Jeff Lawrence on mon 12 oct 98
Hello all,
I am still pursuing my will-o-wisp of salivation-quality copper reds at ^6
and have managed (sly fellow that I am) to intrigue a brilliant retired
scientist here in my home of Los Alamos in the problem. I am attempting to
absorb his comments on Tichane's wonderful book on "Copper Red Glazes" but
thought that others here might also find his comments inspirationally
confutational. In that spirit, here are Ray's comments (sure wish I'd taken
some science courses!).
Jeff
>
>Jeff:
>
>I am nowhere finished with Tichane's book, but the situation is much more
complex than he thinks. He brings up more questions than he proposes
answers. Fascinating.
>
>First, he says that the color is better when reduction is followed by some
oxidation. An hypothesis here. Presumably the glaze becomes a viscous,
vitreous material during reducing conditions. The most important
observation on reducing flames is that they are luminous: the light comes
from incandescent carbon particles! Those carbon particles can be
incorporated in the viscous mass, and they will have a profound effect on
color. They are, however, very small and very reactive (they are not pure
carbon - a long story). Some carbon-rich polymers also form on surfaces
during oxygen-deprived combustion (Maria's pots are coated with vitreous
carbon derived from a polymer of furfural and hydroxymethyl furfural). A
little oxidation can burn them away. How does the oxygen get to them?
Several ways. For example, an anodized aluminum surface is protected by a
coating of Al2O3. But oxide ions diffuse through the coating. They would
diffuse more rapidly through the glaze than would the larger radicals
responsible for reduction (and O2 would hardly diffuse at all). This is
partly why I don't think the final color is totally the result of light
scattering off of particulate Cu metal.
>
>Then on pp. 134-135 ("Conclusion") he shows a chemical equation for the
combustion of propane.. No way! There are some 21 series and parallel
reactions involved in the combustion of pure hydrogen in pure oxygen: there
are hundreds to thousands involved in the combustion of more complex
molecules. The process is usually initiated by the attack of an oxygen
radical on a carbon-hydrogen bond; both an alkyl radical (from the propane
or whatever) and a hydroxyl radical are formed. These radicals are very
active and will react quickly with other molecules and radicals, forming
additional radicals. The process forms a chain reaction exactly comparable
with a nuclear reactor. And the combustion of wood in a kiln would be much
more complicated.
>
>Just to take one part of the chain, hydrocarbons produce many radicals
(like methyl, ethyl, propyl, methylene, ethylene, propylene, etc.), which
are just the parent molecule less one or two hydrogen atoms. These radicals
are very, very active. Paneth and Hofeditz (Berichte 62B, 1335, 1929) found
that alkyl radicals would remove the metal atoms of a mirror surface.
Removal or deposition of mirrors is still used to study free radicals.
Different radicals react best with different metals, but there is a lot of
overlap. For example, "---methylene removes tellurium, antimony, selienium,
and arsenic, but does not attack zinc, cadmium, bismuth, thallium, or lead,
all of which are readily removed by alkyl radicals (eg, methyl)." Or, "---
atomic hydrogen removed arsenic, antimony, selenium, tellurium, germanium,
and tin mirrors, but did not affect lead or bismuth." The bottom line, you
don't need to hand wave chloride for the removal of Cu.
>
>Carbon monoxide can be a radical and/or react as the molecule. Many metal
carbonyls have been studied extensively. I worked with Ni(CO)4 a lot, but I
don't have any direct experience with copper carbonyl. I'll look it up.
Ni(CO)4 is a liquid at room temperature and BOILS at 43C at atmospheric
pressure. It provides a very good way to produce a Ni mirror.
>
>Even good old water becomes hydrogen and hydroxyl radicals at higher
temperatures.
>
>A "reducing" atmosphere will preserve more "strange" things for longer than
will an oxidizing atmosphere, and not all reducing atmospheres are created
equal. Think about what goes on in an internal-combustion engine as you
tune it rich or lean. One important class of compounds that is produced in
hydrocarbon combustion is the peroxides. They are powerful oxidants, and
the rates of their reactions depend on the molecules or atoms they attack.
(Such problems are job security for chemists.)
>
>We know that there are diffusion processes at work, and we know a lot about
how they operate with concentrations and temperature. We also know a lot
about light scattering and color. What we don't know is the nature and rate
of the reactions involving the Cu. A few experiments could be interesting.
>
>Tichane used powdered copper compounds. Particulates require what are
called "zero-order" (surface) reactions. Let's try to change the type of
reaction while keeping the gas-phase reactants the same. I'd like to try a
first shot at using colloidal hydrated copper oxide dispersed as well as
possible in the glaze materials.
>
>I just talked with you. Give me a call when you want to try something.
>
>Ray
>
>
Bill Aycock on tue 13 oct 98
I have some qualifications in the area of science, myself, although not
near those of your friend, Ray.--However, I think he is making a process
that is amenable to simple controls (relatively) into a morass of
obfuscation. He appears to be like the weatherman in a windowless office-
in other words---
He aint never been there. He hasn't singed his eyebrows at a peep-hole.
He expounds at great length on the complexity of the chemistry of
combustion, when that level of detail doesn't matter. If we could measure
those things and make controls to respond- fine- but the everyday Clayarter
cannot, and- more important - SHOULD NOT deal with this level of detail.
In addition, some of his explanations are a little off- The combustion
process starts with heat attacking the bonds- not Oxygen. Heat breaks the
fuel molecules down so the Oxygen, when it gets there, can react. All the
business about alkyls, ethanes, butanes, methanes and radicals is true,
but really makes very little difference. The atmosphere in the kiln has
such a mix of different stages of the process that the exact status of any
one micro-level(small) of the sequence is of no matter- it is all balanced
out at the macro (large) level, which is all me have control over, with our
valves and dampers.
If this type of thing turns you on (as it probably does him) fine- if you
just want to make good pots and glazes, take advantage of what he says, if
it is a suggestion about a practical method, but dont worry about the exact
sequence of the break-up of a propane molecule. The molecule doesn't care
what you think is happening.
Sorry- couldnt resist- "been there- done that"- didnt help.
BTW the color of the red is better with some Oxidization because the raw
red is overlaid with a layer of light blue, which is what gives the
"Oxblood red"
Bill- about to leave Persimmon Hill for the Crafts fair in Asheville, NC
At 09:11 AM 10/12/98 EDT, you wrote:
>----------------------------Original message----------------------------
>Hello all,
>I am still pursuing my will-o-wisp of salivation-quality copper reds at ^6
>and have managed (sly fellow that I am) to intrigue a brilliant retired
>scientist here in my home of Los Alamos in the problem. I am attempting to
>absorb his comments on Tichane's wonderful book on "Copper Red Glazes" but
>thought that others here might also find his comments inspirationally
>confutational. In that spirit, here are Ray's comments (sure wish I'd taken
>some science courses!).
>Jeff
>
Bill Aycock --- Persimmon Hill
Woodville, Alabama, US 35776
(in the N.E. corner of the State)
W4BSG -- Grid EM64vr
baycock@HiWAAY.net
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