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CLOSE THIS BOOKGlazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)
12. Developing glazes
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT12.1. Modifying existing glazes
VIEW THE DOCUMENT12.2. Basic equipment
VIEW THE DOCUMENT12.3. Testing methods
VIEW THE DOCUMENT12.4. Developing a base glaze
VIEW THE DOCUMENT12.5. Modifying a base glaze
VIEW THE DOCUMENT12.6. Colored glaze

Glazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)

12. Developing glazes

Most potters do not know much about glaze chemistry and are usually afraid to develop their own glazes. It is true that glaze chemistry is difficult to understand without a background in chemistry. Still, a simple knowledge of fluxes, stabilizers and glass formers and how they combine to make glaze is useful. It will give even the nontechnical potter an idea of how to approach glaze problems and develop new colors and textures. The reader with more technical background can make use of the Seger formula for a more sophisticated approach to glazes.

Remember that glaze chemistry is something new in the history of ceramics. Before it existed, potters found out how to make glazes by trial and error, without modern methods of analysis or even accurate methods of weighing.

It is also true that once glaze recipes were developed they were closely guarded secrets. A special glaze that no one else could duplicate gave an advantage over the competition. The modern systematic, scientific approach to glaze development has largely made "secret" glazes obsolete because the action of the various glaze ingredients is now well known.

On the other hand, glaze making is very much like cooking. The same recipe may produce very different results when prepared by two different cooks! Standard glaze recipes as published in books often do not work because of differences in raw materials, firing technique etc. Just as a good cook should know how to substitute materials, a good glaze maker can develop an intuitive knowledge of glazes. Like most things, this only comes from hard-won experience.
Potters should not be afraid to experiment with glaze development, although most of them simply do not have the time to take away from production. The following chapters describe standard approaches to glaze development and will be useful to those who want to experiment with glazes.

12.1. Modifying existing glazes

The easiest approach to working with glazes is to start by modifying existing glazes. These may be new recipes from books, or recipes that you are already using. Although you can take a trial-and-error approach, this takes a lot of time and money, and results are largely a matter of luck. It is best to use some of the systematic methods presented below and knowing even a little about the nature of the various glaze materials will help you reach your goal more efficiently.

There are three approaches to modifying glazes:

- You have a goal for a particular surface, color, texture etc.
- You have a glaze problem that needs to be corrected.
- You have no special goal but just want to see what happens when you change the recipe.

12.2. Basic equipment

The minimum equipment required for glaze testing is:

- Accurate balance, either a triple beam balance or a goldsmith's balance with weight. Spring-type scales are not accurate enough for weighing glazes.
- A mortar and pestle for grinding materials. These should be porcelain, so that they do not contaminate tests.
- A 100-mesh sieve. This can be bought ready-made, or you can make your own using brass or stainless steel screen.
- Firing can be done in your regular glaze firing.

12.3. Testing methods



12.3.1. TEST PIECES

The type of test piece you use is largely a matter of personal preference. In order to show glaze behavior under various circumstances, it should have:

- A horizontal and vertical surface.
- A textured area.
- A hole to tie it. This helps to keep similar tests together for future reference.
- An area for labeling. It is best to write the full recipe of the test on each test tile (with a brush and iron oxide and water, or chrome oxide or engobe etc.) since code numbers often get confused and notebooks get lost.

The first step in understanding your materials is to fire all available materials in small bowls at your standard glaze firing temperature.

A small quantity of each material (ground and sieved through 100 mesh) is placed dry in the bowl. This will show you which ones melt alone, and which ones remain as powder. Most of the materials will not melt but may change in color or may react with the clay. Only the strong fluxes will melt by themselves -other materials that do not melt may be fluxes, but only in combination with other materials.

12.3.2. LINE BLENDING

Line blending is a systematic way of finding out the reactions of two different materials (or mixtures of materials).

The easiest way is to prepare the two materials by grinding, sieving and mixing them with water in two separate containers. To make the line blend, the materials are mixed by volume (using a small spoon) and applied on a test tile, starting with one material alone and adding the other material in equal steps. Since the tests are measured by volume it is important that the same amount of water is added to the two line blend materials.

