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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

16.3. Frit calculation

Frit calculation is done in the same way as calculating a glaze, but the calculation is slightly more complicated. As with glazes it is important to follow the recipes accurately.

This also means that you have to make sure that the raw materials are not wet when you weigh them. Also remember that materials like calcined soda and borax will absorb moisture from the air if they are not kept in a sealed container.

16.3.1. MOISTURE COMPENSATION

If you have to weigh materials with a high moisture content you can compensate for this. Weigh 100 g of the material, dry it and then weigh it again. Moisture content is:

(wet weight -dry weight x 100)/dry weight = x%

This x % is added to the amount you are weighing to compensate for its moisture content. Example:

100 g kaolin weighs 92 g after drying.

(100 -92)/92 x 100 = 8.7 %

kaolin in recipe

3500 g

compensation 8.7 % x 3500

304.5 g

total amount needed

3804.5 g


16.3.2. FORMULA RULES FOR FRIT

The practice of fritting was described in section 7. The main reason for fritting is to make glaze materials insoluble, which is possible if the frit materials are mixed in the right proportion. In formula terms they should fall within these limits:

- Ratio flux: SiO2 should be between l: 1.5 and 1:3.

- The sum of K2O and Na2O should not exceed 0.5 molecular parts on the flux side, the rest being other fluxes like PbO, CaO, ZnO, BaO.

- B2O3 to SiO2 not less than 1:2, but with other materials like PbO, CaO, MgO, K2O in the frit the proportion can go down to 1:1.5.

- A little Al2O3 at least 0.05 mol. parts, reduces solubility but it should not exceed 0.2 mol. parts because it reduces the fluidity of the frit melt.



16.3.3. FRIT BASED ON GLAZE FORMULA

We have a glaze formula of an opaque boron glaze for 1100°C:

K2O

.23

Al2O3

.30

SiO=

2.60

ZnO

.27



B2O3

.80

CaO

.50





Initially we calculate the recipe as it was done for the unfritted glaze. We get the K2O from potash feldspar. Borax cannot be used for boric oxide because no Na2O is needed in the formula and so boric acid is required. We get the CaO from whiting and the rest of the materials will be kaolin, quartz and zinc oxide.

We now decide what material to include in the frit batch and what to include in the ball milling only. This is done according to the above rules. We need to include all the soluble boric acid. Along with that we can also include whiting and zinc oxide and some potash feldspar but not all because its Al2O3 will reduce the frit's fluidity.

A frit formula could be:

K2O

.1

Al2O3

.1

SiO2

1.60

ZnO

.27



B2O3

.80

CaO

.50





One problem still remains. When the frit melts, a large amount of H2O and CO2 is lost. Thus loss does not influence the recipe if we weigh the raw frit materials, melt the frit and use all the melted frit in the glaze, adding the other material according to the original amount of raw frit. But it is much more practical to produce a large batch of frit at a time and later weigh the melted frit to produce smaller batches of glaze. We need to find out how much weight is lost.

16.3.4. FRIT LOSS CALCULATION

Practical loss

The loss can be found simply by weighing the amount of melted frit that is produced from a batch of frit. Example:

Raw frit batch weighs in total 500 kg.
After firing the (dry) frit weighs 280 kg.

Loss in % = (500 -280)/500 x 100 = 44 %

Theoretical loss

The loss can also be calculated based on the formula of the frit. On heating, whiting changes to calcium oxide:

CaCO3 + heat ® CaO + CO2

Only CaO enters the melted frit and we can calculate how much this weighs:

The MW of calcium carbonate is 100 and that of calcium oxide is 56 so loss is 44 parts. In percentage this is 44 % The number used to find the amount of oxide entering fusion is called the conversion factor, CF. In the material table in the appendix one column lists the conversion factor for all materials. At the bottom of the left column there is a list for the most common frit materials.

Material

CF

% loss

Barium carbonate

0.777

22.3

Borax (crystal)

0.526

47.4

Boric acid

0.563

43.7

Dolomite

0.523

47.7

Kaolin

0.861

13.9

Laed carbonate (white)

0.863

13.7

Lead oxide (red)

0.977

2.3

Magnesium carbonate

0.478

52.2

Pearl ash

0.682

31.8

Soda ash

0.585

41.5

Soda crystals

0.217

78.3

Whiting

0.561

43.9

Frit glaze example

We can now calculate the loss of our frit from before.

