ester svensson on fri 16 feb 07
and throwing properties
Hello hello Fountain of Knowledge
(i'm finding Clay art very useful, thanks alot to everyone who makes up this
list!)
I work in Afghanistan with a group of potters, where we have a problem with
the clay we are using; or actually quite a few problems. I would greatly
appreciate any input and or suggestions on how we can improve it.
~Alot of our problems stem from the fact that the potters once (raw) fire to
low temperatures (usually between ^013 -^06); it's a question of economics;
the cost of wood to fire the kiln with is by far the biggest cost, so they
pack the kilns as full as possible, single-fire, in about 5 1/2 to 6 hours.
Which means brittle porous clay, crazing, and pinholing. But the kilns are
actually pretty good and efficient, and can fire to over ^03 if for long
enough. At our school here in Kabul we've built a gaskiln, with bricks,
burners and kiln shelves imported from Pakistan, which is cheaper, more
controllable, and we can fire higher.
~I would however love some advice and input on how we can improve the clay
itself; we are using red earthenware dug from the mountains north of Kabul,
which is very porous even when fired to ^02. It is quite short, so the
potters add a plant fibre (i think the equivalent of foxtail), which helps
for throwing, and also makes the ware alot stronger before firing. (Which is
great since it's slipped and glazed raw). But this must also make the fired
clay more porous, even though the fibres are so thin. We don't have a whole
lot of materials available here as you can imagine, but does anyone have any
suggestions on how we could make the clay more plastic/suitable for
throwing?
~After firing, most of the ware develops a white powdery residue/coating on
the glaze. The potters soak all the ware in water for a few hours and dry
in the sun, which removes it. Am I correct in thinking this is lime? There
is also lime-popping on some of the wares. Would grinding the clay before
mixing, or sieving it finer, prevent this problem? Or could we mix something
(like vinegar?) with the clay when we prepare it? Another thing that has
happened to the glaze on the peices fired in the gas-kiln are spots on the
glazes without colour (still glazed, but clear), and it looks like it's from
the clay; could this also be lime? I can send photos off-list to illustrate
if anyone is interested, or if it would help?
I'm sorry for the great length of this! Thanks in advance,
and all the best,
ester
www.turquoisemountain.org
stephani stephenson on sun 18 feb 07
and throwing properties
HI Ester
just want to say hi and wish you well with the
enormous task you have!
I'm no expert on these issues but i think the scumming
is largely calcium deposits but some other minerals
may also be present too.
when these deposits are subject to enough heat and or
fluxing from other agents in glazes, they will act as
fluxes...often this happens somewhere between , say
cone 06/04 and cone 02/2...so that might account
for the clear spots. i myself have never seen the
scumming appear on a glazed surface, only on the
surface of unglazed clay. the soaking is a good idea.
typically here in the US they add a tiny percentage
of barium carbonate to the clay to get rid of
scumming, but that sounds like it would be an
unlikely solution given your location and financial
concerns.
aging the clay in moist condition is always a good
way to improve the plasticity. if it is aged as a slip
or slurry , just making sure there is a way to age it
without letting it dry out. a month of aging will
help, as long as you can will help.
as to the chunks of lime or salts, ...a good low tech
way to do it ,without the necessity of grinding , is
to make a liquid slip of the clay, rather thin, and
put it into cloth bags or some sort of a cloth or
sieve system... and strain out the chunks...burlap,
heavy cotton are good, especially if you do m not
have metal screening...
then the slip or slurry can thicken in concrete
troughs or pools or other evaporative containers
where the clay can be tended...
i have seen the clay scooped with hoes, etc, or just
aged and evaporated in the bags themselves... when the
clay has stiffened the cloth bags are peeled away....
it sounds like you have a lack of fine bodied clay
such as ball clay to make the clay plastic and more
workable, you have to wonder if there is an organic
component you could add , to help the pot get
formed... i don't know.
also , and this may not be an issue, but if the
location of the scumming is a problem you can dry the
ware in a different position. the scumming is brought
to the surface wherever the water is exiting the
ware as it dries, usually the rim, or whichever side
is up, though it depends..just think of the airflow
and where the water leave, that's where the scum
is...i use to dry pieces upside down or choose the
least
important part of the piece and make that be the
high point , or the leeward point
9 think that's correct) if it is drying somewhere in a
breeze... then the scumming would have its effect in a
not so visible place and was less of a nuisance....i
had mixed results with vinegar washes but adding
vinegar ...hmm might help the plasticity, don't know
about the scumming
is the finished ware supposed to hold liquid? is this
why the porosity of the finished ware is a concern?