Glaze half of the test tile twice to show variation in glaze thickness.

MATERIAL

PARTS BY VOLUME (number of spoonfuls)

material A

0

1

2

3

4

5

6

7

8

9

10

material B

10

9

8

7

6

5

4

3

2

1

0

Line blend 10 steps

An example of a line blend in 10 steps, which gives the full range of combinations of 2 materials, is shown in the table above.

The most common use of the line blend is to find out the effect of one material in a standard glaze recipe. If material A is the standard glaze recipe, material B could be the standard glaze + a coloring oxide addition of 5-10%.

Line blend 5 steps

Usually, 5 steps will be enough for the first test. In this example material A is the basic glaze, material B is the basic glaze + 10% copper oxide.


PARTS BY VOLUME

Test Number

A

B

C

D

E

F


GLAZE


0

1

2

3

4

5

GLAZE








+ 10 % CuO

5

4

3

2

1

0


CuO in test

10 %

8 %

6 %

4 %

2 %

0 %


Mixing procedure

- First prepare a line blend mixing card like the one above.
- Prepare the two mixtures as usual and add the same amount of water to each.
- Place the two materials in bowls in front of you, glaze A to your left and glaze B to your right.
- In the middle place an empty bowl into which you pour the spoonfuls from the two other bowls according to the number for each test on your line blend card. Keep track of the spoon counting by marking the line blend card.
- Stir the test mixture well.
- Mark a test tile with the mixture's test number (date + serial number).
- Glaze the test tile.
- Discard the remaining glaze and continue with the other line blend mixtures.

Calculation example

In the case above it was easy to calculate the copper oxide addition in each of the tests. When more complex mixtures are used in a line blend the calculation becomes more complicated. Here is an example of mixing two glazes:

Glaze A:

Frit X

70


feldspar

15


quartz

5


kaolin

10

Glaze B:

Frit Y

80


zircon

10


kaolin

10



PARTS BY VOLUME

Test Number

A

B

C

D

E

F

GLAZE A

0

1

2

3

4

5

GLAZE B

5

4

3

2

1

0

After firing, test number D turned out to be the most interesting. We now want to test a larger amount of this and the recipe is calculated in this way (see table next page):

TEST D

PARTS

FRIT X

FRIT Y

FELDSPAR

QUARTZ

ZIRCON

KAOLIN

GLAZE A

3

210

-

45

15

-

30

GLAZE B

2

-

160

-

-

20

20

TOTAL PARTS

5

210

160

45

15

-

50

NEW GLAZE D

1/5

42

32

9

3

4

10

Materials in glaze A were multiplied by 3 and those in glaze B by 2.

The sums of each material were then divided by 5 and the final recipe is:

Test D:

Frit X

42%


Frit Y

32


Feldspar

9


Quartz

3


Zircon

4


Kaolin

10

The recipe is based on a line blend test measured by spoonfuls. That is not very accurate so, before going any further, the test result should be retested by weighing the dry materials.

12.3.3. TRIAXIAL BLENDING

Triaxial blending is a method of testing varying amounts of three different materials or colors.


Figure 12.3.3.B. A triaxial blending chart system with 10 steps. Composition of a test at an intersection is found by following the lines to the periphery of the triangle.

Each corner of the triangle represents 100% of the material. Each side of the triangle is the line blend of the materials at its ends, and the intersections inside the triangle represent combinations of all three materials. So the result is three line blends, plus all the combinations. Fig. 12.3.3.B is an example of a biaxial system with 66 tests.

The system is better explained by an example. You may have a basic opaque glaze and you want to see how it responds to 3 different coloring oxides: cobalt oxide, copper oxide and iron oxide. In this case we use a simple biaxial blend with only 21 tests as shown in Fig. 12.3.3.C.


Figure 12.3.3.C. triaxial system with 5 steps. The number at each point refers to the test number on the triaxial blending card.

