Digitalfire Ceramics Technical Articles

Changing Our View of Glazes

Section: Glazes, Subsection: Introduction

Description

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

Article

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 Which 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:

• Every fired effect has a formula profile which is to a considerable degree independent of the recipe used to supply the oxides. Oxides display patterns in the way they affect temperature range, expansion, matteness, color, stain compatibility, lead insolubility, crystal development, etc. You can thus predict fired effects from formulas with reasonable certainty. Using a computer, you can then create a recipe using materials at hand to closely fill the oxide needs. You then achieve material independence, meaning that a variety of materials can be used to supply a specific oxide. A formula can be sourced by an infinite number of different recipes (there are subtle fired effects and material variations beyond the scope of calculation, these will occur with this method also). This is a fundamental change in the way formulation projects have been typically executed and documented. While the old recipe and materials level approaches are still valid for many areas (e.g. body formulation), they lead us down a lot of blind alleys in others situations, waste time, and don't teach us much.

Standard Report - LA MATT GLAZE ON M340 (08/29/89)
==================================================
NUM: G1219
                            PARTS  UNIT      %
                              BY    OF       BY
MATERIAL                   WEIGHT  MEAS. WEIGHT
-----------------------------------------------
$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
FORMULA & ANALYSIS
==================
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

• Direct relationships exist between individual oxides and a host of fired properties, making problem solving and adjustment easier. Using the formula and oxide viewpoint, we can mix the same glaze from many different materials and adjust its expansion, color response, firing temperature, leachability, hardness, thermal shock resistance, and many other things. All of these are possible because of the simplicity and intimacy of working with formulas and oxides instead of recipes and materials.

• The oxide formula of a fired glaze or clay is physically very simple, analyzing its properties on this level is easier, more reliable, more intuitive, more documented and more educational; it is even cheaper and more fun. As a result, it doesn't take us years to develop useful glazes because we understand the mechanisms and the way the Kiln God builds with the oxides. Furthermore, there is no need to keep a secret glaze book, the recipes are easy to derive and you will have them in a constant state of improvement so that you won't worry about giving them out.

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... "

In Bound Links

• (Project) Ceramic Oxides Overview

  Ceramic formulation and adjustment technology can ...

INSIGHT is ceramic chemistry
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