Lynne Berman on sat 29 aug 98
Just opened a kiln where half of the pieces have patterns in their terra sig,
somewhat like those Peruvian pots or a brick wall. Anyway, the terra sig has
cracked all over some, just a few cracks in others. It's a grolleg terra sig
I've been using on the ^6 Standard porcelain. Fired to ^010. Can't think of
anything I've done differently but there must be something. Anyone have any
ideas? Did I open the kiln too soon and let them cool too fast? My only idea.
But why are some of them fine? I burnished them all the same way, several
coats of the terra sig, burnish, several more coats, burnish again.
Lynne Berman in Philadelphia
Barbara Lewis on sun 30 aug 98
Lynne, When I used grolleg terra sig I also had problems with chipping. I
incorrectly thought grolleg might produce a whiter white surface. Not that
I could see. So, I'd recommend using ball clay. It works just fine. The
other thing that might have occurred is that it was on too thick. When
you're burnishing the terra sig, if some comes off on your cloth/chamois, it
isn't terra sig yet but merely slip. I burnish between each coat, but never
apply more than 3 applications. Make sure of the specific gravity of the
terra sig. Val Cushing wrote a wonderful article on terra sig in an issue
of Studio Potter dedicated to earthenware. Regards, Barbara
At 09:08 AM 8/29/98 EDT, you wrote:
>----------------------------Original message----------------------------
>Just opened a kiln where half of the pieces have patterns in their terra sig,
>somewhat like those Peruvian pots or a brick wall. Anyway, the terra sig has
>cracked all over some, just a few cracks in others. It's a grolleg terra sig
>I've been using on the ^6 Standard porcelain. Fired to ^010. Can't think of
>anything I've done differently but there must be something. Anyone have any
>ideas? Did I open the kiln too soon and let them cool too fast? My only idea.
>But why are some of them fine? I burnished them all the same way, several
>coats of the terra sig, burnish, several more coats, burnish again.
>
>Lynne Berman in Philadelphia
>
Wellspring Clayworks
5412 Well Spring Road
La Plata, MD 20646
blewis@crosslink.net
Vince Pitelka on sun 30 aug 98
>Just opened a kiln where half of the pieces have patterns in their terra sig,
>somewhat like those Peruvian pots or a brick wall. Anyway, the terra sig has
>cracked all over some, just a few cracks in others. It's a grolleg terra sig
>I've been using on the ^6 Standard porcelain. Fired to ^010. Can't think of
>anything I've done differently but there must be something. Anyone have any
>ideas? Did I open the kiln too soon and let them cool too fast? My only idea.
>But why are some of them fine? I burnished them all the same way, several
>coats of the terra sig, burnish, several more coats, burnish again.
Lynne -
Do the chips expose the base clay, or do they expose the lower burnished
layer? I do not do a lot of burnishing, but in my Ancient Clay workshops we
have consistently achieved the best results by applying repeated coats of
terra sig (thinned to 1.15 specific gravity or less) to bone-dry ware,
allowing each coat to soak in a bit, and burnishing only after we are
satisfied with the buildup of terra sig. We often use a thin smear coat of
lard or glycerin over the last coat of terra sig before burnishing.
What is the condition of the surface of your clay before applying the sig?
Hard-leather-hard? Bone-dry? Smooth? Sanded? What is the consistency of
the sig? How many coats of your sig does it take to get a good opaque
surface? If the sig is not mixed thin enough it will drastically increase
the incidence of cracking and chipping. If the surface of the uncoated pot
is too smooth it will do the same. Tell us more about the condition of the
surface and your sig mixture.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Dai Scott on sun 30 aug 98
>I burnished them all the same way, several>
>coats of the terra sig, burnish, several more coats, burnish again.>
>Lynne Berman in Philadelphia (snip)
Hi, Lynne - my son (a beginner potter) had a similar problem with a number
of pots that he had put various coloured terra sigs on ; only one pot didn't
have flakes of terra sig chipping off - the one pot that he hadn't put
layers and layers on. We deduced that 3 or 4 layers with some buffing in
between is plenty. Maybe your problem was that the t.s. application was
just too thick. Hope you get it worked out!
