feel that the white specks that you see are not the
salts however. These white specks could be a number of things it depends on
where the fireclay came from and the geology of that area. However, the
white specks are probably allied to the existence of salts.
Could it be that the particular firing in question oxidised, that would
produce a pale result but would not account for why the clay refused to
salt. The salts of Calcium or Magnesium, if present, would normally be in
solution in the water that was used to make the clay. That is how they
migrate to the surface as the clay dries. If there is large amounts of these
salts present, particularly calcium sulphate, then it is reasonable to
expect that lime could appear in larger particle size and therefore be
visible under the Mag. glass. Did the pipes lime pop after a period of time
after firing?
Not sure what you mean by handling marks. Do you mean that after drying you
could see finger prints or even hand prints on the surface of the clay? If
so, then, Yes, you do have a clay body with a high soluble salt content and
this is probably the nub of the problem. Try adding around 2% Barium
Carbonate to the clay. The Barium renders the Calcium Sulphate insoluble by
creating the insoluble Barium Sulphate. Remember that Barium is poisonous so
be careful. You will have to do this in a mixer with the clay in a slurry
state and not by hand. An even better solution would be to find another
clay.
Another possibility is a scum caused by the use of coal and the sulphur
contained combining with the oxides in the clay. Hamer suggests that the
glaze fire requires to be slow and clean. Personally, I think it is more
likely that the clay contains Calcium Sulphate. If you buy it ready plastic
then you have a big problem, if you but it dry and you blunge it then I
believe the addition of barium will solve it for you.
It will be interesting to see what others make of this problem.
Phil.
Phil and Lynne Rogers,
Lower Cefn Faes,
RHAYADER.
Powys. LD6 5LT.
Tel/fax. (44) 01597 810875.
philrogers@ntlworld.com
----- Original Message -----
From: vivek patel
To:
Sent: Sunday, September 24, 2000 11:50 AM
Subject: answer wanted to salting problem. probably bad clay
> hi,
> recently i had a problem with the saltglazed stoneware
> sewage pipes that i manufacture. i think there is
> something wrong with the fireclay that i used . this
> is the problem.
> when the pipes were extruded and dried in the sun
> scores of handling marks were visible after drying and
> when i looked at the surface with the magnifying glass
> i could see small white specks very very tiny about
> the size of pin holes on large parts of the surface.
> Now when the kiln was fired and the sample was ripe
> for salt to be added into the kiln, salt was added but
> even after repeated salting attempts the sample could
> not catch salt and ultimately we had to leave the
> firing after wasting so much of coal.
> the fired result was also horrifying. the pipes
> instead of being a bright brown shiny colour were
> white and at the most a light creamy or fawn colour
> and all the handling marks and all sorts of marks were
> visible.
>
> could anyone please advise me as to what may have gone
> wrong. the temperature for salting also had been
> reached and the kiln was fired exactly according to
> our previous experience of about 20 years.
> recently i read in a book by singer and singer that if
> the clay contains more calcium and manganese salts a
> sort of white scum of salt develops or comes to the
> surface of the body thereby reducing the ability or
> receptiveness of the body to salt. Must that be the
> problem? i dont know i am confused.
> the book also recommended quick drying of the ware so
> that these salts do not have enough time to come to
> the surface.
> please help
> any help will be appreciated
> thanks
> vivek patel
>
> __________________________________________________
> Do You Yahoo!?
> Send instant messages & get email alerts with Yahoo! Messenger.
> http://im.yahoo.com/
>
>
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June Perry on sun 24 sep 00
Dear Vivek:
You might try a test with an addition of about 1/2% of barium carbonate.
On one of my trips to Japan, one of the potters doing Oribe told us that he
used barium to prevent this sort of scumming. I don't remember if he gave the
amount used, but you can start with the 1/1% and see if that will do the job.
You might want to check the Clayart archives. I believe this subject has come
up before.
Regards,
June
philrogers pottery on sun 24 sep 00
Vivek,
I have just re read my post and I did not mean to suggest that the water
used to blend the clay body is the carrier of the problem salts although of
course this is possible. I meant to say that the salts are dissolved in that
water when the clay is blended or processed. It is the water that carries
the salt to the surface as it exits the clay during drying.
Phil.
Phil and Lynne Rogers
Lower Cefn Faes,
RHAYADER.
Powys. LD6 5LT.
Tel/fax. (44) 01597 810875.
philrogers@ntlworld.com
----- Original Message -----
From: philrogers pottery
To: > >
>
____________________________________________________________________________
> __
> > Send postings to clayart@lsv.ceramics.org
> >
> > You may look at the archives for the list or change your subscription
> > settings from http://www.ceramics.org/clayart/
> >
> > Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
> >
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
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> You may look at the archives for the list or change your subscription
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Gavin Stairs on sun 24 sep 00
At 06:50 AM 9/24/00, you wrote:
>hi,
>recently i had a problem with the saltglazed stoneware
>sewage pipes that i manufacture. i think there is
>something wrong with the fireclay that i used .
