Peter Jackson on sun 10 nov 96
Salt-Glaze Production Problems
We are firing cone 8-9 salt-glaze, striving for a grey body, with blue
cobalt decorations. Recently, our firings are coming out with the clay more
of a brown tone. We have made some changes in the body to eliminate as much
iron as possible, but still have more of a tan than a grey. What is the
common wisdom out there on salt-glaze in reduction and oxidation? In my
experience, there have been firings where we have kept the reduction as
light as possible, (starting at about cone 08, with body reduction, keeping
a steady, light reduction all the way up. Steady, light in our terms are
flame coming out of the peep holes in "wisps" of 6-8 inches. As you can
tell, we do not have any sophisticated measuring devices for fuel/oxygen
ratios, etc.) the loads have een more grey and blue sometimes, and other
times more tan and black. Any thoughts from fellow salt-glazers would be
appreciated. Any ideas on making the blue bluer through using some other
sodium source that sodium chloride? Borax? Sodium Bicarb? Any surefire
recipes for a grey salt-glaze body? Help!!
Thanks,
Peter Jackson
ROCKDALE UNION STONEWARE, INC.
Richard Burkett on mon 11 nov 96
Peter at Rockdale Union Stoneware asks:
>We are firing cone 8-9 salt-glaze, striving for a grey body, with blue
>cobalt decorations. Recently, our firings are coming out with the clay more
>of a brown tone.
I'm amazed that you guys have been salt firing for so long and haven't
figured this out!
First, why are you firing with a standard cone 10 reduction schedule which is
aimed at producing glazes that look like classic Japanese stoneware? This
type of firing, with the early "body" reduction starting at cone 08-06 will
almost always produce largely brown salt glazes on iron-containing stoneware
clays unless you salt a huge amount or quick cool the kiln at a dangerous
rate.
I'd suggest a couple of things if you're trying to reproduce antique
salt-glazed ware (which, last I knew, you were). I spent 12 years of doing
salt-glazed pottery for a living, and although I wasn't trying to make
historical reproductions, I did try to figure out how to get those surfaces.
First, if at all possible, fire to cone 8, tops, using a fairly low-iron,
nearly-vitreous stoneware clay body. More open bodies with lots of coarse
fireclay and little feldspar to help them vitrify will be tend to always salt
brown. Iron oxide is much more reactive at higher temperatures above cone
7-8. My guess is that a lot of the old ware was fired in the cone 6-8 range
by visual cues and draw trials, not cone 10 as the goal, although those big
old kilns were notoriously uneven.
Second (and this may help you out if you want to keep firing to cone 10),
DON'T do the classic body/glaze reduction which starts at cone 08. Rather try
to fire in oxidation to more like cone 4-6, or maybe even cone 8, then
reduce. The amount that you're reducing sounds like it would be appropriate.
This will help to minimize the amount of iron in the glaze, but should still
give you a a nice gray clay/glaze combination as long as you've got the clay
reduced before you start salting. You'll probably get a clearer blue color
from the slip trailing, too. You didn't mention what you're using for blue or
how you apply it, so it's impossible to give any substantial suggestions
here. Salt would be my preference to get a classic gray glaze if
environmental problems with the chlorine are set aside. Personally, now my
choice would be to use powdered sodium bicarbonate blown into the kiln.
Lastly, avoid long, slow cooling, as that will allow the iron to reoxidize
to red iron oxide - the source of the brown color. Crash cooling shouldn't be
necessary, but letting the kiln cool rather quickly a few hundred degrees
right after the firing should help to keep the color gray, and the glazes
bright. (Note to others: This quick cooling is specific to salt/soda firing,
by the way and not a particularly good idea for regular stoneware firings,
especially if you have matte glazes!)
Hope this helps.
Richard
Richard Burkett - School of Art, Design, & A.H, SDSU, San Diego, CA 92182-4805
E-mail: richard.burkett@sdsu.edu <-> Voice mail: (619) 594-6201
Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
CeramicsWeb: http://apple.sdsu.edu/ceramicsweb/index.html
Marcia Selsor & Matt Benacquista on mon 11 nov 96
Dear Peter,
When we use to fire ^9-10 salt in Phila., we took a few rows of bricks
down from the door after we finished salting. Then when the ware
and atmosphere cooled to red heat we replced the bricks with a kiln
shelf to block any drafts. This is a post firing oxidation, I guess.
Marge Levy, at Pilchuck taught us this method which she picked up in
North Carolina. MARGE, if you're out there, please verify that no harm
was done to the pots.
It works.
