Karl P. Platt on mon 23 sep 96
>>I am told that it is possible to get RED at ^6 (I thought that
wasn't possible). We are also lacking in any purples. <<
>>I know you can get copper red at cone 8--but at cone
6?????<<
The formation of red Cu colors isn't dependent on temperature in that
way. F.H. Norton did extensive work trying to decipher just what it was
that caused the Cu red color much lower than ^6. Composition, firing
atmosphere and cooling rates are the critical variables.
How much Cu is needed depends on how soluble it is in the given glaze.
The solubility is enhanced by the presence of tin (Sn). Sn also plays a
role in how much of the colorant is produced. It has this habit of
accumulating on the surface of the colored particles and after a point
it prevents any more coloring matter from getting there -- sort of a
skin forms.
Red coloration is given by reduced forms of Cu. Usually as a combination
of elemental Cu and Cu2O. The best reds are had when a preponderance of
Cu2O is formed instead of Cu. If you get a muddy Cu ruby it is likely
over-reduced and/or it contains too much Cu and/or too little Sn.
Over-reduction is a good place to start changing thins as there is a
strong tendency to over-reduce -- long hours of having fire belching out
of each crevass in the kiln lends drama, but usually accomplishes
little.
The color forms by having the coloring material come out of solution
with the glaze -- similar to the way in which sugar dissolved into hot
water comes out of solution as it cools. A couple things need to occur
in advance of this process. In short, something needs to be there in
order for the crystals to grow on and there needs to be enough time for
this to occur.
Ancient Chinese reds are characterized by the presence of a small amount
of P2O5. P2O5 is as soluble in a silica rich glaze as oil is in water.
As such it exists as very small regions of P2O5 rich glass within the Si
rich parent. These form seeds or nuclei on which the Cu/Cu2O red
crystals can grow. In many glazes there is also residual crystalline
material on which the red materials can become attached as well. In
fact, SnO2 is poorly soluble itself in glazes -- more so at cooler
temperatures. This is why it finds use as an opacifier and in the
present instance, in addition to its ability to influence the solubility
of Cu and the way it comes out of solution, it can also serve as a
nucleating agent to some extent. Tricky stuff, that tin.
Usual pottery glazes are red when they come out of the kiln. This means
that:
A. Whatever the red crystals formed on came out of solution first
B. The quantity of Cu in the glaze well exceeded the solubility limit
C. Cooling time was long enough for the crystals to develop given the
saturation had.
Here's a program for developing a nice Cu Red using line blends and draw
trials. If you can't be accurate about proportioning your materials and
the fuel/air mix, repeatability will be elusive.
Take some boilerplate ^6 clear glaze -- fritted or no, it doesn't
matter. Make up two batches of, say, 2000 grams. When preparing these,
preferably with a good stirrer or better still for 30 mins in a 1 gal.
capacity ball mill, be very careful to use exactly the same amount of
water -- no more than 600g water/1000g batch. To one batch add nothing.
To the other add 2% CuO, 6% SnO2 and 0.4% Bone ash. Blend these together
using a measuring cup or graduated cylinder or beaker in the proportions
25/75, 50/50, 75/25. If you make a larger amount of glaze you can make
finer divisions, but in any case make enough to prepare some extra 50/50
samples. Apply these to test tiles of the body to be normally used and
place them in the kiln so they'll experience essentially the same
conditions. Ideally you'll set-up to draw out the 50/50 samples at full
heat during the reduction. At some point there'll be no green color
they'll appear sort of straw colored. At this point the reducing
requirement has been satisfied -- at least for the 50/50 draw sample. If
the any of the samples comes out colorless or straw colored, reheat the
sample to 1,300 F or so and the red will probably emerge.
The idea in any case is to use the least amount of Cu and reduction
possible. This is because too much Cu lends to BIG Cu crystals, which
are muddy (livery) looking and too much reduction lends to the
preferential formation of Cu over Cu2O and dingier/livery colors. In
developing a Cu red with the least amount of Cu, slowing the cooling
between 1,600 and 1,300 :F will permit the crystals to grow to adequate
size. Actually, the nicest colors are developed in this fashion.
The rate at which reduction occurs depends on the viscosity of the
molten glaze. Mild reduction at high temperature produces the same
result as strong reduction at lower temperature for the same glaze. As
viscosity is an exponential function of temperature, a temperature 10
degrees lower could require reduction 100 times stronger to produce the
same result -- here one sees how important good controls are to
repeating the best results and that if a flame belching fire is
required, going a little bit hotter could, in fact, save a lot of gas. I
personally prefer lighter reduction throughout the firing and will note
that my experience with reducing additions to the glaze,like SiC, etc,
as has been suggested here and in the literature, has been very, very
poor. Indeed, utterly useless.
Reoxidation of Cu reds can occur and usually does so at high
temperatures -- remember the rate at which these changes might occur
falls off steeply with temperature. Beyond a few hundred degrees below
maturing temperature changes in the amount of reduction locked within
the glaze (redox) is highly unlikely.
OK, I'm burnt on this now even if there is more to add -- such as the
subtleties of Sn, etc. Maybe we'll take this up someother time, but it's
pretty typical to find that a Sno2/CuO ratio of 3/1 is pretty typical,
and at the very least a real good place to start experimenting. More
SnO2 will be needed in B2O3 rich glazes as these have a poorer
solubility of Cu.....(*&^(*&^%$^%#$^$#----------- Enough!
Tchau,
Kat
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