Lezlie Finet on wed 28 jul 04
I hope someone can help me figure out what I=92m doing wrong.
I mixed up my first batch of ball clay terra sigillata (using Vince=92s
method, as best I could without a proper siphon or hydrometer) and
thickened it to 1.14 specific gravity (according to my gram scale). It
seemed too easy.
I=92m getting ready for a garden show next month... I threw some fat birds o=
n
my wheel - fun shapes to raku.
I brushed 5 or 6 thin coats on my bone dry stoneware and buffed to a yummy
smooth shiny white finish - I didn=92t burnish it at all. I was delighted.
Then I bisqued them to ^06 - still satiny & smooooth. I was elated with
this new =91trick'.
I glazed the birds with a matt copper raku glaze, except the breast I
glazed with my favorite clear crackle - 60% Gersley Borate, 20% Neph Sy,
10% Flint & 5% EPK. It likes to be thick.
OK. I fired 3 of them to about 1880 F in about an hour and reduced for 1/2
hour in cans as usual. I do not dunk in water.
Results: beautiful colors and nice white crackle on breast UNTIL I started
to clean the carbon off. The glaze just POPPED OFF the entire breast of one
bird, and partially off another. The third bird was fine. And the copper
matt glaze stayed on just fine, except for a couple small chips. The 2nd
firing included a bird totally covered with the clear crackle glaze without
any problem.
The underlying surface was a smooth webbed texture, looked like naked raku,
so I don=92t think the terra sig came off, just the glaze. This has never
happened before - in the past, I=92ve brushed a layer of porcelain slip over=
the stoneware body to get a really nice large crackle pattern with this
glaze, so I thought the terra sig would do the same.
So WHAT HAPPENED?? Was my clay incompatible with the ball clay? Was the
terra sig layer too thick? No, the problem was only/mostly under the clear
glaze. Why would that be? Does terra sig not like to be fast-fired? Was it
too hot? I read in the archives that it was recommended to sand the
greenware before applying the terra sig, but should I sand the terra sig
before glazing over it? Can I reglaze and fire 'em again?
Anybody have any suggestions? I=92d appreciate any help or hints... on or of=
f-
list... I=92m afraid to fire the rest of the birds.
:-(
Thanks,
Lezlie
in Washington (the REAL one)
Vince Pitelka on wed 28 jul 04
Lezlie -
Do I understand correctly that you glazed over part of the terra sig areas?
If so, that is why you ran into trouble. The terra sig coating simply was
not connected well enough to the clay, and the glaze pulled it off the
surface, especially with five or six coats of terra sig, which may have been
too much, depending on how smooth the clay surface was. Glazing over a thin
coat of terra sig is rarely a problem, but glazing over a thick layer can be
problematic, and seems counterproductive.
There is also the question of whether your measure of specific gravity was
really accurage. The following is probably how you did it, but I'll go
through it anyway.
To get an accurate measurement of specific gravity using a gram scale, you
must use a tall narrow glass or clear plastic weighing container like a
laboratory graduated cylinder, so that you can really make sure that the
level is accurate. The wider the container in proportion to the volume, the
less accurate the estimation of the level.
Once you find an appropriate container, weigh it empty and dry, and record
the weight. Arbitrarily place an absolutely horizontal line near the top of
the container. It is easy to do this. Just anchor a fine-point permanent
marker at the appropriate level in a large lump of clay. With the container
sitting on the table, roatate it against the marker point. Fill the
container with water to exactly that line, and weigh. Subtract the weight
of the container, and record the weight of the water. Fill the container to
the same line with your terra sig, subtract the weight of the container, and
record the weight. Divide the weight of the terra sig by the weight of the
water, and that will give you the specific gravity.
Most ball clay contains small amounts of iron and other fluxes. Any
reduction atmosphere in raku firing will turn the iron to black iron oxde,
which is a powerful flux, and that will cause the terra sig coating to
flake. That could be aggrivated if there was a glaze over the terra sig.
You can ask your supplier for the lowest-iron ball clay, or you can switch
to a kaolin, which will give you a much lower yield of terra sig in
proportion to the dry clay, but it should behave better in raku firing.
But ultimately, try to avoid glazing over the terra sig.
I hope this helps. Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
sdr on wed 28 jul 04
Glaze, especially raku-type glazes, does not adhere
well to pots with terra sig on them, because the clay
surface is too smooth. There is no point, anyway, to
go to all that trouble if you are going to glaze. The
glaze covers the clay, no one knows if it is smooth or
not, eh?