Dai Scott - Kelowna, B.C.
sandra m benscoter on mon 31 aug 98
Hi Lynn!
Is this the same terra sig used down in Loveladies during the raku
workshop. I was given a small jar of it to take home and try. I added
some iron oxide to it, applied 3 coats to a bone dry piece, buffed it
lightly with a soft cloth, bisquit it and put it into a saw dust fire
late this afternoon. Couldn't stick around until the firing was
complete, but I just received a phone call letting me know it came out
great! No flaking problems.
Sande
sbenscoter@juno.com
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Brian Crocker on mon 31 aug 98
------------------
Lynne re the Terra Sig dampen your pot first with a sponge apply the Sig in =
the
usual way when all finished and dry bring it up in the kiln at no faster =
than 50
degrees C per hour to 600 degrees C then go to 100 degrees C per hour to 900=
and
then as fast as you think safe. On the way down close the ports and the flue=
and
let it go as slow as you can.
Kind regards,
Brian Crocker.
4 Erica Street, Tea Tree Gully, S.A. 5091
AUSTRALIA.
=7B e.mail address =7D crocker=40picknowl.com.au
----------
=3E ----------------------------Original message----------------------------
=3E Just opened a kiln where half of the pieces have patterns in their terra=
sig,
=3E somewhat like those Peruvian pots or a brick wall. Anyway, the terra sig=
has
=3E cracked all over some, just a few cracks in others. It's a grolleg terra=
sig
=3E I've been using on the =5E6 Standard porcelain. Fired to =5E010. Can't =
think of
=3E anything I've done differently but there must be something. Anyone have =
any
=3E ideas? Did I open the kiln too soon and let them cool too fast? My only =
idea.
=3E But why are some of them fine? I burnished them all the same way, =
several
=3E coats of the terra sig, burnish, several more coats, burnish again.
=3E
=3E Lynne Berman in Philadelphia
John K. Dellow on mon 31 aug 98
Barbara Lewis wrote:
> ----------------------------Original message----------------------------
> Lynne, When I used grolleg terra sig I also had problems with chipping. I
> incorrectly thought grolleg might produce a whiter white surface.
Are you talking about a colloidal slip ?. Seems to me that Terra sigillata is re
by name !.
John Dellow the flower pot man
Lynne Berman on tue 1 sep 98
First of all, let me thank so many of you for trying to help this frustrated
potter. After four years at this, I finally produced a pot I really liked,
porcelain, terra sigged and burnished, fired in sawdust in the Weber grill
with sulphates. It was a little jewel. Then, with plans to actually repeat
something I had done, this problem!
I'll try to answer Vince's questions. As for the colloidal question John
asked, I thought that was stuff from the health food store.
Okay, I think that the chips expose terra sig, not clay. That's a little hard
for me to answer with certainty because 1) I have chipped off the loose terra
sig and 2) sanded the pots with plans to try to repolish with new terra sig
those that aren't too bad. The rest will get sanded smooth and glazed.
Actually, nothing was exposed when I took them out of the kiln, just sort of
brick shapes made by the terra sig lifting up slightly only in parts of only
some of the pots. Of course, the one I tried to add red iron oxide to was
completed "bricked". But, I could see that was a problem as I did it.
I started with the Val Cushing recipe, using 14 cups of distilled water, 1500
grams of grolleg instead of ball clay, 1 tbsp. Darvan. To answer Sande's
question, it was the same recipe we used at the LBI Foundation in Loveladies
with the substition of grolleg for the ball clay. I had had trouble with the
ball clay recipe on the porcelain down there although it was fine on the
stoneware.
I applied several layers to bone dry pots. The sig was thin but I did not
measure the specific gravity. Was hoping to avoid that but maybe I can't. It
took about six coats to get a good opaque surface.
