Glazes  for the Selfreliant Potter (GTZ, 1993, 179 p.) 
16. Glaze formula calculations 

About 100 years ago a German ceramist, Hermann Seger, developed Seger cones for measuring temperatures in kilns. He also proposed writing the composition of glazes according to the number of different oxides in the glaze instead of listing the raw materials used in the glaze.
For example: Aluminum oxide can be added to the glaze either in the form of clay (Al_{2}O_{3} · 2SiO_{2} · 2H_{2}O) or feldspar (K_{2}O· Al_{2}O_{3} · 6SiO_{2}).
The oxides used in glazes are divided into three groups according to the way the oxides work in the glaze.
Fluxes
This group of oxides functions as melter, and fluxes are also called basic oxides or bases. They are written RO or R_{2}O, where R represents any atom and O represents oxygen. So all the fluxes are a combination of one or two element atoms and one oxygen atom.
Stabilizers
These work as stiffeners in the melted glaze to prevent it from running too much. They are considered neutral oxides and are writen as R_{2}O_{3} or two atoms of some element combined with three oxygen atoms.
Glass formers
These form the noncrystalline structure of the glaze. They are called acidic oxides and are written as RO_{2} or one element atom combined with two oxygen atoms.
Seger formulas allow all glaze formulas to be expressed in a table, keeping the groups separate in order to make comparison of different formulas easy (see below).
In the table form, the sum of the fluxes must always equal 1, which makes different formulas easy to compare.
Examples
The organization of the Seger formula is always
according to the table shown below.
FLUXES 
STABILIZER 
GLASS FORMERS 
RO, R_{2}O 
R_{2}O_{3} 
RO_{2} 
Alkalis: 
Al_{2}O_{3} 
SiO_{2} 
K_{2}O 
B_{2}O_{3} 
TiO_{2} 
Na_{2}O 
B_{2}O_{3}  
Li_{2}O 
 
Alkaline earths: 
 
CaO   
MgO   
BaO   
Other:   
PbO   
ZnO   
Note: B_{2}O_{3} is sometimes listed under stabilizers and sometimes under glass formers, since it has both characteristics.
TABLE OF LIMIT FORMULAS*
NOTE: "KNaO" is a symbol for either sodium or potassium oxide.
c012  08 Lead Glazes      
PbO 
0.7  1.0 
Al_{2}O_{3} 
0.05  0.2 
SiO_{2} 
1.0  1.5 
KNaO 
0  0.3  
  
ZnO 
0  0.1  
  
CaO 
0  0.2  
  
c08  01 Lead Glazes 
  
 
PbO 
0.7  1.0 
Al_{2}O_{3} 
0.1  0.25 
SiO_{2} 
1.5  2.0 
KNaO 
0  0.3  
  
ZnO 
0  0.2  
  
CaO 
0  0.3  
  
c08  04 Alkaline Glazes      
PbO 
0  0.5 
Al_{2}O_{3} 
0.5  0.25 
SiO_{2} 
1.5  2.5 
KNaO 
0.4  0.8  
  
ZnO 
0  0.2  
  
CaO 
0  0.3  
  
c08  04 LeadBoron 
  
 
PbO 
0.2  0.6 
Al_{2}O_{3} 
0.15  0.2 
SiO_{2} 
1.5  2.5 
KNaO 
0.1  0.25  

B_{2}O_{3} 
0.15  0.6 
ZnO 
0.1  0.25  
  
CaO 
0.3  0.6  
  
BaO 
0  0.15  
  
c2  5 Lead Glazes 
  
 
PbO 
0.4  0.6 
Al_{2}O_{3} 
0.2  0.28 
SiO_{2} 
2.0  3.0 
KNaO 
0.1  0.25  
  
ZnO 
0  0.25  
  
CaO 
0.1  0.4  
  
c2  5 Boron 
  
 
KNaO 
0.1  0.25 
Al_{2}O_{3} 
0.2  0.28 
SiO_{2} 
2.0  3.0 
ZnO 
0.1  0.25  

B_{2}O_{3} 
0.3  0.6 
CaO 
0.2  0.5  
  
BaO 
0.1  0.25  
  
c2  5 Lead Borosilicate      
PbO 
0.2  0.3 
Al_{2}O_{3} 
0.25  0.35 
SiO_{2} 
2.5  3.5 
KNaO 
0.2  0.3  

