Karl P. Platt on wed 11 nov 98
In that Cu reds are still being batted about I'm sending this as a
repost. It was first sent out about a year ago.
This and other such stuff can be found at:
www.digitalfire.com
Aproveite!
KPP
Red Copper (Cu) glazes are distinctive and have been highly prized in
history - everyone's
heard about the Chinese guy who died taking the "secret" to the grave
with him, leaving the
Emperor quite disgruntled.
Cu red glazes are based on adding Cu into the glaze as an oxide and then
exposing it to a
reducing firing. If a sample of the glaze is drawn from the kiln at full
heat should show at most a
light straw color and it may turn red on cooling. The red is produced on
cooling by crystals that
come out of solution with the glaze. The composition of these crystals
that has been the source
of controversy.
Up until 1960 or so it was held that the color was due to metallic
copper crystals. Really, this
was accepted as being quite obvious. Then came Atamaram and Prasad, who
suggested that
the red color was actually due to Cu2O (red copper oxide) crystals.
Atamaram and Prasad's paper makes for very interesting reading on a
number of levels - I wish
I had a copy of it here! Recognizing the difficulties had in making Cu
Red, and the desirability
of the color as used in glass bangles Indian women especially like, they
set out to study and
refine the parameters of Cu red development. In the course of their work
they came to the
Cu2O conclusion, but their work was criticized because they added large
amounts of Cu (up
to 5 wt%) to their
glasses.
However, through their work they did obtain delicious and repeatable red
glasses.
Behind Atamaram and Prasad came Rawson who showed that the color of Cu
red was
consistent with the results expected from what is known as the Mie
Scattering Theory for
Cu-metal. Mie theory predicts what wavelengths will be preferentially
reflected from the metal
surface. It's real complicated and we'll leave it to say only that the
red Rawson found by
measuring the spectrum of the red in his glasses gave results that were
consistent the presence
of Cu-metal.
Amal Paul, a guy no-one can say enough about, undertook to sort out the
controversy about
just what it was that made reduced Cu glasses red - Cu2O, Cu-metal or a
mix if the two. He
did his studies in a glass made of 30 Na2O and 70 B2O3 --- this does not
represent either a
useful glass or glaze, but it is easy to melt. No tin was added to the
glass and the amount of Cu
this glass will hold is very low-up to 0.13wt% Cu taken as metal.
Paul concluded that both Cu and Cu2O are present and the "better"-more
pure - Cu reds were
abundant in Cu2O.
Cu belongs to a group of metals known as the Nobel Metals. The Nobel
Metals are Cu, Ag
and Au. In this order they are progressively less likely to form oxides.
Recalling our discussion on Redox, [Editor: a discussion on Clayart
about the mechanisms of
oxidation and reduction] we can say that the outer electrons on these
metals are progressively
more rigidly held moving from copper to gold. Gold stays reduced, silver
resists oxidation, and
copper will go along and ditch an electron or two depending on the crowd
it's in.
Cu oxides, of course, are well known. You can buy Ag2O from the chemical
house, but apart
from that it's not seen much - the film that develops on your Ag
tableware is not predominantly
oxide. Au oxide is never encountered in normal circumstances; this is
the source of value in
gold.
When Ag or Au are added to a glaze, it is not necessary to employ heavy
reduction to produce
metal atoms - Ag and Au would rather be metal atoms. There is a limit as
to how many of
these metal atoms can be in solution (colorless) with the glaze -
naturally this limit is called the
solubility limit. When this critical limit is exceeded, the excess metal
atoms combine with their
kind to form crystals.
These crystals of Ag or Au metal produce color in the glaze.
The color is determined by how these crystals are shaped, how they are
distributed in the
glaze/glass and their size. The number of metal atoms that can be held
in solution with the glaze
decreases as temperature decreases.
As the glaze cools metal crystals will develop -- Often tending toward
being fewer and larger
where big crystals form at the expense of little ones. However, the
little crystals can also
coagulate, and this affects the color observed.