Raku glazes really do not form much of an interface
with the clay - they are designed to crackle, and it
is a fragile firing method. Pop offs don't surprise me.
regards
Dannon Rhudy
----- Original Message -----
From: "Lezlie Finet"
To:
Sent: Wednesday, July 28, 2004 12:27 AM
Subject: Terra Sig + Raku = Pop-Offs??
I hope someone can help me figure out what I'm doing wrong.
I mixed up my first batch of ball clay terra sigillata (using Vince's
method, as best I could without a proper siphon or hydrometer) and
thickened it to 1.14 specific gravity (according to my gram scale). It
seemed too easy.
I'm getting ready for a garden show next month... I threw some fat birds on
my wheel - fun shapes to raku.
I brushed 5 or 6 thin coats on my bone dry stoneware and buffed to a yummy
smooth shiny white finish - I didn't burnish it at all. I was delighted.
Then I bisqued them to ^06 - still satiny & smooooth. I was elated with
this new 'trick'.
I glazed the birds with a matt copper raku glaze, except the breast I
glazed with my favorite clear crackle - 60% Gersley Borate, 20% Neph Sy,
10% Flint & 5% EPK. It likes to be thick.
OK. I fired 3 of them to about 1880 F in about an hour and reduced for 1/2
hour in cans as usual. I do not dunk in water.
Results: beautiful colors and nice white crackle on breast UNTIL I started
to clean the carbon off. The glaze just POPPED OFF the entire breast of one
bird, and partially off another. The third bird was fine. And the copper
matt glaze stayed on just fine, except for a couple small chips. The 2nd
firing included a bird totally covered with the clear crackle glaze without
any problem.
The underlying surface was a smooth webbed texture, looked like naked raku,
so I don't think the terra sig came off, just the glaze. This has never
happened before - in the past, I've brushed a layer of porcelain slip over
the stoneware body to get a really nice large crackle pattern with this
glaze, so I thought the terra sig would do the same.
So WHAT HAPPENED?? Was my clay incompatible with the ball clay? Was the
terra sig layer too thick? No, the problem was only/mostly under the clear
glaze. Why would that be? Does terra sig not like to be fast-fired? Was it
too hot? I read in the archives that it was recommended to sand the
greenware before applying the terra sig, but should I sand the terra sig
before glazing over it? Can I reglaze and fire 'em again?
Anybody have any suggestions? I'd appreciate any help or hints... on or off-
list... I'm afraid to fire the rest of the birds.
:-(
Thanks,
Lezlie
in Washington (the REAL one)
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__
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Lezlie Finet on mon 2 aug 04
Dannon said <pots with terra sig on them, because the clay surface is too smooth.>>
Dannon - OooooKaaaaay, that explains it!
So then explain this: the last time I did Naked Raku I fired a pot I=92d
burnished the heck out of. I slathered on some freshly mixed slip resist
made of fire clay, EPK & Alumina Hydrate (Riggs recipe I think) and fired
it. After a quick post-smoking I tried chipping off the slip - NO luck. I
plunged it in cold water... still stuck TIGHT. Long story short - I
couldn=92t get through that shell with a chisel or sledge hammer, grrrrrr...=
.
and since I was out of dynamite that pot still sits at the bottom of my
pond. (Snort) Ok, so maybe I mighta fired it a tad hot or had a bit too
much alumina....
<glaze. The glaze covers the clay, no one knows if it is smooth or not, eh?
>>
As mentioned, I=92ve usually used a porcelain slip over my stoneware, under
the glaze, in order to achieve the bright white large crackle pattern that
my raku stoneware doesn=92t give me. I (wrongly) assumed the terra sig might=
give me a similar effect. And I know it might be over-kill, efficiency-
wise, but the look of a copper matt or luster over a really smooooth
surface is especially luscious, for my taste.
Working with clay certainly keeps me from getting too complacent about
life...
Thanks a ton for your help!
Lezlie
Lezlie Finet on mon 2 aug 04
Vince -
you said <Any reduction atmosphere in raku firing will turn the iron to black iron
oxde, which is a powerful flux, and that will cause the terra sig coating
to flake.>>
Are you saying that terra sig=92d pots shouldn=92t be reduced? But isn=92t t=
hat
how pit-fired pots are done... or does the lower temperature and/or
burnishing make the difference? Or?
<>
The bare terra sig=92d areas were as sleek as black satin, but the clear-
crackle glazed areas (over the t/s) had tiny dark pockmarks - larger than
pinholes. Might have been the iron, huh?
