Firing in Oxidation to C/4.5.6
by Val Cushing
This article first appeared in
Studio Potter, Volume 5, Number 2 (June 1977).
Copyright © 1977 by Studio Potter. All rights reserved.
May be reproduced with permission of Studio Potter.
The firing range and atmosphere of C/4.5.6 oxidation does not get the
attention it deserves by American potters. There are potters who use it and
understand its advantages, but more often this range is used when other alternatives
are denied. And the alternative most desired seems to be C9/10, high fire reduction
firing. The seductive charms of high fire reduction have continued to lure us into
smoke-filled kiln rooms for the past 30 years. This dominance of high fire reduction
became epidemic during the middle 1940's and was given special prominence
by Bernard Leach in his widely read and important book, A Potter's Book.
Earlier work in high fire reduction had been done by Binns and Charles Harder at
Alfred University, at Ohio State University, and by a few other individuals around
the country. But the Leach book was a turning point and
focused attention on the generally quiet and subtle beauty of early Chinese, Korean,
and more importantly, Japanese pottery. The romance became feverish. Reduction
firing swept the country. Every exhibition, every craft store was flooded with
subdued browns, muted greys, grey-greens, and brown blacks which were the glaze
colors resulting from this rather mysterious way of
firing. It was new and challenging, with little information available about how
to get results.
Formulas from a few soot-smudged notebooks began leaking out. A mystique
developed. In the early 1950's eager students, like myself, were captivated. Studio
production potters were working in reduction and those who went into teaching, as I
did, perpetuated this whole system of firing. High fire reduction is still dominant
today, 30 years later, although many other ways of working and firing are prevalent
in various parts of the country. The most influential
change from the reduction system came through those potters who began working
more with sculpture and less with the functional pot. Many potters in England and
elsewhere in Europe have always fired in oxidation and continue to do so today. My
point is that work in high fire reduction, particularly among functional potters, has
seemed to be almost mandatory; a sign of real professionalism and the only way for
significant expression in pottery. Isn't it time we re-examined these assumptions?
A growing number of potters think it is. I will
try to present some reasons for and advantages of firing in oxidation at the range of
C/4.5.6. At the end of this article I will list
some formulas for clay bodies, slips and glazes for this range which have proved
interesting and may help others get started.
Economic Advantages
The most obvious benefit is economics. Not only is this range about 100°C
(200°F) lower than C9/10, but oxidation is a faster fire than the long, slow
soaking fires of reduction, as practiced by most high fire reduction potters.
The soot and smoke of reduction wastes fuel and is pollution in the
atmosphere, no matter how slight in comparison to industrial violators. Oxidation
firings do not require gas, oil, wood or coal fuels and can actually be done most
successfully in electric kilns. Surely this is an
ecological gain, not to mention the prediction by some scientists that electricity
will soon be the only available source of energy
for most individuals. All the fossil fuels will be needed by generating plants to
make this electricity.
Let's talk about the electric kiln. It is certainly easier and safer to fire, cheaper to
build, more economical of floor space, and easier to locate in the kinds of studios many
potters must use. There is a vast selection of electric kilns available commercially for
those who cannot build their own - a far greater choice of size, shape and cost
range than is possible for any other type of commercial kiln. Building one's own electric
kiln may be a little more expensive than a gas kiln, but lends itself to some of the
technological innovations in materials that are flooding the market. I'm thinking here of
various lightweight insulating materials like hardboards, blankets, castables and systems
of easy pre-fabrication. It should be emphasized at this point that there is a truly
vast body of technical information available on kilns, firing, bodies, glazes, slips,
engobes and all other aspects of oxidation firing in the engineering and scientific
literature. There is little on reduction from these particular sources because reduction
firing is generally considered the enemy by the ceramic industry. Potters will find it
easier to approach engineers, ceramic technologists and scientists with questions and
problems concerning oxidation firing since nearly all phases of the ceramic industry are
exclusively concerned with oxidation. The reason for that is, as all potters know, that
reduction firings are less reliable, less predictable, less controllable and filled with
constant hazards. Oxidation firing does have advantages when it comes to those
last points, if you are looking at practicality. It is certainly easier for potters to find a
studio location near the urban centers, the real market place for pottery, when they are
firing with an electric kiln. The electric kiln decision may have to be made on this point
alone.
Clay Bodies
Our next consideration should be clays and clay bodies. If for some reason a potter
had to be limited to one firing range, I believe the greatest versatility would be found
at C/4.5.6. It is a perfect range in which to combine all the advantages of stoneware
with all the advantages of earthenware. You can combine these two or do either separately.