The size of these crystals is on the order of 50-200 millimicrons -- +/-
a little. If the crystals get
large the glass looks like liver looks (tastes?) and is called livery.
If you made livery glass/glaze,
you blew it. Do not pass Go. No Doughnut. Call the dumpster man. We're
not here to make
livery rubies.
The color in glazes containing nobel metal crystals is mainly produced
by the absorption of light
by the metal crystals. Scattering effects have no role in producing the
color unless the glass is
livery. Big crystals scatter enough light to make the glaze appear
opaque in reflected light and
(densely) colored in transmitted light. We almost always look at rather
than through glazes, so it
is the reflected results which matter to us.
Crystallization occurs within a limited range of temperatures. Above
some temperature there's
too much thermal agitation to allow the metals to organize into
crystals, and below some
temperature the glaze will be too viscous to allow atoms to migrate
towards a developing
crystal. Hold this notion, it'll appear again.
Au (gold ruby) is very rarely used in Studio Ceramics. I can't think of
anyone hand-rolling their
own Au reds - if you're out there please stand-up.
There are, however, Au ruby overglazes and glass enamels commercially
available. Most of
these are based on soft fritted lead glass and they're not cheap - not
because they have huge
amounts of Au in them (there are very tiny amounts), but they're tough
to make. There's nothing
to preclude anyone from making Au ruby glazes except that errors are a
little pricey in terms of
time and providing for precision. In terms of cash cost, it's really not
so terrible as the amount
of gold needed is very small.
Au glass/glaze has its distinctive ruby color. Fenton's sells as
"Cranberry" Glass. Apart from
tableware Au red is often seen in colored sheet glass. There are (really
beautiful) blue Au
glasses which form when the conditions cause the Au crystals to become
large-ish and very
non-spherical.
Silver (Ag) can make almost any color in glaze if you know how to
manipulate it, but usually it
gives yellow. Ag is essentially never used in modern art Ceramics, but
the ancients used it
widely - especially the Persians who developed fantastic lustres after
the collapse of Rome --
the artisans had to go somewhere.
By applying a paste of AgNO3, Kaolin and a little BaSO4 on the surface
of bright soda-rich
glazes and then refiring the pot to 1400 F/650C or so you can "stain"
the glaze locally to nice
effect.
Glassmakers use Ag quite often to develop a number of effects. These
range from yellow glass
to brown glass to a glass that is multicolored in reflected light, but
yellow/amber in transmitted
light. There are also silver glasses that develop a metallic sheen with
reduction.
Essentially spherical crystals of Ag metal cause the yellow and brown
glasses. If the color is
non-uniform, the stuff the Ag is in is probably not uniform. When the
glass turns brown it is also
frequently turbid (milky). This is owing to having formed large and
numerous crystals -
scattering of incident light, and mushy absorption characteristics
tending to longer (more red)
wavelengths.
Multicolored effects seen in silver glasses/glazes are due to the
development of non-spherical
crystals. The mechanics of all of this are not, however, our concern
here, but Peet mentioned
these and I thought it would be worth a brief mention as these effects
could also be developed
in glazes as well.
Silver glazes have fantastic potential that is overlooked in Studio
Ceramics. It is possible to
produce any color in a very pure form using only silver. Accomplishing
this is a delicate dance,
but like Tango, the logic is clear. If you've managed to come to here
without hitting the delete
key, I'd like to solicit a few collaborators in elaborating Ag in
decorative glazes.
But we're really interested in Cu-reds why all this about silver and
gold? Well, the formation of
Cu-reds follows the same lines in terms of the crystallization mechanics
- oversaturation -->
crystal formation --> growth or coagulation of the crystals. However, in
the case of Cu it's not
simply a matter of precipitating metal.