also for glaze fit and delayed crazing?
if you haven't tried using the CLAYART archives yet,
you might find additional info by doing a search in
the archives on scumming, lime pops etc.
anyway, those are some thoughts that come to mind
i wish you well
stephani Stephenson
http://www.revivaltileworks.com
Hello hello Fountain of Knowledge (i'm finding Clay
art very useful, thanks alot to everyone who makes up
this list!) I work in Afghanistan with a group of
potters, where we have a problem with the clay we are
using; or actually quite a few problems. I would
greatly appreciate any input and or suggestions on how
we can improve it.
~Alot of our problems stem from the fact that the
potters once (raw) fire to low temperatures (usually
between ^013 -^06); it's a question of economics; the
cost of wood to fire the kiln with is by far the
biggest cost, so they pack the kilns as full as
possible, single-fire, in about 5 1/2 to 6 hours.
Which means brittle porous clay, crazing, and
pinholing. But the kilns are actually pretty good and
efficient, and can fire to over ^03 if for long
enough. At our school here in Kabul we've built a
gaskiln, with bricks, burners and kiln shelves
imported from Pakistan, which is cheaper, more
controllable, and we can fire higher.
~I would however love some advice and input on how we
can improve the clay itself; we are using red
earthenware dug from the mountains north of Kabul,
which is very porous even when fired to ^02. It is
quite short, so the potters add a plant fibre (i think
the equivalent of foxtail), which helps for throwing,
and also makes the ware alot stronger before firing.
(Which is great since it's slipped and glazed raw).
But this must also make the fired clay more porous,
even though the fibres are so thin. We don't have a
whole lot of materials available here as you can
imagine, but does anyone have any suggestions on how
we could make the clay more plastic/suitable for
throwing?
~After firing, most of the ware develops a white
powdery residue/coating on the glaze. The potters soak
all the ware in water for a few hours and dry in the
sun, which removes it. Am I correct in thinking this
is lime? There is also lime-popping on some of the
wares. Would grinding the clay before mixing, or
sieving it finer, prevent this problem? Or could we
mix something (like vinegar?) with the clay when we
prepare it? Another thing that has happened to the
glaze on the peices fired in the gas-kiln are spots on
the glazes without colour (still glazed, but clear),
and it looks like it's from the clay; could this also
be lime? I can send photos off-list to illustrate if
anyone is interested, or if it would help?
I'm sorry for the great length of this! Thanks in
advance, and all the best, ester
www.turquoisemountain.org
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ester svensson on mon 19 feb 07
and throwing properties
Dear Stephani and Marcia,
Thanks very much for your replies!
Some really helpful information and ideas.
Thanks for the info on scumming, i looked in the archives. We might actually
be able to get barium carbonate, if it is being used here in Kabul for
anything else. It's worth a look.
Also a good idea to use cloth for screening the clay. So far we have used a
metal screen, but the holes are much too big, so it only screens out larger
bits of stone and things. Then we've been drying clay in a simple 'tray'
made from brick, but that sounds like a better solution, to make a big tank
or container from bricks or cement. There's a great market for second-hand
clothes as well, and we've tried drying clay slurry in old corduoroy
trousers, which has worked quite well (and gets a lot of laughs). Up until
very recently it's been so freezing and snowy here, so it's been hard to do
anything outside, but now spring is on it's way, with plenty of
opportunities.
Yes, the ware produced is functional, for holding liquids and foods, and
we're starting to produce tiles as well, so the porosity is a real problem.
I've been reading about talc-bodies, and am trying to find out about the
availability of talc here. How is this for functional ware?
Thanks again for your help, and i'll let you know how we get on.
All the best,
ester
On 19/02/07, stephani stephenson wrote:
>
> HI Ester
> just want to say hi and wish you well with the
> enormous task you have!
> I'm no expert on these issues but i think the scumming
> is largely calcium deposits but some other minerals
> may also be present too.
> when these deposits are subject to enough heat and or
> fluxing from other agents in glazes, they will act as
> fluxes...often this happens somewhere between , say
> cone 06/04 and cone 02/2...so that might account
> for the clear spots. i myself have never seen the
> scumming appear on a glazed surface, only on the
> surface of unglazed clay. the soaking is a good idea.