B_{2}O_{3} 
0.2  0.6 
ZnO 
0  0.1  
  
CaO 
0.35  0.5  
  
c8  12 Stoneware and Porcelain     
KNaO 
0.2  0.4 
Al_{2}O_{3} 
0.3  0.5 
SiO_{2} 
3.0  5.0 
ZnO 
0  0.3  

B_{2}O_{3} 
0.1  0.3 
CaO 
0.4  0.7  
  
BaO 
0  0.3  
  
MgO 
0  0.3  
  
* D. Rhodes: Clay and Glazes for the Potter.
For example, a simple unfritted lead glaze would look like this:
FLUXES 
STABILIZER 
GLASS FORMERS 
RO, R2O 
R2O3 
RO2 
PbO 1.0 
Al2O3 0.1 
SiO2 1.5 
Remember that the flux column always totals 1.0.
A more complicated formula is the unfritted boron glaze:
CaO 
.414 
Al_{2}O_{3} 
.322 
SiO_{2} 
2.291 
MgO 
.414   
B_{2}O_{3} 
.931 
K20 
.172     

1.000     
There are some basic rules for the ratio of oxides in the 3 different groups, according to glaze temperature. These are called limit formulas (see page 139). They should only be considered guidelines, as many glazes exceed the limits m practice.
 Addition of 0.1 part SiO_{2} to a glaze will increase
the melting point by about 20°C.
 Addition of 0.05 part
B_{2}O_{3} will lower the melting point by 20°C.
The formulas of pyrometric Seger cones are listed in the
appendix. These can also be used as a guide for glazes by choosing a cone
formula 4 to 5 cones below the glaze firing temperature. If you need a glaze for
cone 9, 1280°C, you can use the cone 5 formula for the glaze.
16.2.1. BENEFITS OF USING SEGER
FORMULA
The main usefulness of the Seger formula is that it presents glazes in a way that is easy to compare. It is used for:
Originating new glazes
Glazes with desired characteristics of color, mattress etc. can first be written as Seger formulas, selecting oxides that are known to produce the effects.
Comparing glaze recipes
It is difficult to look at two recipes and see how they are different. If they are converted into Seger formulas, the differences can easily be seen.
Substituting materials
If a material is no longer available, other materials can be substituted by working out the quantities in the Seger formula.
Modifying glazes
Glazes that change character, have problems etc. can be analyzed as Seger formulas, and directions for testing decided.
The Seger formula should be considered a guide only, as most
theoretical glazes do not react as expected and still require empirical testing
to develop them fully. If you want to use Seger formulas for your glazes it is
nice to have exact chemical analysis of your raw materials, but this is seldom
the case. Instead you will have to pick one of the materials listed in the
appendix. They may be close enough for practical work.
16.2.2. GLAZE RECIPE FROM
FORMULA
To get the glaze recipe from the formula, there is a standard series of calculations.
Simple lead glaze example
PbO 1.0 
Al2O3 0.1 
SiO2 1.5 
First decide which raw materials to use. For lead oxide, PbO, the choices are red lead, white lead or litharge. Al2O3 is almost always obtained from china clay, and SiO2 usually from quartz powder.
The calculation is helped a table like this:
Material and formula 
Mol. Parts 
PbO 1.0 
Al2O3 0.1 
SiO2 1.5 
Litharge, PbO 
1.0 
1.0   
Kaolin, Al_{2}O_{3} · 2SiO_{2} · 2H_{2}O 
0.1  
0.1 
0.2 
Quartz, SiO_{2} 
1.3   
1.3 
TOTAL  
1.0 
0.1 
1.5 
1.0 molecular part (MP) of litharge provides all PbO needed. We enter kaolin and its formula in the table and write 0.1 for MP. When we take 0.1 part kaolin, we get 0.1 Al_{2}O_{3} and we enter this on the right. In the kaolin formula we have 2 SiO_{2} so when we take 0.1 kaolin we get 0.2 SiO_{2}. We list this under SiO2. We need 1.5 SiO_{2} so 1.3 remains and we get this from quartz.