Yes, that=92s basically how I measured the specific gravity... weigh plastic=
cup, add 100 grams water, mark, fill with same volume t/s, weigh. But my
accuracy is always open to dispute when I get into one of my empirical
testing frenzies. You see, I was also testing the next lower stratum of my
initial mixture of ball clay, as well as the second decanting of the same
mixture, plus corresponding samples of my throwing clay, to compare the
differences of quality, etc. Kinda like virgin vs extra-virgin olive oils,
oui? By the time I was done there was not a pot nor piece of greenware
(nor discarded shard from my reclaim bucket) that escaped being terra sig=92=
d
that night. heh. Anyway,, I love conducting the experiments, I just
don=92t always know how to interpret the results.
I guess a hydrometer would be handy to have... but what was the dowel &
solder tester thingy Mr. Mel made mention of the other day? Do they BOTH
measure density of liquids? or is Density different than Specific Gravity?
Geeez, so many questions leading to more questions... I=92m afraid I=92m
beginning to reveal my OWN density on such matters.
Thanks muchly for all your help,
G=92night,
Lezlie in WA
sdr on mon 2 aug 04
Lezlie said:
.........So then explain this: the last time I did naked raku
.......slathered on some freshly mixed slip resist
made of fire clay, EPK & Alumina Hydrate (Riggs recipe I think) and fired
it. ......tried chipping off the slip - NO luck. ......
When you make a slip for "naked raku", or ANY slip that
you wish to remove after raku firing - test that slip to see if it
chips off of YOUR clay. It may - it may not. It depends on
whether the slip shrinks enough more than your clay shrinks.
Right? So, test it, and if it does not flake off readily, alter the
slip until it DOES flake off easily. One size does not fit all.
I bet you hate this answer. But - that's what you must do.
regards
Dannon Rhudy
terryh on tue 3 aug 04
Lezlie Finet wrote
>And I know it might be over-kill, efficiency-
>wise, but the look of a copper matt or luster
>over a really smooooth surface is especially
>luscious, for my taste.
after quoting dannon as
>>There is no point, anyway, to go to all that
>>trouble if you are going to glaze. The glaze
>>covers the clay, no one knows if it is smooth
>>or not, eh?
my post may be a little out of this thread.
i may not be able to tell the difference of
lusciousness between over terra-sig and over
porcelain slip.
but, difference between black over terra-sig and
un-terra-sigged surface or over white slip is no
need to explain. and if you want have some black
surface on your raku glazed piece and enjoy the
contrast, you'd probably entertain inefficiency/
efficiency of terra-sigging the surface.
in addition, smoothed surface is much easier to
draw figures/patterns on it.
(dip-and-fire people need not ponder :)
actually, i'm very curious about reaction of ash
on smoothed surface. i'm also ignorant how terra-sig
or burnished surface reacts to cone 11 "yakishime"
firing. it won't stay shiny but does it remain smooth
and how fire color behaves differently, if it does,
from rough shigaraki-like clay surface? as a matter
of fact, i wanted to experiment in anagama firing
this week but alas, i lost my only one terra-sig/
burnished test piece during shipment. (OK, next
chance.) maybe this is a stupid question, as
yakishime takes advantage (or appreciate more),
it appears to me, of surface grain sorting.
terry
forecast 100F tomorrow.
terry hagiwara
terryh@pdq.net
http://www.geocities.com/terry.hagiwara
Ivor and Olive Lewis on tue 3 aug 04
Dear Lezlie Finet,
I must say I have paid little attention to this thread. But it struck
me that Vince's comment seemed to be anomalous or contradictory. You
quote Vince as saying <turn the iron to black iron oxide, which is a powerful flux, and that
will cause the terra sig coating to flake>>
It was my impression (from the information in text books) that because
it is melting at a lower temperature than other materials a "flux"
acts as an adhesive. So "...a powerful flux..." would have this
quality enhanced and we might expect the Terra Sigillata layer would
be more firmly held to the fabric of the pot.
Just a thought!
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Lezlie Finet"
To:
Sent: Tuesday, 3 August 2004 3:35
Subject: Re: Terra Sig + Raku = Pop-Offs??
Vince -
you said <fluxes.
.>>
Are you saying that terra sig'd pots shouldn't be reduced? But isn't
that
how pit-fired pots are done... or does the lower temperature and/or
burnishing make the difference? Or?
<>
The bare terra sig'd areas were as sleek as black satin, but the
clear-
crackle glazed areas (over the t/s) had tiny dark pockmarks - larger
than
pinholes. Might have been the iron, huh?
