Craig Martell on tue 1 may 01
Hello Bjorn:
From what I've seen, it's pretty much an impossibility to stop crazing on
the outsides of soda and salt fired pieces unless you intoduce only minute
amounts into the kiln and I'm talking about the total amount of introduced
soda and salt. Not minute amounts in the interval sense.
Sodium is probably the highest expansion oxide that potters use and it's
impossible to predict what pot is going to receive what amount of sodium in
a vapor kiln. The variables are not totally controlable. But, you can
minimize the crazing by reducing the fluxing oxides in the glazes and
adding more alumina. You might even want to add it as alumina oxide
because kaolin will also contribute silica which will attract more
sodium. It has been said too that magnesia resists a salt glaze so perhaps
you could try adding talc and dolomite or magnesium carb to some of your
glaze. Iron also inhibits sodium glazing to a degree but you may not want
to add this for color reasons.
regards, Craig Martell in Oregon
=3D?iso-8859-1?Q?Bj=3DF6rn?=3D Straube on tue 1 may 01
Hi All,
I'm having some problems with crazing in our salt/wood kiln and our
gas/soda kiln. The problem I foresee is that sodium is fluxing out
glazes more in some area than others, and thus the expansion rate is
higher in some areas than others ( if I understand this process
correctly). Even on different sides of the same pot the craze network
is much finer where sodium interactions are the greatest. Now my gut
reaction is that I need to add more silica, I plan on doing a line
blend with Flint and the base glaze. My concerns are two fold. First
how can I compensate for the fact that different areas of the same pot
(let alone the whole kiln) are likely to need different levels of silica
in order to "fix" the glazes? Secondly I suspect that the amount of
silica reacquired may be beyond the limits and or will overly effect the
glaze appearance. I don't know if the second is an issue or not, I
don't own any glaze calculation software (yet). I've listed one of our
glazes below, we fire ^10 wood or gas.
Thank you in advance
Bj=F6rn Straube
Melbourne, FL
Mark's Temoku ( I don't know if this is the original "mark's" or not)
^10 Reduction
Custer 45
whiting 17
EPK 11
Flint 27
----------------------
Red Iron Oxide 10
Bentonite 2
Alex Solla on mon 7 may 01
Couple of things I would like to add to Craig's post
on this idea of crazing in salt/soda:
I did quite a bit of playing with slips and glazes and
claybodies in salt/soda in grad school. As Craig
pointed out, Magnesia does tend to not craze... what
you do get though is a really nice matt to satin
surface, sometimes with really beautiful crystals,
microcrystaline. Dolomite or Mag Carb work in slips to
help towards this. As body fluxes though they are
pretty strong. They can tighten a body up pretty fast,
resulting in a sharp slumping threshhold. Not fun. But
they can also make a dense body really sing.
Tightrope.
The other comment I wanted to throw in is that
Magnesia in bodies, & slips can really change colors.
When you consider that most claybodies are
contaminated if not filled with iron, we are always
looking at the interaction between iron and Mg. In
soda and salt, a body fluxed mainly with spars tends
to be greyish under the layer of glass. Magnesia can
bend the color spectrum to reds, yellows, and browns.
Nice brown pots.
A nice place to start is with your current claybody
mixed as a slurry minus the body fluxes, then add
magnesia of your choice, brush, dip or pour on. Fire.
Nice results to be had from it.
Good luck.
-Alex Solla
--- Craig Martell wrote:
> Hello Bjorn:
>
> From what I've seen, it's pretty much an
> impossibility to stop crazing on
> the outsides of soda and salt fired pieces unless
> you intoduce only minute
> amounts into the kiln and I'm talking about the
> total amount of introduced
> soda and salt. Not minute amounts in the interval
> sense.
>
> Sodium is probably the highest expansion oxide that
> potters use and it's
> impossible to predict what pot is going to receive
> what amount of sodium in
> a vapor kiln. The variables are not totally
> controlable. But, you can
> minimize the crazing by reducing the fluxing oxides
> in the glazes and
> adding more alumina. You might even want to add it
> as alumina oxide
> because kaolin will also contribute silica which
> will attract more
> sodium. It has been said too that magnesia resists
> a salt glaze so perhaps
> you could try adding talc and dolomite or magnesium
> carb to some of your
> glaze. Iron also inhibits sodium glazing to a
> degree but you may not want
> to add this for color reasons.
>
> regards, Craig Martell in Oregon
>
>
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