The procedure is:

- Prepare a biaxial blending card as shown.
- Prepare 3 mixtures of basic glaze with oxide additions:

A glaze + 5% cobalt oxide
B glaze + 10% iron oxide
C glaze + 10% copper oxide


- Add same amount of water, screen 100 mesh.
- Place 3 bowls with the mixtures in front of you: B on the left, C on the right, A in the center. Right in front of you place an empty bowl.
- Have all test tiles numbered and arranged in sequence near by.
- Collect teaspoonfuls of each mixture; A,B,C according to the numbers on the biaxial card. Mark each time you have finished collecting from each bowl.
- The mixture is collected in the empty bowl.
- Stir the mixture, pick the test tile with the right biaxial blend number.
- Glaze the test tile.


Figure 12.3.3.D. Arrangement of bowls for triaxial mixing.

Getting the right number of spoonfuls into the collection bowl for each test takes a lot of concentration. A mixing card as shown below helps you to keep track of your progress with the spoon counting.


TRIAXIAL BLENDING CARD

TEST NUMBER

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

MATERIAL

NUMBER OF SPOONFULS

MIXTURE A

5

4

4

3

3

3

2

2

2

2

1

1

1

1

1

0

0

0

0

0

0

MIXTURE B

0

1

0

2

1

0

3

2

1

0

4

3

2

1

0

5

4

3

2

1

0

MIXTURE C

0

0

1

0

1

2

0

1

2

3

0

1

2

3

4

0

1

2

3

4

5

If you want to know the recipe of one of the tests, say number 14, you calculate this in the same way as for line blends:



PARTS

GLAZE

COBALT O.

IRON O.

COPPER O.

MIXTURE A

1

100

5


-

-

MIXTURE B

1

100

-


10

-

MIXTURE C

3

300

-


-

30

TOTAL


5

500

5

10

30

D RECIPE

total/5

100 %

+ 1 %


+ 2 %

+ 6 %

Once you get used to working with biaxial blends, you will be able to read the percentage directly from the triangular chart.

This biaxial blend was based on only 21 variations. Out of these only 6 were blendings of all three mixtures; the rest were simply line blends involving only two mixtures. A larger biaxial blend system would produce more intermixing of all three materials, but also a lot of extra work. However, you could use a system with 10 steps on each side as shown in Fig. 12.3.3.B, but leaving out the line blends A-B, A-C and B-C, and only blend the 36 tests in the center of the triangle.

12.3.4. KEEPING RECORDS

The key to experimenting with glazes is keeping accurate records and labeling them in such a way that the actual tests can be compared with your notebook.

As mentioned above, it is best to write the entire recipe on the test tile itself, along with the date of testing. This is possible with a simple test like adding coloring oxides to your basic glaze. For more complicated tests and line blend or biaxial blend tests, you will have to rely on the test number on the tile. Mark the date, the number and, if you do more than one test a day, add a serial number. In your notebook, the date and recipe are also written.

Make it a habit to take notes of the fired results immediately after unloading the kiln. Write down firing conditions, location of test tile in the kiln, and your impression of the glaze. Is it well melted, running, pinholes or tendency to crawl? Use a whole sheet of paper for each test or test series. Finally write down your conclusion like "make I kg test batch", "test again with 5% increase of clay". You could make a standard record form like the one in Fig. 12.3.4.B.

Testing is costly and the records help you to avoid unnecessary tests. When planning your next test, first take a look at your earlier results and compare them with your notes. When deciding which materials to add or which to decrease you may check with the oxide list below (page 122). As long as you work with one particular problem or line of research keep the test tiles close by for easy reference. Once you have finished the research you can store all the tests together by hanging them on a string in chronological order.


Figure 12.3.4.B Example of glaze test record form.

12.4. Developing a base glaze

The first step in developing a new glaze is to develop a base glaze, which is simply the combination of materials that melts at the desired temperature (without addition of colorants). Here, we describe an approach to making base glazes without knowing anything about the chemical composition of the materials.

Since all glazes require flux, stabilizer and glass former, these three materials are the starting point. There are a large number of fluxes available (divided into primary and secondary' fluxes), but the stabilizer is usually china clay (kaolin) and the glass former is usually quartz (silica). The main differences are between low temperature and high temperature glazes. Below only the main materials (not chemicals) are mentioned. This list is a rough guide only.

