Lesley Alexander on sun 12 apr 98
Several entries have suggested zinc oxide is not useful in copper
reds. Is there a suggested substitute? Or just not include it? Two recently
posted copper red glazes from McWhinnie include a few % points of zinc. Of
course one could try a line blend! Is there an easier resolution?
David Hendley on mon 13 apr 98
Lesley, all my experiments indicate that slight variations
in the formula for a copper red glaze are not nearly as
important as the way it is fired.
It doesn't really matter if the zinc oxide is volitlized, as some think,
or adds flux to the glaze, when you are speaking of just a couple of
%'s of the total glaze (in my thoughtful opinion).
Mix up tests of the recipes,
1. with the zinc oxide,
2. without the zinc (just leave it out),
3. without the zinc, but increase the other fluxes slightly to keep the
flux/alumina/silica ratio the same.
Be sure to fire the 3 test tiles right next to each other, and, even better,
in several locations in your kiln, and over several firings.
I have done this kind of testing, and concluded that all three variations
make an equally good copper red. There is much greater variation between
different firings and different locations in the kiln than between the
different recipes.
David Hendley
Maydelle, Texas
At 12:18 PM 4/12/98 EDT, you wrote:
>----------------------------Original message----------------------------
> Several entries have suggested zinc oxide is not useful in copper
>reds. Is there a suggested substitute? Or just not include it? Two recently
>posted copper red glazes from McWhinnie include a few % points of zinc. Of
>course one could try a line blend! Is there an easier resolution?
>
Tom Buck on mon 13 apr 98
Zinc oxide readily is reduced to zinc metal and if the kiln temperature is
above 907 deg Celsius the zinc goes to vapour and up the flue. So why put
it there? Well, the zinc oxide is intimately mixed in with the other
ingredients, and since the redox reaction is gas/solid or gas/liquid, then
it simply takes time for all the zinc oxide to become zinc metal and
disappear. During this time, the zinc oxide being a strong flux will bring
about a fluid melt at perhaps a lower temperature than otherwise (if the
ZnO were not present), and the molten glaze then would be more easily
subjected to reduction and the CuO (black) goes to Cu2O (red) and Cu
(metal) penny bright. So, there could be a benefit of putting ZnO
in the glaze recipe. What could replace the ZnO in a copper red recipe?
Perhaps lithium oxide/carbonate, or a small amount of borate frit. Or
reduce the amount of alumina in the mix. Til later.
Tom Buck ) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).
rballou@mnsinc.com on tue 14 apr 98
Tom,
Thanks for this explanation. I've wondered about what exactly happens. But
what concerns me is the poisonous nature of the vaporizing zinc, especially
if the kiln is not properly vented. Of course, in the ideal world all kilns
are properly vented. Also, in my experience, the vaporizing zinc can leave
behind some rather nasty glaze defects that don't disappear upon refiring.
Given these two problems with zinc, I prefer not to use it in reduction,
until someone can show me a real benefit from its presence. I've seen
several different types of reduction glazes with zinc in the recipe. I've
tested them with and without zinc and have yet to see a positive impact
from the zinc. And, I've not seen a negative impact from its absence, by
just leaving it out of the glaze, with no other changes or substitutions.
Ruth Ballou
rballou@mnsinc.com
>----------------------------Original message----------------------------
>Zinc oxide readily is reduced to zinc metal and if the kiln temperature is
>above 907 deg Celsius the zinc goes to vapour and up the flue. So why put
>it there? Well, the zinc oxide is intimately mixed in with the other
>ingredients, and since the redox reaction is gas/solid or gas/liquid, then
>it simply takes time for all the zinc oxide to become zinc metal and
>disappear. During this time, the zinc oxide being a strong flux will bring
>about a fluid melt at perhaps a lower temperature than otherwise (if the
>ZnO were not present), and the molten glaze then would be more easily
>subjected to reduction and the CuO (black) goes to Cu2O (red) and Cu
>(metal) penny bright. So, there could be a benefit of putting ZnO
>in the glaze recipe. What could replace the ZnO in a copper red recipe?
>Perhaps lithium oxide/carbonate, or a small amount of borate frit. Or
>reduce the amount of alumina in the mix. Til later.
>
> Tom Buck ) tel: 905-389-2339
>& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
>(westend Lake Ontario, province of Ontario, Canada).
Richard Burkett on fri 17 apr 98
Tom Buck says:
> During this time, the zinc oxide being a strong flux will bring
>about a fluid melt at perhaps a lower temperature than otherwise (if the
>ZnO were not present), and the molten glaze then would be more easily
>subjected to reduction
Are you sure that this is indeed the case, that a dense, glassy melt will
be more easily subjected to reduction by gases in the kiln? I would argue
just the opposite, that in a fully melted glass it is more difficult to
reduce elements within the glaze than a somewhat more porous, gas permeable
unmelted, or slightly sintered one (although reducing oxides within a glass
isn't impossible by any means).
An important factor for many classic stoneware reduction glazes is that the
glaze melt under reducing conditions which are held until the glaze is
completely melted, thus sealing the surface somewhat from reoxidation.
Obviously glass isn't an impermeable surface to reducing or oxidizing
gasses in the kiln, but it is less so than an unmelted or partly melted
powder.
Richard
Richard Burkett - School of Art, 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/
HyperGlaze@aol.com & http://members.aol.com/hyperglaze/
_ Richard Burkett, Associate Professor of Art
_ The School of Art Design & Art History, SDSU, San Diego, CA 92182-4805
_ http://www.sdsu.edu/art
_ E-mail: richard.burkett@sdsu.edu - voice mail: (619) 594-6201
_ The CeramicsWeb: http://apple.sdsu.edu/ceramicsweb/
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