Jeff Lawrence on tue 24 nov 98
Hello Clayart!
Here are some more paraphrases of Ray's observations on my copper red tests
at ^6, for anyone else interested in what is going on in there...
These observations pertain to two glaze bases:
- a good clear base, high (though within limits) in calcium and alumina,
- a wild and untamed jemez ash glaze which approximates Tichane's
recommended composition for copper reds.
and three types of copper (all ball milled for 6-8 hours in a base glaze):
- red copper oxide
- black copper oxide
- copper carbonate
with and without tin (also milled for 6-8 hours).
The most interesting observation is that there are two distinct types of
coloration we observed:
1. dissolved red color in the glassy phase -- this color was a beautiful
red, allowed a lot of light through and disappeared under polarizing light,
which indicates that it is not crystalline.
2. scattering orangish color from pseudomorphs of Cu2O. These little puppies
were metallic copper the day after the firing when I took them to Ray. He
sectioned the reddest part (orangish if truth be told) and we immediately
saw little crinkled balls of shiny metallic copper. According to his
practiced eye, the appearance of the copper was that of "blister copper," a
form of the metal with a great deal of Cu2O dissolved in it. Over the
course of a week or two, the metallic surface of these spheres became
enhazed in a cloud of red. The smaller spheres seem to have turned entirely
into crystalline Cu2O spheres of the same size and shape (we know it's
crystalline since polarized light enhances the haze. It also bounces around
in the spheres and emerges looking like a perfect four-pointed star). Ray's
tentative hypothesis is that the metallic copper reacted with CuO dissolved
in the glaze to form Cu2O. When the spheres are small enough and when there
is sufficient dissolved CuO, all the metallic Cu disappears into this reaction.
The first type of coloration is deep and pure color, almost to the point of
boring. However, the color itself is so lovely, I can't not like it. The
second kind of coloration is less attractive to me strictly on hue --
orangish and more opaque, but lovely in its sparkly aspect.
Re: Copper
Ball-milling appears to be a good idea for the copper. The milled tests show
a much better dispersion of copper. Even the copper spheres are smaller. At
the same time, it seems much easier to use too much copper when you mill it
fine. I was getting heavy ugly brown coloration from .18% (.0018).
The type of copper doesn't seem to matter much, based on my results. Ray
says the red copper oxide has less transformationto go through to turn into
either of the two colorations we found, but there didn't appear to be much
difference other than the level of coloration. I'll probably continue with
the red, but if I run out of that, I'll substitute with confidence.
Tin: Ray is strongly against any more tin than is necessary for the
protective colloid function discussed by Tichane. He has complained a lot
about the apparent tin crystals that cloud up the color. One batch of
(tin-bearing) ugly brown pots I tossed in for kiln fill because I had
nothing to lose, developed an interesting case of tin-metal acne. The tin
precipitated out into shiny spicular crystal blooms of tin on the same
darned ugly brown background. An effect to try for on a more attractive
background!
Different bases:
Most of my clear base glaze tests were unfortunate by even the most
charitable aesthetic.
The jemez ash glaze was great, with flecks of ruby red, orangish copper,
borosilicate opals, and, most interestingly, spherical lenses of clear glass
that contained no copper at all. Ray by various scientific arguments
demonstrated that these spheres of colorless glass are a different phase in
which the copper is not miscible. Under the lense, they looked a good bit
like balls of oil floating low (they made a slight dimple in the surface) in
a colored soup.
Which makes me conclude that mimicking demonstrably successful compositions
is probably a better idea than grabbing your local clear base.
That's all for now, folks, and Thanksgiving is just around the corner, so
not much more in the near future. And if you are not in the USA, please work
hard this week knowing that here we will all be kicking back for
over-indulgence.
Gluttonously y'alls,
Jeff
David Hendley on wed 25 nov 98
A couple of comments, and thanks again for Ray's observations.
At 09:54 AM 11/24/98 EST, you wrote:
>----------------------------Original message----------------------------
>Here are some more paraphrases of Ray's observations on my copper red tests
>at ^6, for anyone else interested in what is going on in there...
>
>These observations pertain to two glaze bases:
>- a good clear base, high (though within limits) in calcium and alumina,
Jeff, I think it is generally agreed that for copper reds you want a base
glaze that is low (but within the limits) in alumina. I certainly think so.
>The jemez ash glaze was great, with flecks of ruby red, orangish copper,
>borosilicate opals, and, most interestingly, spherical lenses of clear glass
>that contained no copper at all. Ray by various scientific arguments
>demonstrated that these spheres of colorless glass are a different phase in
>which the copper is not miscible. Under the lense, they looked a good bit
>like balls of oil floating low (they made a slight dimple in the surface) in
>a colored soup.
>
My best copper reds have all kinds of flecks, borosilicate opals, and slight
dimples in the surface. I've always thought that these "clear spheres" are
evidence of violent boiling during the molten period. I try to encourage
this action by using ingredients, such as carbonates, in the recipes.
David Hendley
Maydelle, Texas
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