Greg Skipper on mon 2 feb 98
My daughter gave my wife and I a bust of herself which she sculpted in
terracota. We love it and have it in our living room. It is green and
the question of firing it to bisque is at hand. It's now been sitting
around at room temp for about 2 months so it's probably about as dry as
it's gonna be. I could fire it in my electric kiln on my slow ramp
setting (50 degrees per hour then hold at 300 for 5 hours then 75
degrees per hour to 1950) This takes about 30 hours. Is there a
better slow fire formula. The other possibility is to not fire it at
all. I really treasure this piece and don't want to damage it. What do
ya'll think would be best?
Thanks
Greg
Oregon
the Gallagher's on tue 3 feb 98
<. I really treasure this piece and don't want to damage it. What do
ya'll think would be best?
Thanks
Greg
Oregon>
Unfortunately at the dry, but solid state you have it in there is a risk.
Slow firing could make it through, or blow up, leaving it unfired there is
always a risk of chipping.
I did a solid bust 25 years ago, in high school. Never fired it, my dad
coated it with (ugh!) gold spray paint and a polyurethane. It still survives,
but has chipped.
Either way you have risk. The choice is yours.
Good luck,
Michelle
In Oregon, where there have been sightings of this strange, glowing ball in
the sky recently!
David Buck on tue 3 feb 98
Greg Skipper wrote:
>
> ----------------------------Original message----------------------------
> My daughter gave my wife and I a bust of herself which she sculpted in
> terracota. We love it and have it in our living room. It is green and
> the question of firing it to bisque is at hand. It's now been sitting
> around at room temp for about 2 months so it's probably about as dry as
> it's gonna be. I could fire it in my electric kiln on my slow ramp
> setting (50 degrees per hour then hold at 300 for 5 hours then 75
> degrees per hour to 1950) This takes about 30 hours. Is there a
> better slow fire formula. The other possibility is to not fire it at
> all. I really treasure this piece and don't want to damage it. What do
> ya'll think would be best?
> Thanks
> Greg
> Oregon
Hi Greg:
Last winter I made a clay oven for baking bread in an Indian
restruant. The Kiln at the Nepean Sportsplex to fire the oven was too
short so I had to fire in 2 sections. The oven was over 28" wide and 1"
to 2" thick when wet. No top or bottom, it took 4 boxes of clay and
about 10 pounds of grog to make. The top, made in 3 sections, had
nearly a month to dry in a warm room. It cracked baddley in a 20 hour
bisk firing. The bottom was fired a few weeks later and come out with
only a few small cracks where it was 2" thick. If your bust is thick
maybe you could delay firing for a year or so.
David B. Buck
Nepean, Ontario
bobbi on tue 3 feb 98
Greg Skipper wrote:
>
> ----------------------------Original message----------------------------
> My daughter gave my wife and I a bust of herself which she sculpted in
> terracota. We love it and have it in our living room. It is green and
> the question of firing it to bisque is at hand. It's now been sitting
> around at room temp for about 2 months so it's probably about as dry as
> it's gonna be. I could fire it in my electric kiln on my slow ramp
> setting (50 degrees per hour then hold at 300 for 5 hours then 75
> degrees per hour to 1950) This takes about 30 hours. Is there a
> better slow fire formula. The other possibility is to not fire it at
> all. I really treasure this piece and don't want to damage it. What do
> ya'll think would be best?
> Thanks
> Greg
> Oregon
Hi Greg,
I fire sculpture often, the question at hand is, Is it solid, if not are
there any places where air could be trapped? How thick is the thickest
point? Can it placed it your kiln, at least 2" from any coil?
In my experience, there is always the chance that a sculpture will blow
up in the fire. But with proper firing of it and production of the piece
the odds of success increase greatly. The best firing is to candle the
kiln for at least 24 hrs. for really thick and large pieces. Longer if
you can. Take the temp. up as slowly as is possible. If places crack or
parts blow off. You can fix them with, fixall, mortar, or anything that
will stick the pieces together. Then use a non firing patina. Paint,
shoe polish, etc. Might want to think through what will stick to the
glue choice you make. Good luck, Bobbi
Don Prey on tue 3 feb 98
Greg,
My assumption is that a greenware bust will eventually get damaged. It is
just too vulnerarble. So I vote for firing. Maybe a single fire to cone 04?
However, your post raises a couple of questions: 1. You said terra cotta, but
that only means a low fire, red clay body.....what we don't know is whether or
not this is actually a sculpture body with considerable grog, or a smooth,
tight body without grog. When it comes to drying out completely, this is
critical. 2. The other question is, how thick is the thickest section
(assuming it isn't solid. if solid, all bets are off)? The concern is that if
this is a tight clay body and is of sufficient thickness it very likely will
have water trapped in the interior and is destined to blow now matter how slow
you go through 212 deg. F.