> typically here in the US they add a tiny percentage
> of barium carbonate to the clay to get rid of
> scumming, but that sounds like it would be an
> unlikely solution given your location and financial
> concerns.
> aging the clay in moist condition is always a good
> way to improve the plasticity. if it is aged as a slip
> or slurry , just making sure there is a way to age it
> without letting it dry out. a month of aging will
> help, as long as you can will help.
>
> as to the chunks of lime or salts, ...a good low tech
> way to do it ,without the necessity of grinding , is
> to make a liquid slip of the clay, rather thin, and
> put it into cloth bags or some sort of a cloth or
> sieve system... and strain out the chunks...burlap,
> heavy cotton are good, especially if you do m not
> have metal screening...
> then the slip or slurry can thicken in concrete
> troughs or pools or other evaporative containers
> where the clay can be tended...
> i have seen the clay scooped with hoes, etc, or just
> aged and evaporated in the bags themselves... when the
> clay has stiffened the cloth bags are peeled away....
> it sounds like you have a lack of fine bodied clay
> such as ball clay to make the clay plastic and more
> workable, you have to wonder if there is an organic
> component you could add , to help the pot get
> formed... i don't know.
> also , and this may not be an issue, but if the
> location of the scumming is a problem you can dry the
> ware in a different position. the scumming is brought
> to the surface wherever the water is exiting the
> ware as it dries, usually the rim, or whichever side
> is up, though it depends..just think of the airflow
> and where the water leave, that's where the scum
> is...i use to dry pieces upside down or choose the
> least
> important part of the piece and make that be the
> high point , or the leeward point
> 9 think that's correct) if it is drying somewhere in a
> breeze... then the scumming would have its effect in a
> not so visible place and was less of a nuisance....i
> had mixed results with vinegar washes but adding
> vinegar ...hmm might help the plasticity, don't know
> about the scumming
>
> is the finished ware supposed to hold liquid? is this
> why the porosity of the finished ware is a concern?
> also for glaze fit and delayed crazing?
> if you haven't tried using the CLAYART archives yet,
> you might find additional info by doing a search in
> the archives on scumming, lime pops etc.
> anyway, those are some thoughts that come to mind
> i wish you well
> stephani Stephenson
> http://www.revivaltileworks.com
>
>
>
>
>
>
> Hello hello Fountain of Knowledge (i'm finding Clay
> art very useful, thanks alot to everyone who makes up
> this list!) I work in Afghanistan with a group of
> potters, where we have a problem with the clay we are
> using; or actually quite a few problems. I would
> greatly appreciate any input and or suggestions on how
> we can improve it.
>
> ~Alot of our problems stem from the fact that the
> potters once (raw) fire to low temperatures (usually
> between ^013 -^06); it's a question of economics; the
> cost of wood to fire the kiln with is by far the
> biggest cost, so they pack the kilns as full as
> possible, single-fire, in about 5 1/2 to 6 hours.
> Which means brittle porous clay, crazing, and
> pinholing. But the kilns are actually pretty good and
> efficient, and can fire to over ^03 if for long
> enough. At our school here in Kabul we've built a
> gaskiln, with bricks, burners and kiln shelves
> imported from Pakistan, which is cheaper, more
> controllable, and we can fire higher.
>
> ~I would however love some advice and input on how we
> can improve the clay itself; we are using red
> earthenware dug from the mountains north of Kabul,
> which is very porous even when fired to ^02. It is
> quite short, so the potters add a plant fibre (i think
> the equivalent of foxtail), which helps for throwing,
> and also makes the ware alot stronger before firing.
> (Which is great since it's slipped and glazed raw).
> But this must also make the fired clay more porous,
> even though the fibres are so thin. We don't have a
> whole lot of materials available here as you can
> imagine, but does anyone have any suggestions on how
> we could make the clay more plastic/suitable for
> throwing?
>
> ~After firing, most of the ware develops a white
> powdery residue/coating on the glaze. The potters soak
> all the ware in water for a few hours and dry in the
> sun, which removes it. Am I correct in thinking this
> is lime? There is also lime-popping on some of the
> wares. Would grinding the clay before mixing, or
> sieving it finer, prevent this problem? Or could we
> mix something (like vinegar?) with the clay when we
> prepare it? Another thing that has happened to the
> glaze on the peices fired in the gas-kiln are spots on
> the glazes without colour (still glazed, but clear),
> and it looks like it's from the clay; could this also
> be lime? I can send photos off-list to illustrate if
> anyone is interested, or if it would help?