C/4.5.6 is the beginning of the stoneware range which is properly designated as C/4 to
C/10. It is also just a little above the normal range of what we call
earthenware which is ordinarily considered to be about C/06 to C/1 or 2. The C/4.5.6
middle range is the point at which real density and hardness begins to develop in
stoneware clays, ball clays, most fire clays, and the plastic kaolins. Clay body
formulation is easier and more logical when you can base the body on clays which are
in their normal maturing ranges. A little more body
flux, or the selection of a stronger body flux, is all you need to transform C/4.5.6 bodies
into the rockhard, tough, durable and nonabsorbing bodies we associate with the
C/9-10 range. The primary clay body fluxes are feldspars, talcs, frits of various kinds
and iron oxide in various forms. Nepheline syenite, our lowest melting feldspar, is ideal
for C/4.5.6 bodies. Any C/9-10 body can be dropped to C/4.5.6 with an increase in flux
or by switching to nepheline syenite without losing any of the properties or
characteristics it has at C/9-10. (Specific formulas will be given later.)
In this discussion I am making assumptions about clay bodies that I should explain
in more detail. By clay bodies I mean plastic clay bodies, suited for normal wheel
throwing and hand building uses. These bodies should have total shrinkage, from the
plastic stage to the fired stage, of about 12 to 14%. They should also have an absorbency
of under about 6%, which means they will not leak or absorb foods. They will be resistant
to warping and cracking under normal drying and firing conditions. If properly designed
there is no way you could tell a C/4.5.6 body from a C/9-10 body without
putting it under a microscope or through some other scientific testing device. Up to
this point I have been speaking of bringing C/9-10 bodies to the C/4.5.6 range. Now let's
start with earthenware bodies and clays.
Natural earthenware clays are those clays which contain fairly high amounts of iron
oxide in combination with fluxing agents like calcium oxide and sodium oxide. These
clays are plastic and fire to the warm earthy colors of tan, orange, red, brown, etc.,
although their unfired colors may range from red to grey to green to blue to black. Their
normal firing range is approximately C/06 to C/1 or 2. Some clays of this type can
withstand firings of up to C/4.5.6 without bloating or melting or showing other signs
of being over-fired. We use three clays of this type at Alfred and they are easily
available throughout the East. They are Redart, Ocmulgee and Calvert. These clays make
ideal body ingredients for C/4.5.6 bodies. They are fine grained, plastic, richly colored,
and very dense when fired to C/4.5.6. One can find these "common" red iron
bearing clays all over the U.S.A. Deposits of these clays are often found along stream
beds, the edges of lakes, ponds, rivers or in thick layers just below the surface sod and
top soils. These earthenware clays can be the basis of C/4.5.6 bodies in the same way
that stoneware clays can be used. In some earthenware clays you may need to use less
body flux or to add some higher firing clays in order to be certain that they will not
overfire. One very simple body for C/4.5.6 is as follows.
| Any stoneware clay | 50% |
| Any earthenware clay | 50% |
| | -------- |
| | 100% |
This body has much to recommend it, although it could be improved. It is quite
acceptable in plasticity and absorbency. I hope you will see that it is a logical
bringing-together of the high fire and the low fire to the middle range. Naturally this
body may need adjusting to the particular stoneware and earthenware clays you are
using. If you have a very low maturing earthenware clay, you may need to use only 25
to 40% earthenware to 75 or 60% stoneware clay. In many C/4.5.6 bodies the iron oxide
and other fluxes in the earthenware clay will take the place of an added body flux and
give you the rich and warm colors of earthenware in combination with stoneware
and other high fire clays. C/4.5.6 oxidation allows you to take advantage of all the rich
and full earthy color and plasticity of earthenware as well as the tougher, more durable
and "toothy" qualities of stonewares. It is an ideal range for expressive clay
bodies. One last point on body colors: High firing reduction potters know that reduction
firing at any temperature "cools" and greys down the fired color of their bodies.
Iron oxide (Fe2O3) becomes FeO in reduction, which is dark grey to black in color and
stays that color except for the very surface layer of exposed clay which re-oxidizes
(partially) back to Fe203 during the cooling cycle. I think many reduction potters would
admit that they would very much like to retain the warm orange-brown body colors
that are so simple to get and all so characteristic in oxidation - the earthy colors in
oxidation have more zest and brightness.