Low temperature (900-1100°C)

Low temperature glazes require more flux and stronger flux than high temperature ones.

- primary flux:

red lead, white lead, borax or boric acid, soda ash, gerstley borate (calcium borate), or most often frit (either lead, borax, or lead borosilicate).

- secondary flux:

zinc oxide, barium carbonate, limestone, marble dust, talc.

- stabilizer:

china clay or other clay.

- glass former:

quartz, window glass powder.

High temperature (1100-1300°C)

- primary flux:

feldspar, nepheline syenite, fusible clay, wood ash.

- secondary flux:

zinc oxide, barium carbonate, limestone, marble dust, talc.

- stabilizer:

china clay or other clay.

- glass former:

quartz.

In the appendix there is a selection of glazes that can be used as a starting point for developing new glazes.

12.4.1. SELECTION OF MATERIALS

Materials necessarily have to be selected from what is available in your area, as most potters do not have access to suppliers with everything on hand.

When selecting materials to use in glazes, a general rule is to use materials that supply more than one oxide. For example, if magnesia (MgO) and silica (SiO2) are both required, it is better to use talc (3MgO 4SiO2) than magnesium carbonate and quartz. This is because the elements are already combined and contribute to a better glaze melt.

The biggest trouble with glazes is not to develop a nice new glaze but to keep it nice. Most materials vary from batch to batch and some materials may not be in regular supply. Therefore try to base your basic glaze on materials that you can rely on. Chemical stores often have ceramic oxides, but in a chemically pure form that is always very expensive. Instead look for the natural mineral containing the same oxide.

12.4.2. USING GENERAL RECIPES

There are hundreds of books on ceramics, most of which have recipes for glazes. These are limited in their usefulness, as often the raw materials are not available or are different from what you have in your country. Most of the time, these recipes do not work as expected and require modification. Without knowing the chemical analysis of materials, it is still possible to develop good glazes, using standard ones as a starting point and then modifying them systematically using the methods below.

LINE BLEND

PARTS BY VOLUME

TEST NO

A

B

C

D

E

F

G

H

I

J

K

GLAZE

10

9

8

7

6

5

4

3

2

1

0

GLAZE + 30 % ZnO

0

1

2

3

4

5

6

7

8

9

10

ZnO% IN GLAZE TEST

0

3%

6%

9%

12%

15%

18%

21%

24%

27%

30%


12.4.3. TESTING 2, 3 OR MORE MATERIALS USING LINE OR TRIAXIAL BLENDS

Line blends are the best place to start. A recipe for a glaze is made up, and then one material is selected to test in a line blend. It is added in steps, starting with a small amount and working up to perhaps 50% of the total, as described in section 12.3.2. This will give a range that may produce interesting results.

For example, use the following recipe from Ali Sheriff, Tanzania, for an unfritted borax glaze:

Boric acid

30%

Potash feldspar

25

Quartz

15

Dolomite

20

Ball clay

10

You might decide to see the effect of adding zinc oxide to the glaze. As a start a 10-step line blend is usefull (see the table above).

From this line blend you will get a good idea of how zinc oxide works in your basic glaze. Try the same with some more materials that are available like talc, limestone and zircon. From these line blends you will have a general idea of the amount of oxides which can be added.

The next step could be to try 2 or more materials in a biaxial blend. You might decide to try zinc oxide and talc. In this case, one point of the triangle would be 100% glaze, another point zinc oxide and the third point talc.

12.4.4. EVALUATING AND CARRYING OUT TESTS

After you finish a test, the next step is to evaluate it and decide how to proceed. Usually there will be at least one result that looks promising and, if you are really lucky, you might get a usable result the first time. Usually the best result from the first test will be the basis for further tests.
For example if your zinc oxide line blend showed an almost-good glaze with 6% zinc oxide, you might want to try another line blend with smaller variations below and above 6% (see table below).

