Snail Scott on mon 3 jan 11
On Jan 3, 2011, at 12:27 PM, Nancy Spinella wrote:
> I've heard of a lot of potters firing their ^6 and up ware to cone 06-04
> first...but I'm not sure if I should do that. Obviously I want functiona=
l
> pieces to be fully vitrified, so that they're strong. But does the
> vitrification occur only on the first firing, or if it's fired to cone 06
> first will it fully vitrify when later fired to cone 6 in a glaze firing?
The vitrification happens almost entirely in the final firing.
There is relatively little vitrification occurring when a
stoneware body is fired to earthenware temperatures
(the usual range for bisque firing regardless of the clay
type), usually not even enough to cause measurable
shrinkage. You could in fact fire directly from greenware
to ^6 and get identical vitrification results to what you'd get
if you bisque fired first.
The custom of bisque firing to a low temperature arises
for several reasons. One is that it sinters the clay and
prevents it from turning back into mud if the glaze water
penetrates too far, but leaves the surface absorbent enough
to accept the glaze well. Applying glaze to a vitrified surface
is quite tricky by comparison. Earthenware never stops being
absorbent, so there is no reason not to fire all the way to
vitrification in the bisque firing. With stoneware, it's very
seldom done, mainly because of the lack of absorbency
for glaze application. Another reason not to fire to maturity
during the bisque firing (if you need another) is that you
would have to pay the firing cost of two high-temp firings,
which will cost more than one low-temp firing and one high.
So, some reasons to bisque-fire:
1. Clay won't turn to mud if heavily re-wetted,
2. Clay is durable enough to endure handling during glazing,
3. Clay has its impurities burned out before glazing,
4. So you can scrub the glaze off and start over if needed.
Some reasons to bisque to low temps instead of high:
1. It's cheaper,
2. Clay stays absorbent for glazing,
> Is
> vitrification a cumulative effect?
Only if you get hot enough to actually cause some
vitrification to occur. For a ^6 clay, you will need to fire
above ^4 or 5 for even multiple repeated firings to
cause an noticeable increase in vitrification. Even
re-firing to ^6 repeatedly will cause only slight
increases in vitrification. Sometimes that's enough to
make a difference in gaze fit or warping, or maybe
not, depending on the clay body in question. Multiple
low-temperature firings won't cause any variation in
vitrification, though - it's too far below the temperature
at which most vitrification occurs in stoneware clays.
Nothing much melts at those lower temps.
> ... I'm also planning to do
> quite a few unglazed pieces like orchid pots. Should I just fire to cone =
6
> and call it done?
You could. Note the list of reasons to bisque, though,
and decide if any of them apply. Even without glaze, a
bisque firing will allow you to apply engobes or an oxide
wash and mess around with them, without messing
up your object's surface. Also, I often bisque-fire if I
plan to have a lot of small parts or a series of pieces
accumulating around the studio before the vitrification
firing - it reduces the chances of accidental breakage
while sitting on the shelves (or wherever...) Another
reason to bisque is if you are doing very precise
surfaces that would benefit from sanding, grinding or
drilling before vitrification makes the clay too hard to
make these processes practical, when those actions
might cause breakage if performed on greenware.
This is sort of a special-circumstance thing, though,
and not a common practice.
Single-firing can be a very efficient way to work, though,
and it's not just for unglazed work. You can apply most
glazes directly to greenware and fire directly to
vitrification with minimal changes in glazing technique.
It's definitely worth considering.
Hope this clarifies more than it obscures!
-Snail
Nancy Spinella on mon 3 jan 11
I want to start off noting that my background is in cone 04-05, which is
easy: You bisque there, glaze there, no question about it. But then I picke=
d
up some cone 6 clay in the hopes of doing some functional ware, and realize=
d
I wasn't sure about the firing process.
I've heard of a lot of potters firing their ^6 and up ware to cone 06-04
first...but I'm not sure if I should do that. Obviously I want functional
pieces to be fully vitrified, so that they're strong. But does the
vitrification occur only on the first firing, or if it's fired to cone 06
first will it fully vitrify when later fired to cone 6 in a glaze firing? I=
s
vitrification a cumulative effect?
I love the color (Standard's hazelnut clay), so I'm also planning to do
quite a few unglazed pieces like orchid pots. Should I just fire to cone 6
and call it done?
(It's all purely academic at this point, since my kiln is STILL not yet
working even after totally replacing the kiln sitter, but eventually I will
get it up and running. Anyone got a spare working control box for an L&L J1=
8
they don't want??)
Thanks in advance!
Nancy
--
www.twitter.com/nanspins
www.wix.com/nanspin/pottery
William & Susan Schran User on mon 3 jan 11
On 1/3/11 1:27 PM, "Nancy Spinella" wrote:
> I've heard of a lot of potters firing their ^6 and up ware to cone 06-04
> first...but I'm not sure if I should do that. Obviously I want functiona=
l
> pieces to be fully vitrified, so that they're strong. But does the
> vitrification occur only on the first firing, or if it's fired to cone 06
> first will it fully vitrify when later fired to cone 6 in a glaze firing?=
Is
> vitrification a cumulative effect?
Yes, it is essentially a cumulative effect of heat and time, but
vitrification occurs when the clay has been exposed to high enough heat
having particles coming close together and much of the spaces between them
filled with material that has melted ( a very simple explanation).
The bisque firing at 06/04 is only to strengthen the ware to allow easier
handling while applying glaze and burning out organic materials that might
be trapped under a melted glaze causing defects, yet still porous enough to
absorb water from the applied glaze.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Lee on tue 4 jan 11
On Mon, Jan 3, 2011 at 10:22 PM, Snail Scott wr=
=3D
ote:
> Single-firing can be a very efficient way to work, though,
> and it's not just for unglazed work. You can apply most
> glazes directly to greenware and fire directly to
> vitrification with minimal changes in glazing technique.
> It's definitely worth considering.
Yes it can. There is a great misunderstanding about having to bisque
a little higher "because of organics." Actually, in single fire,
you deal with organics by firing slowly in the beginning, like you
would with a bisque firing.
I have singled fired clay that I have wedged rice husks,
rice, flour, coffee grounds and sawdust into, with absolutely no
problems related to "burning out the organics."
--
=3DA0Lee Love in Minneapolis
http://mingeisota.blogspot.com/
=3DA0"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D9=
7that is, =3D
"The
land of eternal youth is behind the house, a beautiful land fluent
within itself." -- John O'Donohue
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