Cu can do a number of things when added to a glaze. In alkaline glazes
it yields a very distinct
blue color (Cu2O). In less alkaline glazes it's green. (CuO). This
indicates the importance of
composition on color development. Red, however, can be produced in
almost any base glass.
In fact, Cu is quite sensitive to its chemical environment and I've
found it to be a good and very
handy indicator as to the acidity or basicity of a glaze - kind of a
litmus test. Simply, if Cu is the
only colorant in a glaze, and the glaze is fired in air, how blue it is
gives a rough idea as to what
you can expect from the glaze in terms of its many other chemistry
sensitive colors.
You can't simply chuck Cu-oxide into any 'ole glaze and expect it to
come out red. The
following factors come into play:
1.Composition
2.The presence of tin oxide
3.Reduction
4.Cooling and sometimes reheating
In terms of composition, the glaze needs to be able to support the
solution of Cu. To achieve
this it needs to have things in it that are friendly to the presence of
metal. The best of these is
PbO - I can hear the gasps of horror now. Yes, that Godzilla of the
Elements. Bismuth is
another good option in some glazes, Zn helps and there is, of course,
tin (Sn).
Sn does a couple things. First it improves the solubility of Cu. Metals,
per se, aren't really very
soluble in glaze and if you can't get the metal dissolved, it can't very
well be precipitated in any
organized fashion. Second, on cooling, Cu tends to attract Sn atoms from
the glaze. These
atoms sort of "coat" the crystals as they are developed and thus serves
to control their size by
limiting the attachment of further Cu atoms to the crystal. This
behavior is that of a protective
colloid and it is of great advantage. Because if the crystals get big,
the glaze turns "livery"
looking, and the doughnut remains elusive. Third, to the
extent that Sn has limited solubility in SiO2 or B2O3 based glassy
material, it probably also
serves to provide nuclei on which the coloring crystals can grow.
Tin oxide is added to all practical non-lead Cu red glazes in amount way
beyond what's
necessary to promote good solution of Cu in the glaze - many
compositions contain up to
4-wt%. Of course, if there's too much tin it doesn't all dissolve --
causing opacity. This may or
may not be desirable.
Tin is volatile at high temperatures and a lot of it leaves the very
thin glaze film by evaporation.
Compensating this evaporation is important to how much tin will remain
dissolved in the glaze.
This explains the large amounts of SnO2 in many reported glaze
compositions. Some kilns have
turbulent atmospheres and a larger amount of evaporation would be
anticipated in these
circumstances. In glassmaking, if you melt in a covered pot, in which
evaporation is not an issue
(in most cases), the amounts of SnO2 required seldom exceed 2 wt%.
If you use too little Sn to promote the solution, Cu will precipitate on
the spot with dreadful
results. This is one of the reasons that application of the glaze is so
important and why really
thin films often fail to develop a nice red - when red color forms at
all in a tin depleted glaze, it
often has the color of liver instead of a crisp red.
The amount of Cu necessary to develop a good red depends on how much of
it can be
dissolved. This depends on how much the glaze would dissolve on its own
and how much this
is improved by the presence of Sn. Many pottery glazes contain what I
feel is a lot of Cu-oxide
in the batch, but that's just an opinion. The best reds always contain
the least amount of Cu.
Reduction is the critical step in producing a nice Cu red.
Amal Paul's 30 Na2O 70 B2O3 glass was melted in a little electric
furnace with a strictly
controlled reducing atmosphere - CO-CO2 mixtures metered into the
furnace with precision
gear. He found that as the "amount" of reduction increased there is a
level below which no red
forms; a (point) range of reduction within which good reds developed and
a point (range)
above which the color is funky.
His conclusions were:
In reduction one wants to achieve an equilibrium which includes only
Cu2O (red copper oxide)
and Cu-metal, with no CuO present. The amount of Cu-metal dissolved in
the glaze is fixed by
the glaze composition and the amount of Cu2O present in the glaze is
fixed by the atmosphere.