Before going on to a discussion of C/4.5.6 glazes we should consider the question of
white firing bodies. Here again we find C/4.5.6 oxidation an ideal range. We can
capitalize on the plasticity and chalky whiteness of white earthenware in combination
with the density, hardness and translucency (if desired) of higher fire china and
porcelain. True porcelain is of course, by technical definition, reduction fired and
cannot be authentically duplicated in oxidation. It should be said that C/4.5.6
reduction firing can exactly duplicate C/9-10 reduction fired porcelain, but that is
another discussion. In my view there is no point
whatever in firing to C/9-10 oxidation for white clay bodies (or stoneware either, for
that matter). Everything you can possibly achieve at C/9-10 is possible at C/4.5.6. In
fact, you can do all high fire effects and much more at C/4.5.6 for less money and
time.
As was the case with stoneware bodies, discussed earlier, making white bodies at
C/4.5.6 is essentially a matter of increasing the amount of the primary body flux or
changing to a stronger flux in order to get the equivalent density, hardness and fired
strength of C/9-10 china. If you are making a truly white body you will be restricted
to the use of kaolins and ball clays - no other clays are white-firing. In many cases you
need only add more flux or change to a stronger flux like nepheline syenite to
transform the very same C/9-10 body to make it have the exact same properties as it
had at C/9-10. If translucency is wanted, you may need to add some additional flux, as
you would need to do at C/9-10. Actual whiteness will be improved at C/4.5.6 because
the kaolins and ball clays have a natural tendency to darken or become
off-white at higher temperatures.
You can also use higher percentages of the plastic kaolins and ball clays at C/4.5.6
because their fired shrinkage is lowered when they are fired at the beginning of their
maturing range. For this reason C/4.5.6 white can be made more plastic and workable
without risk of excessive warping and cracking. The main body fluxes for C/4.5.6
white bodies are nepheline syenite and talc. Where translucency is wanted it may be
necessary to make additions of whiting (CaCO3), or Gerstley borate (colemanite), or
commercial frits to cause more glass to be formed in the body and therefore more
translucency.
The basic fired effects of clay slips and engobes is essentially unaffected by
different firing temperatures and atmospheres. All slips and engobes give specific effects
which can all be duplicated at all temperatures and in oxidation or in reduction. The
main and decided advantage of clay slips and engobes in oxidation is the expanded
possibilities for color development. We all know that color is very much limited by
reduction fire. This applies to clay bodies as well as to slips, engobes, and of course to
glazes. The formulation of slips for C/4.5.6 follows the lines previously described for
making white clay bodies. White base slips for C/4.5.6 are the same general composition
as C/9-10 slips except that fluxes are either increased in amount or switched to
smaller amounts of more powerful fluxes. Frits are commonly used for this purpose
along with Gerstley borate and soluble materials like soda ash and borax. Color
effects will be discussed in the next section in conjunction with glazes.
Glazes
Glazes designed for this temperature and atmosphere have the decided advantage of
a greatly increased color range. And as we have seen with clay bodies, it is possible to
utilize both high fire stoneware effects and the nearly complete range of earthenware
glazes. The material of prime importance for C/9-10 glazes is feldspar. Feldspars are
natural materials that are nearly perfect glazes by themselves. They are mixtures
high in silica, which is the main glass-forming oxide, along with alumina and
small amounts of the important fluxing oxides of sodium, potassium and calcium.
Feldspars form the basis of all C/9-10 glazes. In the earthenware range the primary
glaze ingredients are either frits, which are commercially blended low fire
glasses, or silicates formed with lead, or glasses formed by using B203 (boric oxide),
another glass-forming oxide.
At C/4.5.6 most feldspars are slightly underfired and have not quite reached a
complete glassy state. The exception here would be nepheline syenite which, as
already mentioned, is quite fluid at this temperature. Most of the frits, the lead
silicates and borosilicates are overfired at C/4.5.6.
But you need only combine the high fire feldspars with the lower fired ingredients
just mentioned to make excellent C/4.5.6 glazes. Of course, small additions of clay
and flint may be needed to complete these glazes but that is true for high and low fire
glazes as well. I don't want to seem to be over-simplifying, but I hope you can see the
logic and the parallels between glaze making at C/4.5.6 with the approach already
suggested for clay bodies. A simple example given here may illustrate my point. You
have already seen that an acceptable body can be made by adding 50% stoneware clay
to 50% earthenware clay. The following glaze is a C/4.5.6 glossy transparent:
| Any feldspar | 50% | (high fire ingredient) |
Gerstley borate
(colemanite) | 50% | (low fire ingredient) |
| | -------- | |
| | 100% | |
You could work out a series of similar glazes using approximately 50% feldspar
combined with approximately 50% frit. A simple line blend would be run between
100% spar and 100% frit, going 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, 20/80 to 100%.
