rballou@mnsinc.com on sun 1 mar 98
Unfortunately, not much mystery here. Most of the glaze books don't say
anything about zinc in reduction.Books published in the early 70's (when I
first studied glaze calculation) include zinc as a flux for both oxidation
and reduction. Rhodes' _Clay and Glazes_ is a prime example and it has
long been considered the Bible for glaze formulation. Conrad and Chappell
include reduction glaze recipes with zinc. Parmelee says while zinc easily
reduces to metal, it is useful for starting the melt in high fire porcelain
glazes. Even recently published glaze books, such as the new one by Cuff,
make no mention of the incompatibility of zinc and reduction firing. _Out
of the Earth, Into to Fire_ makes it sound as if there is some debate over
the usefulness of zinc in reduction. The only two references that give a
complete description of zinc oxide are Hamer and Currie. And not only does
zinc vaporize, it is released as a poisonous gas.
It wasn't until a few years ago that I learned about zinc. I was trying to
help a friend with a problem and looked everywhere I could for help. Hamer
provided the solution. For a while after that, I mixed 2 batches of each
zinc containing reduction glaze I had. I made no calculation changes, just
omitted the zinc. I figured since the zinc was vaporizing, it was adding
nothing to the glaze. In every case, without exception, the glazes either
improved (no pinholing, pitting, no lumps under the surface) or showed no
difference in cases where there were no defects to begin with. To further
prove to myself that the zinc was vaporizing, I put a teaspoonful on a tile
in a firing. Nothing remained on the tile. Given the poisonous nature of
the gas, as well as the surface problems zinc can cause, I don't buy the
usefulness of "starting the melt" with zinc in a reduction glaze. The zinc
is probably gone before any melting of the glaze. I guess this is just
another reminder to check all the sources you can. Needless to say, Clayart
is fantastic!
Ruth Ballou
rballou@mnsinc.com
> I believe thois glaze will work well
>without the zinc - when zinc is reduced - (and it is very easy to reduce)
>it gets to be the metal - no oxygen left - zinc boils (vaporizes) at around
>400C so I don't see the point. Anyway you might like to try all three - and
>report back to the list - #1 the original, #2 with more CaO and no BaO and
>#3 with no zinc.
>
>I don't know how this ZnO in reduction glazes got started - anybody have
>any history for us?
Nils Lou on sat 14 aug 10
One reason to not use zinc in a glaze to be fired in reduction
is that, as noted, the oxide reduces early to a metallic form
and vapor (not good to breathe), and is deleterious to the
oxygen sensor on O2 probes. Shortens the life considerably.
nils lou,
Louis Katz on sun 15 aug 10
I believe Zinc Oxide like many other chemicals with high melting points d=
=3D
oes
have an impact on melting temperature. Calcium Oxide lowers the melting
point of silica significantly, so does alumina. Taken together they lower=
=3D
it
even more. Zinc oxide however because it increases the viscosity of the
glaze melt slows the process as it reduces the temperature.=3D20
He is an example from the archives:
----------
Mary, here's a glaze you might like for comparison. It's from a book
by John B. Kenny:
Bristol Glaze, cone 4-8
Zinc oxide - 7
Whiting - 10
Feldspar (potash or soda) - 68
China clay - 8
Flint - 7
Marian
Neon-Cat=3D20
------------
The thread on zinc has occurred in various forms over the years. Zinc cou=
=3D
ld
serve many purposes in a glaze even if nearly all of it disappears.=3D20
In copper red glazes it could by promoting an early melt help protect cop=
=3D
per
from oxidation at midrange temperatures. There are lots of questions I h=
=3D
ave
about this, but most of the copper reds that call for it seem to be from =
=3D
the
70's, a time when people routinely aimed for what they called "neutral" f=
=3D
ire
in the midrange. I think we have become more subtle in our firing skills
since then.
As it escapes the glaze, boiling zinc could help stir the glaze, although=
=3D
I
am not sure that it does this.
Metalic zinc may have the ability to scavenge oxygen from copper oxides. =
=3D
I
don't know. It is a question for someone who listened better in chemistry=
=3D
and remembers.
Given all that, and the expense of zinc, It make sense to test copper red=
=3D
glazes without zinc.
David Hendley on sun 15 aug 10
I agree with Louis: Zinc oxide can indeed have an effect on
glazes fired in reduction. I know that theoretically it is vaporized
in the firing, but I'll go with observation over theory, and I have
tested and used glazes that yield different results with and without
zinc in the recipe.
In the 90's, I did extensive testing of a copper red glaze and
systematically removed the zinc oxide as well as the barium
carbonate, both leaving them out and replacing them with
more calcium or strontium. I fired all the variations over 6
firings. I could not detect any consistent differences in the
formulae. There were variations, but they were traceable
to the firing, not the glaze composition. The results were
published under the title "Simply Red" in the October 1999
issue of Ceramics Monthly.
Some of my other glazes, however, are changed if the zinc
oxide is removed from the recipe. Like Louis, I don't know
what role the zinc plays in these reduction glazes, but it is
doing something.
Don't just remove the zinc oxide from reduction glazes - as
always in glaze formulation, test first!
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
I believe Zinc Oxide like many other chemicals with high melting points doe=
s
have an impact on melting temperature. Calcium Oxide lowers the melting
point of silica significantly, so does alumina. Taken together they lower i=
t
even more. Zinc oxide however because it increases the viscosity of the
glaze melt slows the process as it reduces the temperature.
The thread on zinc has occurred in various forms over the years. Zinc could
serve many purposes in a glaze even if nearly all of it disappears.
In copper red glazes it could by promoting an early melt help protect coppe=
r
from oxidation at midrange temperatures. There are lots of questions I hav=
e
about this, but most of the copper reds that call for it seem to be from th=
e
70's, a time when people routinely aimed for what they called "neutral" fir=
e
in the midrange. I think we have become more subtle in our firing skills
since then.
As it escapes the glaze, boiling zinc could help stir the glaze, although I
am not sure that it does this.
Metallic zinc may have the ability to scavenge oxygen from copper oxides. I
don't know. It is a question for someone who listened better in chemistry
and remembers.
Given all that, and the expense of zinc, It make sense to test copper red
glazes without zinc.
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