The other general question I have for the list as a whole: What is the
usefulness of a ramp program that holds for 5 hours at 300 deg. F?
Don Prey in Oregon
Leslie Ihde on wed 4 feb 98
Greg- why not make a cast of your bust and then use that to make a more
fire worthy piece. I'm not the best to give technical info on this, but I
imagine many on this list could.
Leslie
amy parker on wed 4 feb 98
>The other general question I have for the list as a whole: What is the
>usefulness of a ramp program that holds for 5 hours at 300 deg. F?
>Don Prey in Oregon
>
don - that is the program for a crockpot.
amy parker Lithonia, GA
amyp@sd-software.com
Greg Skipper on wed 4 feb 98
It is a red clay and appears to have some grog quality and is hollow. The
thickest part is at the chin and is about an inch thick. Just picking it up
yields a bit of red dust so I appreciate the advice to fire it eventually. I
guess I'll wait a few more months just to be on the safe side. I also would
appreciate advice re the best ramp program for thick sculpture firing. I believ
you're correct re the fact that 300 degrees may be too high for the first hold
stage. I just picked this out of the air, but now that you mention it, it is
pretty obvious that 212 is the critical point. I maybe should hold at 180 or 20
for hours.
Greg
Oregon
Don Prey wrote:
> ----------------------------Original message----------------------------
> Greg,
> ...1. You said terra cotta, but
> that only means a low fire, red clay body.....what we don't know is whether or
> not this is actually a sculpture body with considerable grog, or a smooth,
> tight body without grog. When it comes to drying out completely, this is
> critical. 2. The other question is, how thick is the thickest section
> (assuming it isn't solid. if solid, all bets are off)?
John Rodgers on thu 5 feb 98
Don Prey wrote:
>
> ----------------------------Original message----------------------------
> Greg,
> My assumption is that a greenware bust will eventually get damaged. It is
> just too vulnerarble. So I vote for firing. Maybe a single fire to cone 04?
> However, your post raises a couple of questions: 1. You said terra cotta, but
> that only means a low fire, red clay body.....what we don't know is whether or
> not this is actually a sculpture body with considerable grog, or a smooth,
> tight body without grog. When it comes to drying out completely, this is
> critical. 2. The other question is, how thick is the thickest section
> (assuming it isn't solid. if solid, all bets are off)? The concern is that if
> this is a tight clay body and is of sufficient thickness it very likely will
> have water trapped in the interior and is destined to blow now matter how slow
> you go through 212 deg. F.
> The other general question I have for the list as a whole: What is the
> usefulness of a ramp program that holds for 5 hours at 300 deg. F?
> Don Prey in Oregon
OK, OK, I just have to get my 2 cents worth in here on this!
It sounds like a fairly high risk of losing the piece in firing. If the
original sculpture itself is the most valuable aspect, you probably
will have to risk the firing of it. You could take a different approach,
and consider the work an interim form until it is transfered to a medium
that is more durable in nature, ie, make a rubber mold of it and have a
bronze cast made, or other cold setting material cast, or make plaster
molds, and make pressed clay components of more uniform thickness and
re-assemble then fire. In my own experience this is the approach that I
had to take, because it is not unusual at all to lose the original work
in a firing. I do a lot of slip casting and I consider what the
finished medium is going to be - in most cases fired slip-cast
porcelain, and when the work reaches that stage of completion, then and
only then do I consider the work done. Everything else I consider to be
only steps to arrive at the final work in it's finished medium.
I am often brought objects'd'art and asked to "do something" with it
and "It" can be anything from paper miche' to carved alabaster. Client
expects his "art" to turn out wonderful when it is reproduced, and
doen't expect the original to be damaged. But have I got news!! One does
ones best but it will happen sooner or later, that something will go
wrong, and it will be damaged or even lost entirely. And boy, is that
egg on the face bright yellow when the original is damaged or broken
beyond repair. From hard experience, client gets told up front .... the
process may destroy the original! We try our best.....but can happen. I
rarely ever do a piece any more with out first making a rubber mold of
the original to ensure the ability to replace the form, in the event of
a catastrophic failure somewhere along the way. I emphasize the
permanence of the medium in which the final piece will be done in. And
try to eliminate from consideration those mediums that do not lend
permanence. So, I have been fairly sussessful with that approach, for
others work and my own.
Respectfully suggest you give a close look at this approach.
John Rodgers
Nasty Weather in Alabama
Dave Spangenberg on fri 6 feb 98
In response to Greg Skipper's query-
I have been firing sculpted terracotta heads for more than 10 years - 15 to
20 heads every three to four months for a sculpting class.