Yes, that's basically how I measured the specific gravity... weigh
plastic
cup, add 100 grams water, mark, fill with same volume t/s, weigh. But
my
accuracy is always open to dispute when I get into one of my empirical
testing frenzies. You see, I was also testing the next lower stratum
of my
initial mixture of ball clay, as well as the second decanting of the
same
mixture, plus corresponding samples of my throwing clay, to compare
the
differences of quality, etc. Kinda like virgin vs extra-virgin olive
oils,
oui? By the time I was done there was not a pot nor piece of
greenware
(nor discarded shard from my reclaim bucket) that escaped being terra
sig'd
that night. heh. Anyway,, I love conducting the experiments, I just
don't always know how to interpret the results.
I guess a hydrometer would be handy to have... but what was the dowel
&
solder tester thingy Mr. Mel made mention of the other day? Do they
BOTH
measure density of liquids? or is Density different than Specific
Gravity?
Geeez, so many questions leading to more questions... I'm afraid I'm
beginning to reveal my OWN density on such matters.
Thanks muchly for all your help,
G'night,
Lezlie in WA
______________________________________________________________________
________
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
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Snail Scott on tue 3 aug 04
At 11:21 AM 8/3/04 +0930, Ivor wrote:
>...You
>quote Vince as saying <>turn the iron to black iron oxide, which is a powerful flux, and that
>will cause the terra sig coating to flake>>
>It was my impression...that because
>it is melting at a lower temperature than other materials a "flux"
>acts as an adhesive. So...we might expect the Terra Sigillata layer would
>be more firmly held to the fabric of the pot.
I'm not sure that iron would have any significant
fluxing effect at raku temperatures, but if it
did, I'd be inclined to think that the sigillata
would bond to itself and create a shell, but not
have as strong a bond with the clay underneath,
creating a discontinuity between the clay and the
coating which would be vulnerable to unequal
rates of shrinkage in cooling. In fact, the outer
shell of fused sigillata might actually cool more
rapidly while insulating the inner clay core, as
heat might not be conducted as freely across even
a hairline gap induced by discordant shrinkage
rates. Such a discrepancy would tend to feed back
on itself as the outer shell would cool even faster
once it began to separate, and shrink even more
than its underlying clay, widening the gap still
further until full separation occurred.
-Snail Scott
Ivor and Olive Lewis on thu 5 aug 04
Dear Snail Scott,
You suggest <<.. I'm not sure that iron would have any significant
fluxing effect at raku temperatures, but if it did, I'd be inclined to
think that the sigillata would bond to itself and create a shell, but
not have as strong a bond with the clay underneath,...>>
Speculatively I would agree with you because of the moderate
temperature at which the raku might be fired, well below the melting
point of Black Iron Oxide (K & L 1420=BA C). But if black iron oxide is
present, its ability to enter into sintering reactions leading to
strong bonds could commence as low as 750=BA C.
What I could not understand in Vince's information,quote...<reduction atmosphere in raku firing will turn the iron to black iron
oxide, which is a powerful flux, and that will cause the terra sig
coating to flake>>....was that a flux would prevent rather than assist
bonding. This seems to be incongruous.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Snail Scott"
To:
Sent: Wednesday, 4 August 2004 2:36
Subject: Re: Terra Sig + Raku =3D Pop-Offs??
> At 11:21 AM 8/3/04 +0930, Ivor wrote:
> >...You
> >quote Vince as saying <will
> >turn the iron to black iron oxide, which is a powerful flux, and
that
> >will cause the terra sig coating to flake>>
>
> >It was my impression...that because
> >it is melting at a lower temperature than other materials a "flux"
> >acts as an adhesive. So...we might expect the Terra Sigillata layer
would
> >be more firmly held to the fabric of the pot.
>
>
> I'm not sure that iron would have any significant
> fluxing effect at raku temperatures, but if it
> did, I'd be inclined to think that the sigillata
> would bond to itself and create a shell, but not
> have as strong a bond with the clay underneath,
> creating a discontinuity between the clay and the
> coating which would be vulnerable to unequal
> rates of shrinkage in cooling. In fact, the outer
> shell of fused sigillata might actually cool more
> rapidly while insulating the inner clay core, as
> heat might not be conducted as freely across even
> a hairline gap induced by discordant shrinkage
> rates. Such a discrepancy would tend to feed back
> on itself as the outer shell would cool even faster
> once it began to separate, and shrink even more
> than its underlying clay, widening the gap still
> further until full separation occurred.
>
> -Snail Scott
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
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> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
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