The ideal melt would be found somewhere in that blend. (Remember that small
amounts of clay and flint may also be needed.) There are literally hundreds of
different frits that could give an infinite variety of glazes, along with any one of
several different feldspars. The particular choice of
a feldspar and the particular choice of a frit is crucial to the development of specific
colors. For example, copper blues require alkaline mixtures, uranium oxide reds require
lead mixtures, etc. What I am trying to illustrate is that C/4.5.6 glazes can result
from fairly simple blending of the essential high fire glaze-making ingredients with the
essential ingredients of low fire glazes. It is obviously not possible to go into extensive
glaze theory and lectures in this article. My only aim is to give some insight into the
question.
As I stated earlier the wide color range is the main advantage of C/4.5.6 oxidation
over high fire. As for general glaze characteristics there is no difference between
C/4.5.6 and C/9-10. You can make glazes with any degree of light transmission from
transparent, to translucent, to semiopaque, to opaque. You can make matte
glazes, satin glazes, glossy glazes and glazes of any other desired surface texture.
You can make wood ash glazes at C/4.5.6, as well as Albany slip glazes, crystalline
glazes, adventurine glazes, fired in lustres, salt glazing, soda glazing, residual firings,
majolica glazes and nearly any other effect that I can think of.
If I have expressed my thoughts clearly then we can now all agree that C/4.5.6 oxidation
firings will save us money. We will be able to use both high fire stoneware and
low fire earthenware glazes and bodies. We will be able to use a much fuller range of
colors than is possible at high fire. We will also be able to develop the tough and
durable feldspathic glazes of C/9-10 at C/4.5.6. Our firings will be more reliable, more
dependable, more predictable. We will be able to get more technical help from industry.
We will have an easier time in locating a studio, because electric kilns are less trouble
in this regard than fuel kilns. We will be safer, less polluting and more ecological.
Will that help convince you?
I wish I could end this paper by telling you that I have also made the conversion to
C/4.5.6 oxidation. Maybe some day!
Val Cushing
November 1976
Cone 4.5.6 Oxidation Plastic Clay Bodies
(Throwing or Handbuilding)
All have approximately 12-14% shrinkage and 6% or less absorption.
1. C/4.5.6 White Body
| E.P.K. kaolin | 30 |
| Tenn. #9 ball clay | 30 |
| Kingsley kaolin | 10 |
| Nepheline syenite | 10 |
| Talc | 8 |
| Flint | 12 |
| | -------- |
| | 100 |
| + Bentonite | 2% |
2. C/4.5.6 Off-White Stoneware
| Goldart stoneware | 20 |
| Pine Lake fire clay | 15 |
| E.P.K. kaolin | 20 |
| Tenn. #9 ball clay | 20 |
| Nepheline syenite | 20 |
| Flint | 5 |
| | -------- |
| | 100 |
3. C/4.5.6 Light Sandy-Tan Stoneware
| Goldart stoneware | 30 |
| Pine Lake fire clay | 15 |
| Kentucky OM #4 ball | 20 |
| Ocmulgee red clay | 25 |
| Talc | 10 |
| | -------- |
| | 100 |
4. C/4.5.6 Medium Dark Red, Brown Cinnamon Stoneware
| Goldart Stoneware | 20 |
| Pine Lake fire clay | 20 |
| Ocmulgee red clay | 35 |
| P.B.X. fire clay | 20 |
| Nepheline syenite | 5 |
| | -------- |
| | 100 |
+ Grog (20/30 mesh)
(if desired) | 6% |
5. C/4.5.6 Sculpture Body
| A.P Green fire clay | 30 |
| Pine Lake fire clay | 25 |
| Ocmulgee red clay | 15 |
| P.B.X fire clay | 15 |
| Wollastonite | 15 |
| | -------- |
| | 100 |
| + Grog (20/30 mesh) | 30% |
| + Sand | 15% |
| | |
|
Low shrinkage. Not very plastic. Good for large heavy pieces.
6. C/4.5.6 White China Translucent
| E.P.K. kaolin | 20 |
| Grolleg china clay | 20 |
| Kentucky ball clay | 5 |
| Nepheline syenite | 30 |
| Gerstley borate | 5 |
| Flint | 20 |
| | -------- |
| | 100 |
| + Bentonite | 3% |
Not too plastic. Fire to C/6 for translucency.
7. C/4.5.6 Dark Brown (Very Dark) Stoneware
| Ocmulgee red clay | 50 |
| Pine Lake fire clay | 15 |
| Goldart stoneware | 15 |
| Kentucky OM #4 ball | 10 |
| Barnard clay | 10 |
| | -------- |
| | 100 |
| + Iron oxide | 4% |
| + Grog (20/30 mesh) | 6% |
Some Glazes for Cone 4.5.6 Oxidation
Not all have the full range of 4.5.6. All should work at c/5,
some may be best at c/4.5, some at c/5.6.