Frit Recipe:

Raw

CF

Melted

Potash feldspar

55.6


55.6

Whiting

50.0

x 0.561

28.1

Quartz

60.0


60.0

Zinc oxide

21.9


21.9

Boric acid

98.4

x 0.563

55.4

Total

285.9


221.0

Theoretically we get only 77.3 % melted frit from our raw frit batch. We found that 286.3 parts raw frit equal 221.2 parts melted frit so finally we can establish our glaze recipe based on melted frit:

Final glaze recipe:

Frit

221.0

69.9%

Potash feldspar

72.3

22.9%

Kaolin

18.1

5.7%

Quartz

4.8

1.5%


16.3.5. GLAZE RECIPE WITH STANDARD FRIT

Very often a ceramics producer gets the frit from a commercial supplier or wants to use only a few standard frits. Above we calculated a new frit based on the glaze formula. We will now calculate a glaze recipe from formula using a standard frit instead.

Example of a standard frit formula:

K2O

.26

Al2O3

.05

SiO2

2.5

ZnO

.13



B2O3

1.0

CaO

.61





We will try to use the frit for the following glaze:

K2O

.30

Al2O3

.40

SiO2

3.5

ZnO

.20



B2O3

0.8

CaO

.50





The calculation is done as with the unfritted glaze. First oxides are entered at the top of the table and we start to select materials to satisfy them. Before starting, we need to know the formula weight of the frit. In the appendix we get the MW of all the oxides and these we total.

K2O

.26 x 94

=

24.4

Na2O

.13 x 62

=

8.1

CaO

.61 x 56

=

34.2

Al2O3

.05 x 102

=

5.1

SiO2

2.5 x 60

=

150.0

B2O3

1.0 x 70

=

70.0

Frit MW



291.8

This we round off to



292

The frit is entered in the calculation table like other materials with many oxides. The MP is selected according to the need of B2O3 It takes 0.8 MP of frit to get the needed 0.8 B2O3 and all the oxides listed in the frit formula are multiplied by this number and the results entered on the right of the table.

Glaze Recipe

Parts

%

Frit

233.6

61.7

Potash feldspar

51.2

13.5

Soda feldspar

50.3

13.3

Kaolin

42.7

11.2

Whiting

1

0.3


16.3.6. HINTS FOR USING UNKNOWN LOCAL MATERIALS

We have already discussed above calculating local materials by guessing their closest theoretical formula. This will usually give a good starting point for making line blends, which then can be used to get a working glaze or frit.

What do you do when you have a recipe or formula but do not know the analysis of your local materials and cannot get pure ones? Usually you can create a glaze using the formula or recipe as a starting point, but it is unlikely to match the description in the book.

The most common local materials are usually:

Clays

Common clays can be used in most glazes instead of kaolin, since they all contain Al2O3 and SiO2. But they will have lower melting points and probably change the glaze color, since they will introduce K2O, Na2O, Fe2O3, CaO, MgO and perhaps other fluxes. Probably the easiest way to work with them is simply to substitute directly for the kaolin, fire a sample and then use it as the basis for line blends to get a working glaze.

Feldspars

There are a tremendous number of different feldspars, all of which vary in the relative amounts of K2O, Na2O, CaO, MgO, Al2O3 and SiO2 they supply. This means that directly substituting feldspars will affect the melting point of the glaze, and possibly its color response. Try them out as direct substitutions, and then the result can be altered using line blends. If the new glaze seems underfired (dry surface), the fluxes can be increased. If it seems overfired (too fluid), the clay content can be increased.

CaO sources

Calcium is introduced into glazes from a large variety of raw materials: calcium carbonate, whiting, limestone, marble, seashells, coral, agricultural lime, etc. Usually, substituting will not make much difference, but again the result can be developed using line blends of the new material.

Glass cullet

Glass cullet means waste glass, which can be used as the basis for cheap glazes. The best glass to use is window glass, which can usually be obtained free of charge or cheap from glass suppliers. Window glass consists of soda-lime-silica and can be used as a frit in glazes. It melts at about 1100°C. With the addition of some flux and clay, it can be made into a low temperature glaze. However, because of its high CE, it will usually craze.

Unknown materials

If you find new materials that are completely unknown, the easiest way to find out what they do is to first fire a small sample of the material alone, to see if it melts or not and what color it becomes. If it melts, it is a strong flux. If it does not melt, it may still be a flux. Check the test carefully to see if it has reacted with the clay body. If it develops a strong color, it will probably affect the glaze colour.

The material should also be tested by adding it to a known glaze recipe as a line blend.