The surface of the pots was smooth, perhaps too smooth. I had heard that terra
sig won't stick well if the clay is too smooth but then I saw Don Thieleman's
pots this summer, gorgeous, large, burnished and saggar-fired. He burnishes
the clay first, giving it a head start and has no problem so I have been
trying that. He applies three coats, burnishes, then applied three more and
burnishes.
I wish that when one does the same thing, the same result would occur. Why
were some of the pots fine and others not? I think that the two that weren't
quite bone dry were two of the perfect ones, not the slightest hint of chip.
All of them, however, looked fine when they went into the kiln. I did notice
that sometimes there was a moveable air bubble that had to be rubbed out while
I was burnishing. Didn't understand what was happening but they were kind of
fun to move around until they disappeared.
I think I have told you everything I know about the process. It's really so
nice to have potters with experience working on my problems. Thank you.
Lynne in Philadelphia
Vince Pitelka on wed 2 sep 98
>Okay, I think that the chips expose terra sig, not clay. That's a little hard
>for me to answer with certainty because 1) I have chipped off the loose terra
>sig and 2) sanded the pots with plans to try to repolish with new terra sig
>those that aren't too bad. The rest will get sanded smooth and glazed.
>Actually, nothing was exposed when I took them out of the kiln, just sort of
>brick shapes made by the terra sig lifting up slightly only in parts of only
>some of the pots. Of course, the one I tried to add red iron oxide to was
>completed "bricked". But, I could see that was a problem as I did it.
Lynne -
I agree that there are potters out there who burnish in stages, applying
more sig, then burnishing more. I have had serious problems with chipping
and peeling when I do this. If, as you say, the chips exposed a smooth
terra sig beneath, then this certainly supports the theory that you should
wait until you have applied your six coats of terra sig before beginning
burnishing/polishing. Also, your description of being able to "chase" a
bubble around beneath the sig coat is grounds for concern, because if the
sig coat is well-affixed to the surface you should never be able to do this.
It reminds me of the kind of problems I have occasionally had when
burnishing pieces which were coated with sig without pre-sanding, or which
were coated with a simple slip rather than a terra sig. You are going to
have to do some experimenting here. If you use a ball clay sig, make sure
you are applying it in very thin layers. I would not use a grolleg sig, for
the simple reason that the average particle size is too coarse. If you
prepare sig by the long, involved method (my method, as described on Tony
Hansen's IMC webpage at http://digitalfire.com/education/glaze/terasig.htm )
then you would likely have a sig which would work fine, but the tradeoff is
that you will only be using the very finest particles, which is but a small
fraction of the original clay, and that could get very expensive with
grolleg. I'd go back to the ball clay sig, but make sure it is very thin
(best of all, get a hydrometer and measure the SG, and make sure it is less
than 1.15.
>were some of the pots fine and others not? I think that the two that weren't
>quite bone dry were two of the perfect ones, not the slightest hint of chip.
This indicates to me that the problem is with the sig sticking to the
surface of your clay. You always get a better shine with terra sig when it
is applied to bone-dry clay, but the physical connection is more tenuous
unless the surface is sanded. On those hard-leather-hard pots the sig
attached itself more firmly, thus no peeling problems.
Good luck -
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Bruce Girrell on thu 3 sep 98
> -----Original Message-----
> From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.UKY.EDU]On Behalf
> Of Vince Pitelka
> Sent: Wednesday, September 02, 1998 9:53 AM
> To: CLAYART@LSV.UKY.EDU
> Subject: Re: Chipping Terra Sig
>
> This indicates to me that the problem is with the sig sticking to the
> surface of your clay. You always get a better shine with terra
> sig when it
> is applied to bone-dry clay, but the physical connection is more tenuous
> unless the surface is sanded. On those hard-leather-hard pots the sig
> attached itself more firmly, thus no peeling problems.
> Good luck -
> - Vince
>
But doesn't applying this much water to the surface of bone dry clay cause
it to crack due to the rehydration? Every time I have ever applied any
significant amount of moisture to a bone dry piece it has destroyed the
piece. Am I missing something here?