LINE BLEND

PARTS BY VOLUME

TEST NO

A

B

C

D

E

F

G

H

I

J

K

A: GLAZE + 3% ZnO

10

9

8

7

6

5

4

3

2

1

0

B: GLAZE + 9 % ZnO

0

1

2

3

4

5

6

7

8

9

10

Zn % IN GLAZE TEST

3%

3.6%

4.2%

4.8%

5.4%

6.0%

6.6%

7.2%

7.8%

8.4%

9%

If this is still not satisfactory, you might take the best result as the new base glaze and try to improve it in a new line blend, using another raw material. When deciding which materials to try, study the oxide list (page 122). Under each oxide you will find a list of its effects and you then choose accordingly. If your glaze is too stiff (high viscosity) you look for materials with low viscosity etc.

12.5. Modifying a base glaze



12.5.1. MATT GLAZE

Matt glazes have non-reflecting, dull surfaces, like eggshell, paper or river rocks (page 35). This kind of surface is called "matt". Matt glazes are especially popular for decorative ware, and for floor tiles because they are not slippery.

Matt glazes are developed in several different ways:

Underfired matt glaze

Most glazes that are fired below their maturing point become matt. In a similar way, overloading the glaze with a glaze material will produce a matt surface, because the material will act as a refractory that cannot be dissolved in the glaze melt.

-alumina matt:

The addition of kaolin will produce a rather dull matt, but above 1200°C a smooth, pleasing matt is possible.

-silica matt:

Excess amount of silica will cause small silica crystals to settle out of the melt during cooling. Alumina content should be low. If silica content is too high, the glaze will be matt from underfiring.

Crystalline matt glaze

During slow cooling, the glaze develops small crystals on the surface, which break up light and appear matt. These glazes are usually smoother than underfired matt glazes. If cooling is too rapid, crystals may not have time to develop, and the glaze will be glossy.

- barium matt:

Barium carbonate is a common material to produce matt glazes, usually in amounts of 15-40 %. It is almost impossible to achieve a transparent matt glaze, but with luck it can be done with barium carbonate. Barium matt glazes are sensitive to firing conditions and it is better used together with other matting agents like zinc oxide and titanium dioxide.

- zinc matt:

For low temperatures zinc oxide is a reliable agent for matt glaze. At temperatures above 1150°C it tends to build too large crytars, but a high alumina (Al2O3) content will reduce the size of the crystals. Pure zinc matt glazes are soft and not acid-proof, so for dinnerware it should be used in combination with other matting agents.

- titanium matt:

Addition of 8-15% titanium dioxide will make a transparent glaze matt. The oxide easily combines with any iron in the body producing yellow to brown colors.

- calcium matt:

The range of addition is 10-30% whiting (CaCO3) or 20-40% wollastonite (CaO SiO2). Bone ash (Ca3(PO4)2) will produce smooth matt glazes for low temperatures when added to the frit.

- magnesium matt:

Magnesium carbonate (magnesite MgCO3), talc (3MgO 4SiO2 H2O) 10-18%, dolomite (CaCO3 MgCO3) often produce smooth, "buttery" matt glazes above 1 100° C.

With a high amount of matting agent, the surface may turn too dull matt. This can be countered by either adding clay (alumina) that will reduce the crystal size or by reducing the matting agent.

Combining matting agents

A combination of matting agents will produce matt glazes less sensitive to firing conditions, harder and with better acid resistance. Below recipes of four different mixtures are suggested. The materials are premixed and added together to the glaze in amounts of 10-30%:

- Zinc oxide

50

Kaolin

50

Mixed and calcined above 800°C.


- Titanium dioxide

40

Whiting

30

Zinc oxide

30

- Titanium dioxide

30

Tin oxide

30

Zinc oxide

30

- Barium carbonate

40

Whiting

20

Zinc oxide

20

Talc

20

The different mixtures are added to the glaze in line blends.

12.5.2. OPAQUE GLAZE

"Opaque" means you cannot see through the glaze. Opacity is developed by:

Opacifiers
These are finely ground materials that do not enter the glaze melt but remain as small white particles suspended throughout the glaze. They reflect light and make the glaze opaque. Standard opacifiers are:

- Tin oxide (SnO2), addition 3-10%. Tin oxide is very expensive and is hardly used in the ceramics industry. It works well in combination with other opacifiers and produces a soft white color.

