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Changing Our View of Glazes

Section: Glazes, Subsection: Introduction


A big secret to getting control of glazes is to begin looking at them as formulas of oxides rather than recipes of materials.

Article Text

The science of glaze formulation, adaptation and adjustment is about to take a leap. The reason is the computer. It is not the advent of calculations; we have done them for a century. No wait, we haven't always done them; we just knew how, but didn't actually do them. Computer software like INSIGHT has put ceramic calculations within easy reach of anyone that can work through the step-by-step examples at the beginning of the manual and it has given great new powers to seasoned technicians. Ordinary people have a new viewpoint and they are developing practical formula interpretation skills, they are successfully tackling the glaze challenges of their new business in a more technical way than even engineers in large companies did in the past. We are on the leading edge of a bit of a revolution in the understanding and making of glazes and clay bodies.

A typical textbook recipe (that has been used for years by thousands and given all of them trouble).

LA Matt 
Potash Feldspar 51.6 
Silica           5.6 
Whiting         18.8 
Zinc Oxide       8.6 
EPK             15.4 
Bentonite        1.0

First, I would like to propose some conventions and new terms and clarify more correct usage of existing ones. Let us take a little trip from where we have been to where I think we are heading.

Where do babies come from? Where does milk come from? To the average three year old, the obvious sources are the stork and the grocery store! Most potters can look back on a time when they were content to view glaze recipes in the same superficial manner. Recipes all came from textbooks and we felt a sense of great discovery after mixing 200 textbook recipes and finally stumbling onto one that worked in our kiln. Many became experts at remembering the exotic names. Since our most intimate knowledge reached far enough to distinguish between different recipes, I will refer to this as working at the "recipe level." Also, let's call a mix of materials a "recipe," not a "formula." That term is needed later.

As time went on, more and more people noted problems with this blind use of secondhand recipes. The method was expensive, labor intensive and the textbook recipes never seemed to behave in our own kilns (even more frustrating, whenever a mix didn't work, we learned nothing or were lead down some blind alley).

Just as children soon trace milk back to a dairy farm and babies to the hospital, many of us became dissatisfied with the recipe level approach and began to ask pointed questions. Where do these textbook recipes come from? What is each material for? What principles guided formulation? Could anyone do it?

Answers were quickly available, in the form of seminars and books extolling the virtues of material blending. this method entails firing line and triaxial blends of selected materials and picking a point somewhere on the blend that looks right. Refinement and adjustment are achieved by substitution trials and conditioning with special purpose materials. We will call this the "material level" approach, since it requires a good working knowledge of individual materials (hundreds of which are available) and their functions and interactions to form recipes.

The ancient masters probably used a painstaking trial and error technique, to both create and adjust, as do the vast majority of practitioners today. Many feel it provides control over color and surface development, while retaining a considerable element of discovery and surprise. However, most of us seem to get much more of the latter. Deriving a glaze recipe this way can be quite labor intensive and most potters still learn little about the real contribution made by each material. Further, the glaze may not fit your clay or be what you want and who knows if it is safe.

There is, however, a real problem when this material level approach is the only one employed. Formulation and adjustment projects are not usually wild stabs, but are specific problems with a narrow range of solutions (e.g. utilizing a native material in a glaze, dropping the melting temperature of a glaze while retaining its character, replacing a material with a blend of other minerals, dealing with crazing, hardness, gloss, devitrification, and stain compatibility problems). Material level analysis proves to be of little more value than the recipe level approach in dealing with these problems. There are few direct relationships between the presence or absence of a specific material and any of the above problems (e.g. more feldspar might increase or reduce crazing). Yes, years of blending experience, with the hundreds of available materials, often fails to provide any solution strategies, yet these are the very kinds of issues which usually face us.

Again, the inquiring mind asks questions. The seven year old has traced milk back to the cow, and babies to mummy's tummy. Can glazes be traced further back? It doesn't appear possible.

However, by adopting another viewpoint, two more levels of intimacy are achieved, reaching down to the standpoint of the Kiln God himself! Think about it for a minute. What are materials made of? A bag of glaze powder is made from ceramic materials, but the fired glaze is not. It can never be returned to these materials. The fired glaze, like the materials in its raw powder, is built of oxides, about 12-15 of them (e.g. SiO2 (silica), Al2O3 (alumina) etc.). A "formula" expresses the mix of these oxides. The action of the kiln fires (personified as our Kiln God) decompose materials to liberate their basic oxides, which can then move about freely in a glaze melt, rearranging themselves. This melt freezes to solidify a new oxide glass structure. By viewing a glaze recipe as if it were already a fired formula of oxides, you are looking at it from the same viewpoint as the Kiln God. Let's refer to this as the "formula level." The 12-15 individual oxides typical of most formulas, therefore, become our new material suite. When considering individual oxides and their effects, we will be working at the "oxide level." Since a recipe refers to material mixes, you don't "formulate" glazes at the recipe or material level, you actually "recipate" them, but let's not get carried away!

Do you remember all the sections in the ceramic texts describing the functions of each oxide and the make-up of each material? You may have skipped these before, but now they are going to be very useful. Consider some of the possibilities these new formula and oxide viewpoints provide:

Standard Report - LA MATT GLAZE ON M340 (08/29/89)
NUM: G1219
                            PARTS  UNIT      %
                              BY    OF       BY
$POTASH FELDSPAR            51.60         51.10
 SILICA                      5.60          5.50
 WHITING                    18.80         18.60
 ZINC OXIDE                  8.60          8.50
 EPK KAOLIN                 15.40         15.20
$BENTONITE                   1.00          1.00
                           101.00 KG
CaO          0.49  11.92
K2O          0.24   9.75
MgO          0.00   0.04
Na2O         0.00   0.01
ZnO          0.27   9.52
TiO2         0.00   0.07
Al2O3        0.39  17.13
P2O5         0.00   0.04
SiO2         1.97  51.34
Fe2O3        0.00   0.17
L.O.I.             10.54
 EXPAN -    8.80

The claim that calculations are too complex is no longer valid. Remember, their main purpose is to convert from formula to recipe and back, a utilitarian task easily handled by a computer. The formula viewpoint is simple and has proven in most cases to be valid and dependable. To demonstrate, think of the practice of doing flux substitutions to bring life to glazes. Most technicians would work at the material level, substituting flux containing materials on a gram-for-gram basis believing the only variable to be the identity of the flux, but the oxide viewpoint clearly shows that different oxides have drastically different formula weights so flux sourcing materials vary widely in the amount of oxide they contribute per gram. A complete upset in the balance of the glaze results where the materials also contain SiO2 and Al2O3 .

I want to make one last point again: Kiln disappointments usually teach us little on the recipe level, but we must learn from mistakes. Most of us are lacking in oxide formula interpretation skills, but by viewing glazes as oxide formulas we will learn new things every time something doesn't succeed.

Can you go back yet further? The milk can be traced back to the grass eaten by the cow. And the baby? Well, it can be traced all the way back to... "

Ceramic Oxide Periodic Table

All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).

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In Bound Links

By Tony Hansen

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