1. C/5.6 Stone Matt, Whitish Base
| Nepheline syenite | 50 |
| Barium carbonate | 10 |
| Ferro frit #3124 | 5 |
| Whiting | 5 |
| Lithium carbonate | 5 |
| E.P.K. kaolin | 10 |
| Flint | 15 |
| | -------- |
| | 100 |
Good matt glaze for color; run tests with copper, iron, chrome, etc.
3% iron oxide gives yellow-green.
2. C/5.6 Albany Slip Glaze
| Albany slip | 72 |
| Barium carbonate | 10 |
| Cryolite | 6 |
| Gerstley borate | 12 |
| | -------- |
| | 100 |
Smooth matt, yellow, orange-brown.
3. C/5.6 Alkaline Matt
| Nepheline syenite | 30 |
| Ferro frit #3110 | 25 |
| Barium carbonate | 10 |
| Whiting | 15 |
| E.P.K. kaolin | 10 |
| Flint | 10 |
| | -------- |
| | 100 |
Base glaze is a whitish, stony point matt.
Colors:
- 1% copper carbonate - light blue
- 5% iron oxide - yellow with orange-greenish streaks
- 1% manganese dioxide - pinkish-purple
4. C/5.6 "Weird" Matt, Speckled
| Gerstley borate | 50 |
| Talc | 35 |
| Rutile | 15 |
| | -------- |
| | 100 |
| + Granular ilmenite | 0.25% |
Yellow, orange, tannish, use thick and thin.
5. C/4.5.6 Leadless, Glossy Transparent
| Kona F-4 feldspar | 35 |
| Gerstley borate | 23 |
| Barium carbonate | 8 |
| Whiting | 8 |
| E.P.K. kaolin | 8 |
| Flint | 18 |
| | -------- |
| | 100 |
Add 1% copper carbonate for blue/green.
6. C/5.6 Barium Stone Matt
| Nepheline syenite | 60 |
| Barium carbonate | 24 |
| Lithium carbonate | 5 |
| E.P.K. kaolin | 6 |
| Flint | 5 |
| | -------- |
| | 100 |
Very good for intense color, particularly over a white body.
Colors:
- 3% copper carbonate - very strong blue
- 2% iron oxide - brownish, yellow with green
- 1% chrome oxide - green
- 6% Mason's vanadium stain - bright yellow
- 1/2% manganese dioxide - pinkish violet
7. C/5.6 Satin Matt, Smooth Whitish
| Nepheline syenite | 25 |
| Ferro frit #3124 | 25 |
| Whiting | 20 |
| E.P.K. kaolin | 15 |
| Flint | 15 |
| | -------- |
| | 100 |
Interesting glaze for color. Run tests.
8. C/5.6 Smooth Satin/Matt Glaze
| Nepheline syenite | 45 |
| Whiting | 18 |
| E.P.K. kaolin | 20 |
| Zinc oxide | 12 |
| Flint | 5 |
| | -------- |
| | 100 |
Color may be added to this base.
9. C/4.5.6 Wood Ash Glaze
| Wood ash (sifted) | 40 |
| Nepheline syenite | 30 |
| Talc | 15 |
| P.B.X. clay (Valentine) | 15 |
| | -------- |
| | 100 |
Should be strong fluxing type ash, like elm, etc.
White Base Slips (Engobes)
for C/4.5.6 Oxidation
1. C/4.5.6 White Base Slip
| E.P.K kaolin | 30 |
| Ky. OM #4 ball | 30 |
| Nepheline syenite | 10 |
| Ferro frit #3124 | 10 |
| Flint | 10 |
| Zircopax | 5 |
| Kingsley kaolin | 10 |
| | -------- |
| | 100 |
For wet to leather-hard application.
Color tests can be run to any white slip.
2. C/4.5.6 White Base Slip
| Kingsley kaolin | 10 |
| Calcined kaolin | 10 |
| Ky. OM #4 ball | 10 |
| Nepheline syenite | 15 |
| Ferro frit #3124 | 10 |
| Flint | 25 |
| Borax | 5 |
| Zircopax | 15 |
| | -------- |
| | 100 |
For dry to bisque application.
3. C/4.5.6 Dark Slip
| Albany slip | 50 |
| Barnard clay | 35 |
| Gerstley borate | 15 |
| | -------- |
| | 100 |
Use under glazes.
Will work nicely coming up through glazes for dark iron texture.
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