- Zircon (zirconium silicate, ZrSiO4) is the main opacifier, addition 10-30%. It is used instead of the more expensive zirconium oxide (ZrO2). Soda and potash content should be low. Very fine grinding promotes opacity. Commercial opacifiers are normally extremely finely ground zircon. It is better to add the zircon to the frit, but this may not be practical.

- Titanium dioxide (TiO2), addition 5-10%. Produces a creamish color and combines easily with iron in the body. Works well in combination with oxides of zinc, calcium and magnesium, especially in boron glazes. Opacifying effect depends on crystals forming during cooling.

- Bone ash (calcium phosphate, Ca3(PO4)2), addition 5-15%. In amounts above 5% it may cause blistering and crawling in low temperature glazes and it is better added to the frit.

A variety of combinations of zinc, calcium, magnesium and titanium dioxide produces opacity in boron glazes. Zircon may be added (5-10%) to increase opacity further. By such combinations it is possible to produce a reliable zircon-based opaque glaze without the pinholing trouble otherwise seen with zircon glazes.

12.5.3. CRYSTAL, CRACKLE GLAZE:

Crystal (or crystalline) and crackle glazes are used for special effects.

Crystalline glazes

Crystals develop in glazes that are low in alumina and that are cooled slowly. Usually these are small crystals that produce matt glazes.

Very large crystals, from a few mm to several cm long, can be formed in special glazes. These glazes are fired to their maturing point, soaked for several hours and then cooled very slowly. That gives the crystals time to grow. To further increase the size of the crystals, the temperature can be kept slightly below the glaze's maturing point for several more hours. The outcome is very uncertain and many test firings are needed before the right firing and cooling method is developed.

Large crystals only grow in a very fluid glaze melt. So the glaze should contain little alumina and little silica but a large amount of flux. The best fluxes are lead, lithium, soda and potash.

The main agents for crystal formation are zinc oxide (20-3D%) and titanium dioxide (5-15%). Lithium, calcium, magnesium and barium are supportive additions.

Crackle glazes

These are glazes that craze, which are popular for decorative pottery. Crackle glaze should not be used on pots for food.

Most glazes can be made to craze by decreasing the quartz or increasing high-expansion oxides like soda and potash (see page 95 on crazing). Rapid cooling of the kiln helps to produce fine patterns of crazing.

To enhance the crackle, pots can be soaked in strong tea, or ink can be rubbed into the lines. Reglazing and refiring crackled pots with a contrasting glaze sometimes result in interesting patterns.

12.6. Colored glaze

Colored glazes are developed by adding coloring oxides. These are added to the base glaze as a percentage, based on the range for each oxide as listed below. Different oxides have different strengths, so some of them are used in much larger amounts than others.

For example, you might want a brown glaze. Looking at the list of oxides, you find that brown can be developed with iron oxide from 5-10%. This can be done as a line blend, adding 5,6,7,8,9 and 10% to the base glaze. The percentage is in addition to the total base glaze weight:

Glaze

100 g

Iron oxide

6%

100g x 0.06 = 6g

Glaze + oxide = 106 g

Ready-made glaze pigments, called glaze stains, are also used to develop colors that cannot be made easily with oxides alone.

12.6.1. LIST OF OXIDE ADDITIONS

It is more or less impossible to give an accurate guide to colors in glaze, because there are so many variables of chemical reaction in different base glazes.

The firing conditions, temperature and oxidation/reduction also greatly influence the color of the glaze.

The table below should be considered a rough guide. See also chapter 13 for color reactions in different types of base glazes.

Single oxides

Percent

Effects

Iron oxide, Fe2O3

1 - 5 %

Green, cream, light brown


5 - 10 %

Brown, red-brown


10 - 15 %

Dark brown, black

Cobalt oxide, CoO

0.2 - 3 %

Blue

Cobalt carbonate, CoCO3



Manganese dioxide, MnO2

2 - 10 %

Brown, purple-brown

Manganese carbonate, MnCO3



Rutile, TiO2

1 - 10 %

Yellow, tan, mottled colors

Chrome oxide, Cr2O3

1 - 5 %

Green

Copper oxide, CuO

0.5 - 5 %

Green, blue, red in reduction

Copper carbonate, CuCO3



Nickel oxide, CuO

0.5 - 3 %

Gray, green-brown

Ilmenite, magnetite (contains iron)

1 - 10 %

In granular form produces speck and spots in the glaze.