Next the required molecular parts, MP, of each material are multipled by their molecular weights, MW, to get the batch weight of each material:
Material 
MP 
MW 
Calculation 
Batch weight 
Litharge 
1.0 
223 
223 x 1 
223 
Kaolin 
0.1 
258 
258 x 0.1 
25.8 
Quartz 
1.3 
60 
60 x 1.3 
78.0 
To change the recipe into percentages, all the figures are divided by the total:
Litharge 
223/326.8 = .68 = 68% 
Kaolin 
25.8/326.8 = .08 = 8% 
Quartz 
78.0/326.8 = .24 = 24% 
Boron glaze example
A more complicated formula is the unfritted boron glaze.
CaO 
.414 
Al2O3 
.322 
SiO2 
2.291 
MgO 
.414   
B2O3 
.931 
K2O 
.172     
Again, the first step is to select materials. Because materials that supply more than one oxide usually work better in glazes, they are preferred if available. We need both CaO and MgO, which are supplied by dolomite, CaCO3 · MgCO3. Potash feldspar supplies K2O along with Al2Ok3 and SiO2. Quartz provides SiO2. For boron, boric acid is selected.
CALCULATION PROCEDURE
1. Enter formula at top of calculation table.
2. Select
materials, enter formula and MW.
3. Multiply each material's MW with its MW
and enter result in part's weight.
4. Enter MP of each oxide of the material
under the formula to check oxide balance.
5. Convert parts' weight into a
percentage recipe.
As before we change the recipe to percentage:
Dolomite 
76/384 x 100 = 19.8 
20% 
Potash   
feldspar 
96/384 x 100 = 25 
25% 
Kaolin 
39/384 x 100 = 10.2 
10% 
Quartz 
58/384 x 100 = 15.1 
15% 
Boric acid 
115/384 x 100 = 29.9 
30% 
When calculating from formula to recipe, there is no need to carry out results beyond round figures, particularly when we do not know the exact chemical analysis of our materials.
Figure 16.2.2.A Copy this example of
a calculation table.
16.2.3. FORMULA FROM GLAZE
RECIPE
Calculating from a recipe to the Seger formula is the same process in reverse. We will use the same raw boric acid glaze as an example. Again we use the calculation table and the following steps:
1. Enter recipe materials and their formulas in the left column and MW and recipe figures in MP's weight column.2. Write oxides of the materials at top of table.
3. Divide each recipe figure with its MW and enter result under MP.
4. Multiply MP with each oxide in material formula and enter result under respective oxide in the right columns.
5. Add together all oxides and list them according to ROR2O3RO2.
6. Add oxides in RO and divide all RO figures with the total.
Note that from dolomite only CaO and MgO are entered in the formula. CO2 is released during heating and does not take part in the glaze melt. H2O of kaolin and boric acid likewise evaporates.
The oxides are set up in the standard Seger formula:
K_{2}O 
.045 
Al_{2}O_{3} 
.084 
SiO_{2} 
.598 
CaO 
.109   
B_{2}O_{3} 
.244 
MgO 
.109     

.263     
The formula is brought to unity by dividing all the figures by the total, .263, in the left column.
K_{2}O 
.171 
Al2O3 
.319 
SiO_{2} 
2.27 
CaO 
.414   
B_{2}O_{3} 
.928 
MgO 
.414     
NOTE: The figures are not exactly the same as the original formula above, due to rounding off the figures. This is accurate enough for practical work.
If you have a chemical analysis of materials you want to use in
a glaze, you first have to calculate the formula of the material as described on
page 137. Then you enter this formula and its formula weight in the table under
MW.