It should be mentioned that by using the term Cu2O it is not meant that
molecules of Cu2O are
floating about in the glaze. On the contrary, it means that Cu+1 ions
are in the glaze and that on
cooling Cu2O crystals (which are red) are formed. There is some degree
of reduction at which
Cu2O solubility is at a maximum. This is the point you want to find.
If the reduction applied to the glaze is too weak CuO forms together
with Cu2O and Cu-metal.
This is not where you want to be at all because at this level of
reduction, the glaze will be
unsaturated in Cu2O and Cu-metal. As a result nothing will crystallize
and you won't see any
red. However, owing to poor mixing of the glaze batch it frequently
happens that red patches
develop in the presence of green CuO. This can be a nice effect.
If reduction is too strong an abundance of Cu metal is formed at the
expense of Cu2O, the
glaze will precipitate Cu-metal on the spot, as its solubility will have
been exceeded, and the
color is murky. This is an important distinction between Cu red glazes
and Cu red glasses. In
an over-reduced glass the metal comes out of solution and sinks to the
bottom of the crucible -
often forming little beads that drill holes in the crucible. In glaze,
the Cu metal can't wander
away. Sometimes it manages to oxidize again depending on the firing
conditions and it can also
form a metallic film on the surface of the finished glaze. The extreme
character of this behavior
is widely exploited in raku.
The thermodynamic considerations of all of this are tedious, and it's
not worth going into all of it
here, but the sum results are:
In reduction you can produce three forms of Cu in the glaze. These are:
CuO, Cu2O and Cu-metal.
The ideal degree of reduction will be a little different for each base
glaze. I don't have time to
crunch the numbers to give some precise fuel/air mix, but if someone
else wants to go through
it, feel free. In a little more absolute terms, you want to produce an
oxygen pressure
somewhere between 10^-12 and 10^-15 atmospheres. This corresponds to a
CO/CO2 ratio
around 1/10^5 - I pulled these numbers off of an Ellingham diagram, they
were not calculated.
Of course, rich combustion yields Hydrogen, too, and this needs to be
considered. Also,
Ellingham diagrams say nothing about the "activity" of the metal/oxide
in glaze and these effects
can be profound.
Apart from applying heavy science to making Cu red glazes we all know
that arriving at the
best fuel/air ratio can, of course, be derived by trial and error - as
has been done for millennia.
This takes us to the importance of having a stable combustion system
with means for metering
the amount of fuel and air entering the kiln. Precise combustion is
elusive in natural draft kilns
subject to wind, variable atmospheric pressure and so on. It can be
achieved by experienced
operators constantly tending the fires. High-pressure gas "venturi"
burners will furnish better
repeatability in natural draft kilns in all circumstances. Forced air
combustion can be metered
very precisely by metering orifices.
We should note again that oxygen analyzers cannot meter reduction - they
are sensitive to the
presence, not the absence of oxygen. As such other means, like metering
orifices, or even a
decent pressure gauge are preferred.
We've established that there is some degree of reduction at which the
amount of Cu2O is at a
maximum and that this is where we will get the best color. Establishing
this condition in the
glaze can be done several ways. Some like to begin reduction early in
the firing - around 1700
F or so and maintain this degree of reduction through the end of the
firing. This is fine. Others
like to reduce the pi$$ out of the kiln for a short time at high
temperature. This can work, too,
but it never goes as well as the former approach.
In very practical terms, a glaze film is really thin - usually way less
than a millimeter. While glaze
is usually pretty viscous stuff, it
doesn't take terribly long at high temperatures for equilibrium between
Cu/Cu2O and the
atmosphere to be established. I'd suggest that firing in neutral
conditions until the last couple or
three hours of the firing, and then adding the necessary reduction will
be a more economical
approach to obtaining the best results. Three hours is plenty long
enough to establish
equilibrium. Moreover, really prolonged reduction has other affects
which may not be desirable
at all - like depleting the glaze surface of Na, messing with the body
color in undesirable ways,
deteriorating the kiln's refractories, and so on.
