Outline of
the Grid





 Outline of the Grid Method

The 3 main variables in the base glaze are fluxes, alumina and silica. The power of this method comes from the systematic variation of these variables using a standardized and simplified experimental approach.

 The Standard Grid


Varying Alumina and Silica: The central experimental format is the "grid" of 35 glazes on one large tile in a 5 X 7 layout displaying 2 of the 3 main variables:
1: alumina increasing from bottom to top (as a result of increasing kaolin) and
2: silica increasing left to right.
Varying Fluxes: In any one set, the proportion of flux materials, one to another, remains constant. To alter this third variable, we have to do a different set of 35 glazes.

In the second diagram we can see four zones that result automatically from the way we have increased the alumina and silica. The zone in the middle of the set can usually be regarded as "well balanced" glazes, or in other words, not "high" or "low" in anything. Don't forget that all the glazes contain fluxes as well.

If we vary Al2O3 with something other than kaolin, or vary SiO2 with something other than "silica" or quartz or flint, then we call this a non-standard grid.

Starting Point

The Starting Point example used here (70% Feldspar plus 30% Whiting) does indeed contain some alumina and silica even though they are both "flux materials". Looking at the formula for feldspar we can see that it contains Al2O3 and SiO2 as well as the flux K2O:
Potash Feldspar - K2O.Al2O3.6SiO2. Usually at least one of the flux materials chosen for a set will contain alumina and silica like this.

If having trouble reading the formulae, see the short list in the book Introduction.

An examination of these two diagrams will show that corner C has minimum alumina and silica. To understand the grid method this is where we start; the glaze in this corner is a mix of just "flux materials". We refer to Glaze C as the "Starting Point" for the set. For example it might consist of 70% feldspar plus 30% whiting.

Varying Alumina and Silica

Starting in Corner C made from just flux materials we add in silica (or quartz or flint - these are all more or less pure SiO2 ) to move from left to right across the grid. We add kaolin (our source of variable Al2O3 ) to go from bottom to top. The 35 glazes are always related to each other in exactly the same way. Essentially, the kaolin and added silica percentages across the grid are the same in every set. These percentages are laid out in the Standard Recipe Grid (following).

 The Concept:
   To understand the relationship between the 35 glazes, imagine this:
Initially we decide on the composition of glaze 31, the starting point glaze, which is composed of 100% flux materials. We could obtain the rest of the 35 glazes in the set by adding kaolin and/or silica to Glaze 31. Therefore the flux materials don't change as kaolin and silica increase. But this is not how we actually prepare the glazes.....

 The Method:
 To prepare a set, we make up the 4 glazes at the corners of the set, numbers 1, 5, 31 and 35, and blend them to produce the rest.

One set represents the "story" of the set of flux materials originally chosen for glaze 31, examining how it changes as we increase alumina and/or silica.


The Standard Recipe Grid

Glaze Recipes and Glaze Numbering: This diagram shows the recipes for each glaze in the grid (to the nearest 1%). It also shows the standard layout of the glaze numbers from 1 to 35. This layout does not change.
[*Note below: " Fl.Mtl." = "Total Flux Materials"]