Vivek,
Is this a new clay to you? What is the difference between this clay and
what you used previously?
You call it fireclay. That may mean a high alumina content, which could
account for all that you report. The salt will attack silica, and should
not be bothered much by excess flux, although you might get marking from
calcium on the surface.
You also report it came out pale, rather than red or brown, which probably
means low iron. That may mean that iron is usually fluxing your clay
body, and it is missing from this batch for some reason.
Since you report no glazing at all, I would suspect that you didn't reach
the glazing temperature for that clay. Did you change anything about the
firing or kiln? Since you fire with coal, it might be some complication
from sulphur in the coal, but I doubt it.
I understand that you are trying to install a pyrometer, so you probably
don't have temperature data for this firing. Can you say what temperature
you reached by any other means? Hot colour perhaps? I wonder at what
temperature you salt: how do you tell the right salting point?
I'm sorry to have more questions than answers. I do sympathize with your
problem. A whole kiln load of useless ware could be ruinous. You don't
say much about the fired pipes, except to note the additional marks. They
may be from salt effect on uneven silica content. It is very hard to
diagnose without first hand information. If any of these marks are
glazing, then it will tell you that you reached salting temperature at
least. If these glazed marks are light, not dark, it will tell you that
you are missing iron. And the rest of the marking may be from lack of
plastic material in the clay, or something like that.
Good luck. If you would like more help, you will need to supply more
information, technical details of the firing and the clay, how it feels
when wet, etc. Either to the list, or to me off list.
Gavin
Louis H.. Katz on mon 25 sep 00
This sounds like magnesium salts, possibly calcium salts. Either of these leaves a
noticable scum on the surface. In my limited experience, at cone ten it is easy to
salt through a noticable scumming but as the temperature lowers the scum seems to
resist salting.
The obvious solution is to add barium carbonate to the clay in some form. If you
are already suing an Additive A product to help your extrusions, the same company
makes it with barium carbonate already added. My experience also leads me to
believe that magnesia resists salt more than calcia.
If your problem is new, what has changed in your operation. Are you recylcing more
water, has your water source become more hard? Is cement, or plaster or old
magnesia bricks getting into your clay body? If it is not the water and not an
additive than it sounds like your clay. Perhaps the deposit has changed.
Although it is hard on your kiln, you may be able to overcome the scum with a
small addition of borax to your salt mixture. Use it at the beginning of salting.
There may be more details on the use of borax and wether or not it will help in a
Borax Consolidated Limited Techinical Service Bulletin from May 1948, Boric Oxide
as a Constitutent of Salt Glazes.
Also C.M. Lampman and H.G. Schurecht:
Effect of Variable Lime and Alkalis in Clays. Journal of the American Ceramic
Society 23, 167, 1940
MAgnesia and lime tend to give dull straw colored slat glazes. Calcium scummed
clays give a yellow green surface( I have seen this in the cone 4-8 range), I like
it, but I am not making sewer tile.
Hope some of this helps
Louis
vivek patel wrote:
> hi,
> recently i had a problem with the saltglazed stoneware
> sewage pipes that i manufacture. i think there is
> something wrong with the fireclay that i used . this
> is the problem.
> when the pipes were extruded and dried in the sun
> scores of handling marks were visible after drying and
> when i looked at the surface with the magnifying glass
> i could see small white specks very very tiny about
> the size of pin holes on large parts of the surface.
> Now when the kiln was fired and the sample was ripe
> for salt to be added into the kiln, salt was added but
> even after repeated salting attempts the sample could
> not catch salt and ultimately we had to leave the
> firing after wasting so much of coal.
> the fired result was also horrifying. the pipes
> instead of being a bright brown shiny colour were
> white and at the most a light creamy or fawn colour
> and all the handling marks and all sorts of marks were
> visible.
>
> could anyone please advise me as to what may have gone
> wrong. the temperature for salting also had been
> reached and the kiln was fired exactly according to
> our previous experience of about 20 years.
> recently i read in a book by singer and singer that if
> the clay contains more calcium and manganese salts a
> sort of white scum of salt develops or comes to the
> surface of the body thereby reducing the ability or
> receptiveness of the body to salt. Must that be the
> problem? i dont know i am confused.
> the book also recommended quick drying of the ware so
> that these salts do not have enough time to come to
> the surface.
> please help
> any help will be appreciated
> thanks
> vivek patel
>
> __________________________________________________
> Do You Yahoo!?
> Send instant messages & get email alerts with Yahoo! Messenger.
> http://im.yahoo.com/
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
--
Louis Katz
NEW EMAIL ADDRESS Louis.Katz@mail.tamucc.edu
NCECA Director of Electronic Communication and Webmaster(Ad-Hoc)
Texas A&M-CC Division of Visual and Performing Arts
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