Re-oxidizing, as it were, can occur if the circumstances are right.
Avoiding this can be done by
cooling rapidly to 1,700 F or so. Maintaining reduction during cooling
is sometimes necessary
to control the redox balance in the glaze.
We know that the color is formed by the precipitation of Cu and Cu2O
crystals on cooling.
Usually a kiln will cool slowly enough so that these crystals have
plenty of time to form in the
natural cooling of the kiln -- net cooling rates of 1-3 degrees F/minute
are common. Cooling
will be faster at first and then proceed at a progressively slower rate
owing to the diminished
temperature gradient between the interior of the kiln and the air.
There will be some temperature at which the crystallization rate is
highest. Above this
temperature the thermal agitation within the glaze is too great to
permit crystals to organize and
below it the viscosity of the glaze retards the progress of Cu or Cu+1
to developing crystals.
There will also be some temperature at which the precipitation of Cu2O
is at a maximum on
cooling. If you find that temperature and hold it for a spell you get
better reds.
There may be the weird instance where cooling was too fast and no color
appears. In such a
case one can reheat the pot to 1,600-1,800 F and the red will form -
assuming you reduced
correctly.
The presence of P2O5 lends to ruby formation and this was well known to
potters in early
civilizations. It has a reducing influence by its presence and it is
highly insoluble in the host glaze.
As a result P2O5 rich droplet regions will form within the glaze and
these will promote
development of the red crystals. An excess of P2O5 gives opalesence -
this can be beautiful.
It would be useful to assemble and examine studio results a lot more
carefully. There's a great
deal of experimentation in the archives of the last 25 years we could
probably learn something
from.
Getting the quantities of Sn and Cu right is something that can be
worked out using line-blend
methods. Any well made shiny glaze will serve as a host for Cu Red. The
maximum Sn and Cu
you'd want to use are around 3%. Excellent Cu reds have been made with a
lot less of both
elements. Sn is typically add to the batch in larger amounts than Cu -
3:1 is a frequent ratio.
The amount of Cu is fixed its solubility in the glaze - you want to use
just enough. The minimum
amount of Sn is that
necessary to evade having it all evaporate.
The amount of Cu that crystallizes depends strictly on it's
concentration. Less is usually more in
Cu reds.
Someone else might have the patience to elaborate how this line-blend
should go together. I'd
do it on a triaxial in 5 divisions (20% increments) with the corners
being: Neat glaze, 3% Cu,
3% Sn.
Make enough glaze to make 3 or 4 test tiles (of the same clay body) of
each mixture and fire
these in separate firings in the same place in the kiln. This'll give
you an idea what the quality of
the firing is like - assuming you made homogeneous glaze in the first
place.
Adding SiC to the glaze to furnish reduction in electric kilns gives
results ranging from
moonscape to colorless glaze. In the main, it doesn't work real
well....at all. You can also toss
organic material into the kiln - I've always been amused by the use of
mothballs - they work,
but the stench.... Charcoal is a good alternative. You could, as well,
be really anal and go out
and get tanks of CO and CO2 to inject into the kiln. Talk to your
welding gas supplier. And if
you do this stuff, don't do it in a confined space.
Remember to do an oxidizing firing after reducing in the electric kiln
to build up the Al2O3 film
on your Kanthal elements. Nonetheless, element life will be diminished
by reduction firings.
Elemental Si is something that should be tried as an in-glaze reducing
agent. Maybe someone's
already done it. It's cheap, widely available - especially to those who
live near a steel works -
and very potent in its effects. The advantages of Si as a reducing agent
are several fold, but the
big one it that you can melt with more correct combustion, and a few
grams of Si has always
been a lot cheaper than 6 or 7 hours of bad combustion.
Alright, I don't know about you, but I've had quite enough of all of
this.
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