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 clay50%
Any earthenware clay50%
 --------
 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 feldspar50% (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. kaolin30
Tenn. #9 ball clay30
Kingsley kaolin10
Nepheline syenite10
Talc8
Flint12
 --------
 100
+ Bentonite2%


2. C/4.5.6 Off-White Stoneware

Goldart stoneware20
Pine Lake fire clay15
E.P.K. kaolin20
Tenn. #9 ball clay20
Nepheline syenite20
Flint5
 --------
 100


3. C/4.5.6 Light Sandy-Tan Stoneware

Goldart stoneware30
Pine Lake fire clay15
Kentucky OM #4 ball20
Ocmulgee red clay25
Talc10
 --------
 100


4. C/4.5.6 Medium Dark Red, Brown Cinnamon Stoneware

Goldart Stoneware20
Pine Lake fire clay20
Ocmulgee red clay35
P.B.X. fire clay20
Nepheline syenite5
 --------
 100
+ Grog (20/30 mesh)
  (if desired)
6%


5. C/4.5.6 Sculpture Body

A.P Green fire clay30
Pine Lake fire clay25
Ocmulgee red clay15
P.B.X fire clay15
Wollastonite15
 --------
 100
+ Grog (20/30 mesh)30%
+ Sand15%
  

Low shrinkage. Not very plastic. Good for large heavy pieces.


6. C/4.5.6 White China Translucent

E.P.K. kaolin20
Grolleg china clay20
Kentucky ball clay5
Nepheline syenite30
Gerstley borate5
Flint20
 --------
 100
+ Bentonite3%

Not too plastic. Fire to C/6 for translucency.


7. C/4.5.6 Dark Brown (Very Dark) Stoneware

Ocmulgee red clay50
Pine Lake fire clay15
Goldart stoneware15
Kentucky OM #4 ball10
Barnard clay10
 --------
 100
+ Iron oxide4%
+ 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 syenite50
Barium carbonate10
Ferro frit #31245
Whiting5
Lithium carbonate5
E.P.K. kaolin10
Flint15
 --------
 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 slip72
Barium carbonate10
Cryolite6
Gerstley borate12
 --------
 100

Smooth matt, yellow, orange-brown.


3. C/5.6 Alkaline Matt

Nepheline syenite30
Ferro frit #311025
Barium carbonate10
Whiting15
E.P.K. kaolin10
Flint10
 --------
 100

Base glaze is a whitish, stony point matt.
Colors:

  1. 1% copper carbonate - light blue
  2. 5% iron oxide - yellow with orange-greenish streaks
  3. 1% manganese dioxide - pinkish-purple


4. C/5.6 "Weird" Matt, Speckled

Gerstley borate50
Talc35
Rutile15
 --------
 100
+ Granular ilmenite0.25%

Yellow, orange, tannish, use thick and thin.


5. C/4.5.6 Leadless, Glossy Transparent

Kona F-4 feldspar35
Gerstley borate23
Barium carbonate8
Whiting8
E.P.K. kaolin8
Flint18
 --------
 100

Add 1% copper carbonate for blue/green.


6. C/5.6 Barium Stone Matt

Nepheline syenite60
Barium carbonate24
Lithium carbonate5
E.P.K. kaolin6
Flint5
 --------
 100

Very good for intense color, particularly over a white body.
Colors:

  1. 3% copper carbonate - very strong blue
  2. 2% iron oxide - brownish, yellow with green
  3. 1% chrome oxide - green
  4. 6% Mason's vanadium stain - bright yellow
  5. 1/2% manganese dioxide - pinkish violet


7. C/5.6 Satin Matt, Smooth Whitish

Nepheline syenite25
Ferro frit #312425
Whiting20
E.P.K. kaolin15
Flint15
 --------
 100

Interesting glaze for color. Run tests.


8. C/5.6 Smooth Satin/Matt Glaze

Nepheline syenite45
Whiting18
E.P.K. kaolin20
Zinc oxide12
Flint5
 --------
 100

Color may be added to this base.


9. C/4.5.6 Wood Ash Glaze

Wood ash (sifted)40
Nepheline syenite30
Talc15
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 kaolin30
Ky. OM #4 ball30
Nepheline syenite10
Ferro frit #312410
Flint10
Zircopax5
Kingsley kaolin10
 --------
 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 kaolin10
Calcined kaolin10
Ky. OM #4 ball10
Nepheline syenite15
Ferro frit #312410
Flint25
Borax5
Zircopax15
 --------
 100

For dry to bisque application.


3. C/4.5.6 Dark Slip

Albany slip50
Barnard clay35
Gerstley borate15
 --------
 100

Use under glazes.
Will work nicely coming up through glazes for dark iron texture.