Antimony oxide, Sb2O3

1 - 5 %

Cream to yellow in lead glazes


12.6.2. LINE, TRIAXIAL BLEND PLANNING

The most interesting colors often come from combining 2 or more oxides in the same base glaze. Usually it is best to test the base glaze first with various oxides alone and to use the best results in combination with each other. Line blends are useful for this kind of test, and biaxial blends can also be used for 3 oxides in combination (see page 105, 107 for details on line and biaxial blending).

One-color line blend
For testing a color oxide, you prepare two mixtures for a line blend.

Example:

Mixture A:

100 parts your basic glaze

Mixture B:

100 parts basic glaze


10 parts titanium dioxide

Make line blends with all the coloring oxides you have. After firing, you will have a good idea of the color range you can get with your basic glazes. Maybe you will already now have all the colors you need. If you want to try a combination of several oxides you can do this by line blends or biaxial blends.

Two-color line blend

Choose one of the colors you got from your first set of line blend testing. Make this your basic glaze and then try another coloring oxide in addition to this.

Example:

Mixture A:

100 glaze

Mixture B:

100 glaze


4 copper oxide


5 iron oxide

Note that when mixing several coloring oxides their total amount should normally not exceed 10% of the glaze.

This type of Line blending can be continued with any combination of oxides. Do it one step at a time with only one or two line blends at a time in your regular glaze firing. After firing you can choose the best results and do more tests along those lines.

Triaxial blend

From your first set of line blends choose three coloring oxides and test their combinations in a biaxial blend. When setting up the biaxial blend, make the points A, B and C with oxide additions about 30% higher than what you expect to use in the final glaze.

You can even try four color oxides in one biaxial blend.

Example:

Your line blend showed that 1.5% addition of cobalt oxide produced a nice blue, but you want to modify it with other color oxides:

Base glaze: glaze + 1.5% cobalt oxide

A: base glaze + 6% iron oxide
B: base glaze + 5% copper oxide
C: base glaze + 8% titanium dioxide

After doing the tests you have to calculate the final recipe. This is done by setting up a calculation table as shown on page 109.

12.6.3. COLOR PIGMENTS

Glazes can be colored by adding metallic oxides directly to them. Some oxides can be used as on-glaze colorants by painting them directly on the unfired glazed object.

Ceramic pigments are produced from the same coloring oxides, but other materials are added in order to change the colors and make them more stable or cheaper.

The materials used for pigments can be divided into four groups:

Color agent
- metallic oxides; for example, iron oxide, copper oxide.

Modifier
- influences coloring effect of oxides. Examples of modifiers: titanium dioxide, zinc oxide, zirconium oxide, antimony oxide.

Filler
- raises melting point of the pigment and stabilizes the coloring oxides. Examples of fillers: alumina, quartz, feldspar, clay body.

Flux
- lowers melting point of the pigment. Examples of fluxes: borax, lead, frit or glaze.

Fluxes are added according to the use of the color pigment. The pigments can be adjusted for use as:

Under-glaze colorant:
- The pigment is painted directly on the raw or biscuit-fired body and a glaze is applied on top.

Maiolica or on-glaze:
- Decoration on the unfired glaze layer.

Overglaze enamel:
- Applied to the already fired glaze.

In-glaze colorant:
- Added to a basic glaze as a coloring agent.

Production of Color pigments

Close production control, accurate weighing and the use of the right materials are especially important when producing color pigments. Even slight deviations may result in the change of a fired colour.

Four main processes are used in the production:

1) Mixing of raw materials
2) Calcination
3) Washing
4) Grinding

Mixing

If all raw materials of the recipe are already finely ground mixing can be done manually ensuring good mixing by screening the batch twice through 60 mesh. Normally materials will be coarse, so after weighing out the pigment recipe the batch is ball-milled.

