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close this bookGlazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)
close this folder16. Glaze formula calculations
View the document(introduction...)
View the document16.1. Glaze formula chemistry
View the document16.2. Seger formula
View the document16.3. Frit calculation

16.2. Seger formula

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 (Al2O3 · 2SiO2 · 2H2O) or feldspar (K2O· Al2O3 · 6SiO2).

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 R2O, 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 R2O3 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 RO2 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, R2O

R2O3

RO2

Alkalis:

Al2O3

SiO2

K2O

B2O3

TiO2

Na2O

B2O3


Li2O



Alkaline earths:



CaO



MgO



BaO



Other:



PbO



ZnO



Note: B2O3 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

Al2O3

0.05 - 0.2

SiO2

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

Al2O3

0.1 - 0.25

SiO2

1.5 - 2.0

KNaO

0 - 0.3





ZnO

0 - 0.2





CaO

0 - 0.3





c08 - 04 Alkaline Glazes





PbO

0 - 0.5

Al2O3

0.5 - 0.25

SiO2

1.5 - 2.5

KNaO

0.4 - 0.8





ZnO

0 - 0.2





CaO

0 - 0.3





c08 - 04 Lead-Boron





PbO

0.2 - 0.6

Al2O3

0.15 - 0.2

SiO2

1.5 - 2.5

KNaO

0.1 - 0.25



B2O3

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

Al2O3

0.2 - 0.28

SiO2

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

Al2O3

0.2 - 0.28

SiO2

2.0 - 3.0

ZnO

0.1 - 0.25



B2O3

0.3 - 0.6

CaO

0.2 - 0.5





BaO

0.1 - 0.25





c2 - 5 Lead Borosilicate





PbO

0.2 - 0.3

Al2O3

0.25 - 0.35

SiO2

2.5 - 3.5

KNaO

0.2 - 0.3



B2O3

0.2 - 0.6

ZnO

0 - 0.1





CaO

0.35 - 0.5





c8 - 12 Stoneware and Porcelain




KNaO

0.2 - 0.4

Al2O3

0.3 - 0.5

SiO2

3.0 - 5.0

ZnO

0 - 0.3



B2O3

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

Al2O3

.322

SiO2

2.291

MgO

.414



B2O3

.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 SiO2 to a glaze will increase the melting point by about 20°C.
- Addition of 0.05 part B2O3 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, Al2O3 · 2SiO2 · 2H2O

0.1


0.1

0.2

Quartz, SiO2

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 Al2O3 and we enter this on the right. In the kaolin formula we have 2 SiO2 so when we take 0.1 kaolin we get 0.2 SiO2. We list this under SiO2. We need 1.5 SiO2 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 RO-R2O3-RO2.

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:

K2O

.045

Al2O3

.084

SiO2

.598

CaO

.109



B2O3

.244

MgO

.109






.263





The formula is brought to unity by dividing all the figures by the total, .263, in the left column.

K2O

.171

Al2O3

.319

SiO2

2.27

CaO

.414



B2O3

.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.