>
> I'm sorry for the great length of this! Thanks in
> advance, and all the best, ester
>
> www.turquoisemountain.org
>
>
>
>
> ____________________________________________________________________________________
> Sucker-punch spam with award-winning protection.
> Try the free Yahoo! Mail Beta.
> http://advision.webevents.yahoo.com/mailbeta/features_spam.html
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>
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>
Ben Shelton on mon 19 feb 07
and throwing properties
Scumming and lime popping. That is typical for a lot of earthenware clays
throughout the world. There are 3 solutions for lime popping. One is to
remove the lime pieces. As has been suggested, sieving is a good plan. You
will have to test but if the lime pieces are below a certain size they will
pose little problem. The second solution is to pulverize the chunks to
reduce their size to an acceptable range. A ball mill is in order for this
task and while a simple machine, it may be beyond you currently. If you can
get books I wuold suggest both "The Potters Alternative" and "Pioneer
Pottery". Both have a wealth of info that would prove useful in your
situation regarding machines and clay chemistry. Third method is levigation.
yu can use a levigation trough where a shallow, stepped water way is used to
separate the clay by particle size leaving bigger pieces along the way at
the steps, or you can use a washing method. In washing a great excess of
water is used and the added clay is agitated well. Let this settle for 2~4
hours the pour off the top layer. Repeat at least once. The larger bits will
settle faster and be left behind.
One added advantage of the ball mill would be to make the greatest use of
the plasticity available in your local clay.
Wet processing also allows a washing effect on the clay. If the raw clay is
mixed with a great excess of water then allowed to settle the excess water
can be drained off and it will carry away any soluble materials it has
picked up. I don't know how this will effect the scumming issue but a test
would be simply made to determine its effect.
The issue of fuel for firing in your situation can be a huge hurdle too. You
may want to look into used motor oil as a fuel source. I don't know what the
normal means of disposal is in your area but it would be worth a look. There
is a great book on used oil as a pottery fuel but the title escapes me at
the moment. Try looking it up on the web.
I hope this is of some help,
Good luck,
Ben
Linda Arbuckle on tue 20 feb 07
~After firing, most of the ware develops a white
> powdery residue/coating on the glaze. The potters soak
> all the ware in water for a few hours and dry in the
> sun, which removes it. Am I correct in thinking this
> is lime?
This sounds like alkaline solubles in a glaze that is out of balance. There
is a Water Blue recipe in the states that uses a lot of frit 3110, highly
alkaline, that makes a great turquoise color w/copper from the high amount
of alkaline flux, but "grows" a white scum on the glaze after firing under
humid conditions. There are solubles in the glaze not bound into the glass,
probably sodium/potassium. This is different than scumming of the clay body.
The glaze needs to be re-formulated. Here, using some 3195 in place of some
of the 3110 will cure the problem, as it's less alkaline, and the result
seems to allow the alkaline solubles to be melted into the glaze in a more
stable way. The molecular amounts in the glazes are below. Looks like more
calcium, less KNaO (sodium/potassium), among other things. Even when it
doesn't grow, it's not a very sturdy glaze for hard tableware use. Soaking
the piece in water would leach out the solubles in the glaze, to some
extent. Re-formulating the glaze is the thing, I think.
Water Blue glaze
Na2O 0.58 Al2O3 0.22 SiO2 3.28
K2O 0.09 B2O3 0.21 P2O5 0.00
MgO 0.01 TiO2 0.00
CaO 0.32 Fe2O3 0.00
Alumina:Silica ratio is 1.00:14.89
Neutral:Acid ratio is 1.00:7.63
Alk:Neut:Acid ratio is 1.00:0.43:3.28
Expansion: 94.9 x 10e-7 per degree C
Oxides causing abnormal expansion:
B2O3
Water Blue w/18 3195 in place of 18 of the 3110
Na2O 0.49 Al2O3 0.31 SiO2 3.07
K2O 0.06 B2O3 0.46 P2O5 0.00
MgO 0.02 TiO2 0.00
CaO 0.42 Fe2O3 0.00
Alumina:Silica ratio is 1.00:9.79
Neutral:Acid ratio is 1.00:3.96
Alk:Neut:Acid ratio is 1.00:0.78:3.07
Expansion: 86.9 x 10e-7 per degree C
Oxides causing abnormal expansion:
B2O3
Linda Arbuckle, Univ. of FL Ceramics
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