Steven Branfman on wed 17 jan 01
Friends,
I won't make this long. There is really no need to argue about the fragility
or functional use of raku. Due to the nature of the firing process raku is
both fragile and not suitable for functional use. Do I sound didactic?
Whether or not one speed cools the ware in water, the fast firing and
relatively fast cooling of the ware does indeed impart a fragility to the
pots. Yea, yea, some raku is more fragile than other
raku................details, details. Vince is certainly correct about copper
glazes and when he says that crazing in and of itself is not cause for alarm.
But in the context of raku, the glaze surface is fragile and repeated
utilitarian use will cause glaze to flake and the clay body will absorb food
and liquid. Why subject yourself to this? How about the occasional/ceremonial
functional use of raku? I would say not to worry. However, I tell everybody
that buys my work that it is fragile and not to be tempted to use the pot in
a utilitarian context other than to hold or possibly serve dry foods.
Anybody want to fight?
Steven Branfman
The Potters Shop
31 Thorpe Rd.
Needham MA 02494, USA
781 449 7687
fax: 781 449 9098
Paul Gerhold on fri 19 jan 01
Steve,
Sure I'll fight but first let's define the arena. When discussing strength it
seems to me there are actually two separate discussions. The first is
fragility of the glaze and second is fragility of the ware minus glaze.Now I
will readily concede that Raku will never have the glaze/clay interaction
that high fire work has and therefore glaze on Raku will be fragile and not
very suitable for functional ware.
The more interesting discussion and perhaps more relevant to those of us who
lean toward sculpture is the strength of the piece itself. It is my
contention that with a minimum of effort Raku can at least be as strong as
most earthenware and probably as a lot of the stoneware out there. Things to
bear in mind.
1. Raku can be bisqued a lot higher than the Raku firing temp. giving much
additional strength.
2. You can formulate a body that is vitrified and yet porous which will
withstand Raku firing.Now vitrified may not be the right technical term but
you know what I mean- a body that is glassy but porous.
3.I see discussions on the effects of slow cooling which some people call
tempering. My question for clay guru's would be does the physical state of
the clay actually change much during slow cooling or is the reason for slow
cooling to avoid introducing stresses in the body. Does slow cooling create
bonds at the molecular level or is slow cooling primarily to reduce effects
of quartz inversions etc.
4. Final question is does anyone out there have the equiptment to test for
strength or is this all subjective anyway.
Yours for stronger Raku. Paul
Steven Branfman on sat 20 jan 01
Paul and others,
Here's my stance on this fragile raku issue; Determining the actual
"strength" and fragility of raku relative to other clays and firing methods
doesn't interest me in the least. I have no plans to produce utilitarian raku
nor do I plan on making monumental raku sculpture for public outdoor
placement. If the issues of strength and utility, food safe and function, are
of importance in your work then by all means go for it. But for me, this
issue takes me way beyond what the raku method, process, and technique mean
to me and my work.
Yours in constructive and respectful discussion,
Steven Branfman
The Potters Shop
31 Thorpe Rd.
Needham MA 02494, USA
781 449 7687
fax: 781 449 9098
vince pitelka on sat 20 jan 01
Paul -
It seems to me that your points here are irrelevant. No one does these
things. I mean, you might, but no one else does. You are not talking about
how fragile raku is, you are talking hypothetically about how strong it
could be, and that has nothing to do with this. We are speaking in general
terms, and most raku ware is much more fragile than earthenware, and it
never, ever, achieves close to the strength of stoneware.
> 1. Raku can be bisqued a lot higher than the Raku firing temp. giving much
> additional strength.
That would add strength, but it would eliminate the thermal shock resistance
you need for raku firing.
> 2. You can formulate a body that is vitrified and yet porous which will
> withstand Raku firing.Now vitrified may not be the right technical term
but
> you know what I mean- a body that is glassy but porous.
No, I don't know what you mean. In my experience there is no such thing as
a body which is glassy and porous, unless it is foaming from flux
volitization. And if there was, it would have very low thermal shock
resistance, because the porosity would not do any good unless it was present
at a a particulate level, such as it does in low-fired or bisque-fired clay.
> 3.I see discussions on the effects of slow cooling which some people call
> tempering. My question for clay guru's would be does the physical state of
> the clay actually change much during slow cooling or is the reason for
slow
> cooling to avoid introducing stresses in the body. Does slow cooling
create
> bonds at the molecular level or is slow cooling primarily to reduce
effects
> of quartz inversions etc.
Slow cooling is relative. I think people were just referring to the
practice of leaving the wares in the post-firing smoking chambers instead of
quenching them in water. That constitutes slow cooling in comparison to the
quenching, but it is still pretty quick cooling in comparison to the
annealing period in a normal cooling ramp in other kinds of firings. So
there would still be considerable stress introduced unless the claybody is
porous and highly thermal shock resistant.
> 4. Final question is does anyone out there have the equiptment to test for
> strength or is this all subjective anyway.
Testing laboratories have the equipment - sensitive hydraulic presses which
measure modulus of rupture or whatever they call it. But it seems to me the
best tests are in the home, the kitchen, the dining room. Raku wares in
functional use don't last very long. Stoneware, porcelain, and well-fired
earthenware do.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dekalb.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
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