Bruce Girrell
Kayo Parsons-Korn on thu 3 sep 98
The only time I had terra sig chip after firing was when I had put on
too many coats. I had also burnished between coats as an experiment on
that one, so that may have been part of the problem too. I had never
heard of burnishing between coats, but it sound like this has worked for
some people on this list. I found it a lot of extra work. I get
extremely high shines on mine just burnishing the top coat. I burnish or
polish once or twice with either a smooth rock, old nylon or plastic
grocery bag. Then I apply a light coat of grease, and polish with a
flannel cloth. I think it works better to apply the grease after the
initial burnishing even though Vince Pitelka does it the other way
around.
Lynn desribes bubbles floating around as she is burnishing. Egads! Makes
me wonder if the sig is actually bonding to the coat below. I've been
applying sig to BC6, which is also has a very smooth surface and found
it is difficult to get it to build up. I now let each coat dry a little
before applying the next.
If you are working with porcelain, you might want to try just burnishing
the clay body itself. I saw this done by Hisi Quotskuyva at a demo at
the Heard Museum early this year. Her family doesn't apply terra sig,
(or slip as they call it) to their pots. I didn't get to stay long
enough to see her burnish the pot. Was there with sister-in-law and baby
in tow. She explained the process to me. The pot she was about to
burnish was well sanded and bone dry. A very fine clay. She said she
just applys water to the surface and starts burnishing with her rock. If
the clay drys out, she adds a little water as she goes up. She said you
still had to do it all in one shot though, or you could see where you
had stopped and started again. The clay looks very grey but fires out a
beautiful reddish/orange.
I tried this on a small test pot of BC6, greased it after a couple of
burnishes and burnished once more. It came out beautifully. As high a
shine as any of the sigged pots I'd done. There was a slight pinkish
cast to it. Maybe just a little iron in the clay? But I imagine
procelain would come out very white.
--
Kayo Parsons-Korn
Kayo Parsons-Korn Design
kayo@kayodesign.com
http://www.kayodesign.com
(503) 647-5464
P.O. Box 436
North Plains, OR 97133
John K. Dellow on thu 3 sep 98
>
>
> I'll try to answer Vince's questions. As for the colloidal question John
> asked, I thought that was stuff from the health food store.
>
> Ok lynne i`ll bite. I am being perdantic ,(spelling, spell check has passed it
> but can`t find my spelling in oxford dictionary) i find terms white " terra
> sigillata " and white " terra cotta " to be a misnomer.
John Dellow the flower pot man :)
Vince Pitelka on fri 4 sep 98
>But doesn't applying this much water to the surface of bone dry clay cause
>it to crack due to the rehydration? Every time I have ever applied any
>significant amount of moisture to a bone dry piece it has destroyed the
>piece. Am I missing something here?
Bruce -
Applying the sig does not seem to have the same effect as applying water. I
guess the secret is in applying the sig in very thin layers, allowing each
one to soak in before applying the next. It may also depend on the
claybody, but I have never had a problem with the piece disintegrating with
any claybody I have used in this process. I like to make pinch pots very
thin, as little as 1/16", and I sand them when dry, then sig-coat and
burnish them. It takes a delicate touch, as the added moisture does make
them more fragile at that stage.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Vince Pitelka on fri 4 sep 98
>> Ok lynne i`ll bite. I am being perdantic ,(spelling, spell check has
passed it
>> but can`t find my spelling in oxford dictionary) i find terms white " terra
>> sigillata " and white " terra cotta " to be a misnomer.
John -
There is no such thing as white terra cotta. Terra cotta is of course an
iron red earthenware body. Although Roman terra sig was always red, modern
terra sig color is determined by whatever clay it is made from, or whatever
colorants have been added to the sig.
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
John K. Dellow on sat 5 sep 98
> John -
> There is no such thing as white terra cotta. Terra cotta is of course an
> iron red earthenware body. Vince I know that ,but is there a name for the
> white slip prepared in the fashion of terra sig .
Have had people ask me about white terra cotta & white earthenware, some of thes
are seasoned clay workers.
John Dellow the flower pot man
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