Marcia in Montana
Peter Jackson wrote:
>
> ----------------------------Original message----------------------------
> Salt-Glaze Production Problems
>
> We are firing cone 8-9 salt-glaze, striving for a grey body, with blue
> cobalt decorations. Recently, our firings are coming out with the clay more
> of a brown tone. We have made some changes in the body to eliminate as much
> iron as possible, but still have more of a tan than a grey. What is the
> common wisdom out there on salt-glaze in reduction and oxidation? In my
> experience, there have been firings where we have kept the reduction as
> light as possible, (starting at about cone 08, with body reduction, keeping
> a steady, light reduction all the way up. Steady, light in our terms are
> flame coming out of the peep holes in "wisps" of 6-8 inches. As you can
> tell, we do not have any sophisticated measuring devices for fuel/oxygen
> ratios, etc.) the loads have een more grey and blue sometimes, and other
> times more tan and black. Any thoughts from fellow salt-glazers would be
> appreciated. Any ideas on making the blue bluer through using some other
> sodium source that sodium chloride? Borax? Sodium Bicarb? Any surefire
> recipes for a grey salt-glaze body? Help!!
>
> Thanks,
>
> Peter Jackson
> ROCKDALE UNION STONEWARE, INC.
--
Marci Selsor
Matt Benacquista
http://www.imt.net/~mjbmls/
mjbmls@imt.net
Jack Troy on mon 11 nov 96
Typing this post is giving me something to do while waiting for the cones in our
salt kiln at Juniata College to go over:
A few things come to mind regarding the problems you're having with
"braunstein," the murky, chocolatey look of stoneware that appears over-reduced
and under-salted. (Typically, also, the cobalt slip lacks brilliance).
A lot of the 19th century wares from western PA, West Virginia, Kentucky, and
parts of the South have these qualities, compared to the stonewares of New York
State, and New England, which are generally much brighter and probably embody
the values you're seeking.
1. The body itself is critical, and should be low in iron-bearing clays. When
working in this mode, we've never added any Redart, or similar clays, for
example. Even so, you may need to lighten the body you're using if you like its
working properties. Try adding 5-25% kaolin in increments of 5% and check the
fired tests using your present slip and firing schedule. (Naturally, the more
kaolin you add, the less the body will feel like its familiar self).You might
also consider slipping the pieces with a terra sigilatta that fires well when
applied to the body you're using. (many times the finer particle sizes will
accept vapor glaze better). All of this may involve major changes in your
production, so perhaps look at the firing schedule first.
2.We've always avoided any murkiness in the kiln while firing up, since salting
itself reduces the body sufficiently for what we're after, and as c/8 is about
half way over we usually add some slivers of wood prior to salting - just enough
to avoid any black smoke coming from the kiln. The long wood flame helps
distribute the vapors and also flashes the pieces nicely on a good day.
3.Your message didn't mention your cooling cycle, which is critical to both body
color as well as the brightness of the cobalt slip. Usually the slower the kiln
is cooled - especially at the upper end of the cycle - the more prounounced will
be the dark colors that are vexing you. An hour or so after you've stopped
firing and sealed up the kiln pull out some of the top door-bricks for a couple
of hours or more. Close it up again before it's lost red heat.
4. You might think about brightening up the slip, too, with some frit.
(Left-brainers fasten seat-belts here). We add a handful of frit to a 2000 g.
batch of our Old Timey Slip formula, and the frit closest to the scales always
seems to work. (I can see the Math Police suiting up in riot gear at this
point).
Old Timey Slip:
50 Grolleg
25 flint
25 potash spar
4 cobalt carb
2 yellow ochre
Here's another blue slip being used by a pottery that's making ersatz 19th
century wares:
300 g. dry clay body
150 silica
120 nepheline syenite
30 cobalt carb
16 Redart
450 g. water for brushing
300 g. water for slip-trailing
Good forms; good firings,
Jack Troy
SLPBM@cc.usu.edu on mon 11 nov 96
As to how to get your clay body more grey-
The presence of potash in your feldspar will aid in getting grey.
Titania or magnesia will tend to make the clay body more brown.
Cooling will also effect this.
Reduction before shutting down also promotes the grey you are looking for.
Alex Solla
slpbm@cc.usu.edu
Tracy Dotson on tue 12 nov 96
Peter.....If you find that over a period of time, glaze and body color may
change gradually because your chimney bricks have been moving around letting
in a little more outside air than before. some times just plugging the
cracks will help or if it is dry stacked just rebuild it. This brick
movement also applies to the floor allowing air to come through the brick.
I usually apply a 1 to 2 inch coat of a high alum castable on top of the
floor if I have a problem. This will eventually crack but can be easily
repaired.
Good luck
Tracy
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