60% Fl.Mtl.*
40% Kaolin
00% Silica
54% Fl.Mtl.
36% Kaolin
10% Silica
48% Fl.Mtl.
32% Kaolin
20% Silica
41% Fl.Mtl.
29% Kaolin
30% Silica
35% Fl.Mtl.
25% Kaolin
40% Silica
67% Fl.Mtl.
33% Kaolin
00% Silica
59% Fl.Mtl.
30% Kaolin
10% Silica
52% Fl.Mtl.
27% Kaolin
21% Silica
45% Fl.Mtl.
24% Kaolin
31% Silica
37% Fl.Mtl.
21% Kaolin
42% Silica
73% Fl.Mtl.
27% Kaolin
00% Silica
65% Fl.Mtl.
24% Kaolin
11% Silica
57% Fl.Mtl.
22% Kaolin
22% Silica
48% Fl.Mtl.
19% Kaolin
33% Silica
40% Fl.Mtl.
17% Kaolin
43% Silica
80% Fl.Mtl.
20% Kaolin
00% Silica
71% Fl.Mtl.
18% Kaolin
11% Silica
61% Fl.Mtl.
16% Kaolin
23% Silica
52% Fl.Mtl.
14% Kaolin
34% Silica
42% Fl.Mtl.
13% Kaolin
45% Silica
87% Fl.Mtl.
13% Kaolin
00% Silica
76% Fl.Mtl.
12% Kaolin
12% Silica
66% Fl.Mtl.
11% Kaolin
23% Silica
55% Fl.Mtl.
10% Kaolin
35% Silica
45% Fl.Mtl.
8% Kaolin
47% Silica
93% Fl.Mtl.
7% Kaolin
00% Silica
82% Fl.Mtl.
6% Kaolin
12% Silica
70% Fl.Mtl.
5% Kaolin
24% Silica
59% Fl.Mtl.
5% Kaolin
36% Silica
48% Fl.Mtl.
4% Kaolin
48% Silica
100% Fl.Mtl
00% Kaolin
00% Silica
87% Fl.Mtl.
00% Kaolin
13% Silica
75% Fl.Mtl.
00% Kaolin
25% Silica
62% Fl.Mtl.
00% Kaolin
38% Silica
50% Fl.Mtl.
00% Kaolin
50% Silica

 Note the following in the Standard Recipe Grid:
1. Glaze 31 is 100% flux materials. This is Corner C.
2. The bottom row contains no kaolin.
3. The left-hand vertical row contains no added silica.
4. The maximum amount of kaolin is 40% in Glaze No.1 (Corner A).
5. The maximum amount of added silica is 50% in Glaze No.35 (Corner D).

Working out the Flux Material Breakup

One of the important features of this method is that the percentages in this recipe grid do not change, irrespective of the flux materials used. If the set uses only one flux material, then these are the final recipes... for ANY set. Usually however there is more than one and we have to find the breakdown of flux material. There are several easy ways of doing this, and these will be given in a later section.

One Glaze Recipe Explains the Whole Set

Another result of using this approach is that all the glaze recipes can be worked out once we know the recipe for the Corner C glaze. This is a natural consequence of standardizing the method. This means the only recipe we need to record is Glaze C. From this anyone understanding the method can work out the rest of the 35 recipes if and when required. It also means that we can think of a whole set of glazes just by stating one recipe, Glaze C. We can compare whole sets of glazes one with another just by comparing the C-corner glazes; the difference between sets can be fully understood by examining the difference between the two C-glazes. This is because a particular glaze number always has the same amounts of kaolin and silica in the recipe.


Colourants and Opacifiers in the Standard Grid

Studying Colourant and Opacifier Addition

If we wish to study in depth the variation of colourant or opacifier in glazes, we would not use the grid method with its focus on fluxes, alumina and silica. Normally we would choose one or more base glazes for characteristics like melting point, surface quality, opacity, crystal formation, crazing, runniness etc. We would then systematically add colourants/opacifiers using line blending, or perhaps a biaxial or triaxial experiment.

The grid method is needed only in the initial search for the base glaze(s) used.

 It is important when designing an experiment to separate out the variables. In a two-dimensional grid format experiment we can cope with a maximum of 2 variables - for example, silica increasing from left to right and alumina from bottom to top. If we decide to add colourant or opacifier to the standard grid as outlined here, we simply add the same percentage to each of the 35 glazes. This means that the addition of colourant or opacifier is not a variable in this experiment of 35 glazes.
The general rule that will apply for this method is that colourants and opacifiers will be added as a percentage on top of the 100% of the base glaze. See the following example.
Glaze Recipe:
100% Base Glaze
+10% Red iron oxide

Revealing Glaze Principles

As well as discovering interesting glazes, the standard grid method also reveals glaze principles because wherever possible we are separating out the variables. This reveals cause and effect. For example, a glance at a fired standard grid tile reveals immediately the effect of altering alumina or silica on a given set of fluxes. Also, we can see the effect of altering fluxes by comparing the grid tiles for 2 different sets, if they have been fired on the same clay under identical firing conditions.