After milling drying and calcination follow.

Calcination

The calcination will burn away carbonates' water, sulfates and the coloring oxides will form new crystalline combinations with the other materials in the batch. This will stabilize the colors so that they will not be easily dissolved in the glaze.

The temperature of calcination is in the range of 700°C to 1400°C. In general, the color pigment should be calcined at least to the temperature at which it is going to be used and preferably higher. Some colors will disappear if fired high whereas other colors will only develop correctly at 1300°-1400°C.

Calcination is done in small saggers or clay pots with a lid. The pigments are fired in a small kiln (e.g. test kiln) to the desired temperature or in the hot spots of the normal production kiln.

Washing

After calcination the sintered pigments are crushed to sand size and then washed with water in order to remove any soluble materials that may remain. The washing is normally not important except for pigments to be used in delicate decorations where possible soluble materials may cause a blurred final image.

Grinding

The pigment is ground in a small ball mill. For enamel overglaze decorations it should be ground very fine. In normal practice it should pass 250 mesh. When used as a glaze colorant, 150 mesh is fine enough, but in general the coloring quality is better with fineness.

For special decorative speckled effects the pigment can be made coarse "rained.

After grinding the pigment is dried, packed and labeled and a color test made before releasing for sale or production.

The basic pigment can now be used for mixing of underglaze, on-glaze or enamel colorants with additions of fluxes, clay, silica etc. as described below.

Underglaze

These colorants are applied to raw body, body covered with engobe or to biscuit-fired body. Colored engobes can also be termed underglaze colors.

The colorants should not react with or be dissolved by the overlying glaze. A high content of clay, feldspar or whiting prevents this.

If applying to raw clay, shrinkage should be adjusted to fit with that of the body. For biscuit body some 5-10% raw clay will give better adhesion and strength to the dried surface. 3-5% raw borax reduces tendency of glaze crawling over the decoration and adds strength to the decoration before glazing. 10-20% addition of the glaze used for final glazing is normally also added.

Addition of glue like sugar, dextrin, CMC helps adhesion.

Maiolica or on-glaze

These colorants are applied onto the already glazed but unfired pot. The colorants sink into the glaze during firing and melt together with the main glaze. More fluxes are added to maiolica colorants than to underglaze colorants and the lower the viscosity of the colors and the glaze is, the more the decoration will run and the contours of the decoration will be blurred.

About one part frit is added to one part pigment. With a low melting frit or a pigment containing a high amount of copper oxide the frit content is lowered.

Maiolica colorants can be made by adding a little glaze or frit to the raw color oxide. The maiolica technique can also be used by decorating with coloured glazes on top of the basic glaze. To prevent running, the melting point of the colored glaze can be raised by adding silica and clay. Color oxide mixed with water can also be used when thinly applied. The oxide will then melt together with the glaze. If the oxide layer is too thick the glaze cannot "wet" the oxide and the decoration will be dry and dark in color after firing.

Overglaze enamel

Overglazes (or "China paints") consist of frit and pigment and they are fired at low temperatures of 700°-850°C. The flux content is 70-90% of the enamel color.

Examples of lead-free fluxes for 400-600°C

1)

Borax

380

2)

ZnO

37


Quartz

100


Borax

60





Whiting

7

The flux and the color pigment are melted together and ground.

The ground colorant is mixed with about 50% organic oil (linseed oil, olive oil) as a medium for painting on the fired glaze surface. Turpentine is used for thinning. If no proper oil is available turpentine which has had some of its volatile parts removed by boiling can be used as a medium. Another medium for suspending the colorant is water with the addition of white carpenter's glue.

Glaze colorant

The color pigments can also be used for coloring basic opaque or transparent glazes. Coloring can be done by directly adding color oxides to the glaze. However, there are some benefits from doing the coloring with prepared pigments:

- The color effect of oxides is increased and thus cost of expensive oxides like cobalt can be reduced.
- Colors can be made more stable so they will be less influenced by kiln atmosphere and glaze materials.
- More colors can be produced.
- Blisters and pinholes produced by coloring oxides (MnO) can be avoided.

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