I allow the wall thickness to get up to about 2 inches thick using
Cambell's Red Low Fire Clay with Grog or Standard's 104. Air drying time
is a minimum of 3 weeks, any doubt on moisture content gives the head a
position on top of a kiln while going through a firing cycle. Beyond a
month drying time in open air with relative humidity less that 80%, I
consider a waste of time. The steam from retained moisture when "dry" and
the steam from the dehydration process (from clay - a hydrated material) as
well as other gases released provide the mechanism for blowing them up. I
would not consider firing a solid clay head (6 to 8 inch thickness} made
with these clays. Double the grog and I could fire them solid.
My current firing profile (in Celcius) that I believe is conservative and
very safe for my circumstances:
Heat at 100C/Hr to 95C - hold for 6 hours
Heat at 100C/Hr to 750C - no hold
Heat at 200C/Hr to 1000C - no hold
Heat at 20C/Hr to 1025C - Finish point Cone 05
I would go faster than the 200C/Hr above 750C but that is about the maximum
rate the kiln will go at 800C; setting the controller higher achieves
nothing. Also note that the finish point was 1004C before I replaced the
thermocouple (end point needs to be calibrated for your kiln and
controller).
Dave Spangenberg 75463.1027@compuserve.com
Janet H Walker on fri 6 feb 98
...best ramp program for thick sculpture firing...
I had a sculptor bring something over to be fired. It was a completely
solid figure about 6-8 inches through at the widest and with various
appendages dangling off. Nice piece. He had dried it on top of his
wood stove for a week. Yikes I said. But he said, oh just try it; it
isn't that important.
So I fired it to 04. I could go look up the firing log but it is 50
miles from here and I mostly remember probably. The bottom line is
that I fired it slower than i do my usual loads of pots but not that
much slower. It might have been 6 hours to 120C (about 250F) and then
hold about 2 hours then take it up the usual sort of speed. I do 50C
from 700C to and then soak for one hour.
To my amazement, the piece was fine.
This all happened before I read important things like Lawrence and
Norton -- the ceramic science texts that explain about drying, porosity,
firing speed, ceramic change, and all the stuff that goes on during a
firing. A long hold after you get well above the boiling point of
water gives the water vapor a chance to make it through all the inter
particle spaces to get to the surface.
Up until i read the texts, I had no real knowledge to go on except that
"everyone" seemed to say "The thicker it is the longer it has to wait
before firing and the less likely it is to fire without exploding."
I now believe that opinion is the result of master-to-apprentice word
of mouth transmission of practise, absent the reasons. Obviously the
people who fire big thick toilets don't wait months for them to dry.
So there must be more to this than gets explained in most studio art
courses on clay. I recommend some reading on the topic of clays,
drying, and firing to find the combination that best fits what you
want to be able to do. (Like fire the piece before the millenium.)
Regards,
Jan Walker
Cambridge MA USA
Janet H Walker on tue 10 feb 98
Back to the "can I fire this or will it blow up" question with
respect to solid sculpture busts. A few days ago i reported
having fired a solid bust that had been dried over the sculptor's
wood stove for a week. Here is the actual firing schedule that
I used with a controller and the piece did not blow up:
Temp rise
per hour to Temp Hold (time)
40C 120C 2:00 [this gives roughly 5 hours warmup]
100C 500C 0 [let's get it going]
50C 600C 0 [slowly while it changes volume]
100C 700C 0 [why not zip through this part]
50C 900C 30 [slowly while it burns out organics]
100C 1000C 30 [getting impatient now, to ^05]
A lot of this fiddling around between 50/100C for the temperature
rate increase simply is a result of having the capability to do so.
It doesn't do anything important. The whole firing could as well be
done at 50C/hr. (Give a hammer to a 10-year old and everything
looks like a nail; give a computer controller to a geek...)
Whether or not this works for any particular sculpture will have as
much to do with
- what kind of clay it is (this looked like some kind of terracotta)
- how it was constructed
- how it was dried
That means the details of the construction and drying phases. For
most clays, if the clay isn't all the same moisture content or else
very thoroughly joined together, it won't stay together when you
fire it. If the clay doesn't dry evenly, then stress cracks inside
the piece will open up when it fires. A piece that sits on a wooden
shelf, for example, doesn't have a hope of drying evenly unless it
is covered for a long time. Dry it on newspaper or fabric on
drywall and it can be allowed to go much faster because moisture can
get out of the base directly. The piece over the woodstove probably
had roughly the same atmosphere all around it and so it dried
evenly.
For the technical ceramics types, this is simplistic I know but it
is correct enough to provide the other side to the "it has to dry
forever" coin.
Jan Walker
Cambridge MA USA
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