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CLOSE THIS BOOKGlazes - for the Self-reliant Potter (GTZ, 1993, 179 p.)
5. Simple glaze theory
VIEW THE DOCUMENT5.1. Basic chemistry
VIEW THE DOCUMENT5.2. Glaze structure
VIEW THE DOCUMENT5.3. Effect of heat
VIEW THE DOCUMENT5.4. Melted glaze behavior
VIEW THE DOCUMENT5.5. Interface between glaze and body
VIEW THE DOCUMENT5.6. Cooling and crystal formation
VIEW THE DOCUMENT5.7. Transparency and opacity
VIEW THE DOCUMENT5.8. Shiny or matt glaze

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

5. Simple glaze theory

5.1. Basic chemistry

Chemistry is the science which describes what substances are made of and how they combine with each other. This science uses special names and symbols which are described below.

5.1.1. ELEMENTS/COMPOUNDS

Elements

An element is made of only one kind of atom. It cannot be broken down into more simple substances. Oxygen (O) is the most common element on earth.

Compounds

A compound is composed of more than one element combined chemically. Water (H2O) is a compound made up of two atoms of hydrogen (H) and one atom of oxygen (O). Silica (SiO2) is another compound and consists of one atom of silicon (Si) and two atoms of oxygen (O). This is the most abundant material in the earth's crust. Two or more atoms combined form a molecule.


Figure 5.1.1.A. Water is two elements combined. A molecule of water consist of two atoms of hydrogen and one of oxygen. Figure 5.1.1.B. A molecule of the compound silica (sand) has two atoms of oxygen and one of silicon

Ceramic raw materials are usually in the form of oxides: an oxide is a compound that includes oxygen (O). Minerals are compounds.

5.1.2. SOLID, LIQUID, GAS

Solid, liquid and gas are the three states of matter. Most materials can exist in all of these states, depending on their temperature. A familiar example is water, which is solid below 0°C, liquid from 0°C to 100°C, and gas above 100°C.

Making glaze depends on mixing solids together, applying them on a pot and then changing them to liquid in the kiln. Some of the glaze materials also become gas during firing and leave the glaze. On cooling, the glaze again becomes solid.

Mixture

A mixture is a physical, not chemical, combination of compounds (and sometimes elements) and each compound remains chemically unchanged in the mixture. Air is a mixture of oxygen, carbon dioxide, nitrogen and other gases. A glaze made of feldspar, quartz and lime is prepared by combining the compounds as a mixture, but during firing a chemical combination takes place and the fired glaze becomes a compound.

Chemical symbols

There are about 100 elements, and each of these has a name and a chemical symbol, which is used as an abbreviation of its name. Some of these symbols are the same as the first letters of the English name, but some are not!

For example:

Oxygen is "O"
Hydrogen is "H"
Silicon is "Si"
Alumina is "Al"
Sodium is "Na"
Lead is "Pb"

Compounds are written in a similar way with capital letters marking the individual elements: for example, water is "H2O" and salt is "NaCl"

The small number "2" in "H2O" indicates that there are two atoms of hydrogen for each atom of oxygen in water. If there is no number, it is understood that there is only one atom -so salt is one atom of sodium and one atom of chlorine.

The formulas of complex ceramic materials are written as compounds of oxides with a raised period (·) between them to show they are chemically combined. For example potash feldspar is written:

K2O Al2O3 6SiO2

In the appendix the chemical formulas of other materials are listed.

5.1.3. CHEMICAL REACTIONS

The formation of clay from feldspar can be written in chemical symbols:

K2O Al2O3 6SiO2

+ H2O

(feldspar)

+ (water)

®Al2O3 2SiO2

2H2O + K2O

+ SiO2

(clay)

+ (potash)

+ (silica)

All materials are built up of elements which are chemically bonded together. When heated to a high temperature, chemical bonds can break down and the material will change its properties. The production of quicklime by heating limestone to 900°C is an example of this:

CaCO3®

CaO

+ CO2

(limestone)

(quicklime)

(carbon dioxide)

Carbon dioxide (CO2) goes into the air, and the remaining quicklime (CaO) is slaked with water and can then be mixed with sand to form mortar for house construction. The mortar sets when the calcium oxide (CaO) takes back carbon dioxide (CO2) from the air and thereby regains the hardness of the original limestone (CaCO3):

CaO

+ CO2

®CaCO3

(soft mortar)

(from air)

(set mortar)


5.1.4. SOLUTION/SUSPENSION

Solution

A solution is a mixture of molecules. For example, sugar completely dissolves in water: the separate particles consist of molecules of sugar and water. Sugar and water remain a solution until the water evaporates.

The higher the temperature of the liquid, the more solid material can dissolve in the liquid. When no more solid can be dissolved the solution is called "saturated".

Suspension

In a suspension the particles are bigger than molecules. A mixture of clay and water is a suspension. The clay particles are not changed by the water, and after some time the clay will settle at the bottom of the vessel. The clay is insoluble in water.

5.1.5. CRYSTAL STRUCTURES

Crystal structure

If we heat water to 90°C and add salt (NaCl), it will become dissolved in the water. If we continue to add salt until no more salt can be dissolved, the suspension is saturated with salt. If we let the solution cool to room temperature (20°C) the water can hold much less salt in solution, with the result that some of the salt will separate in the form of salt crystals.

All minerals have the form of crystals. When the water cools, the excess salt molecules start to combine with one another in regular patterns like small building blocks. The way the salt molecules connect to one another is very orderly and produces a cube-shaped crystal. Different materials will produce crystals of different shapes. The shape of a mineral's crystal is used to identify it.


Figure 5.1.5.A. The cubic shape of a salt crystal.

5.2. Glaze structure

Glaze is similar to glass. Making glazes is confusing because there are so many raw materials that can be used. However, all of these raw materials can be broken down into three categories:

- flux
- glass former
- stabilizer.

All glazes require these three components. The main glass former is silica, the main stabilizer is kaolin, and the rest of the glaze is composed of one or more fluxes.

5.2.1. GLASS STRUCTURE

Silica (SiO2) alone will make an excellent glaze if it is fired to its melting point (1715°C). Since this temperature is too high for ordinary kilns, other materials are added to lower the melting point of silica. Quartz is a crystalline form of silica found in nature. If a glaze forms quartz crystals when it cools, it will not be transparent, since light is refracted in many different directions by the crystal faces. Because glass or glaze is not usually crystalline, this does not happen.

A glaze or glass is a mixture of compounds that melts when heated. The melted liquid glass is like a solution. When the liquid cools, crystals start to form in a similar way as in a salt solution. However, the liquid glaze is very viscous (meaning sticky and semifluid) and the molecules cannot easily move around to form a regular crystalline pattern. So normally no crystals form during cooling, and the glaze remains clear like a liquid.

Glaze is, therefore, like a solid solution and is sometimes called a supercooled liquid.

5.2.2. FLUXES

Fluxes are the materials which lower the melting point of a glaze. They can be called melters.

Silica melts by itself but at a very high temperature. Therefore it needs additions of flux to make a practical glaze. The most common flux for temperatures below 1100°C is lead oxide (PbO), but since it is poisonous it is no longer used in modern crockery glazes. Another powerful flux is boron or boric oxide, B2O3, which is not poisonous and is used in glazes in the form of borax or boric acid. There are many other fluxes which contribute various properties of hardness, opacity, color response etc.

Fluxes are also called basic oxides or network modifiers.

5.2.3. GLASS FORMERS

Silica forms the main part of all glazes and is called a glassformer. The other glass-former is boron. Silica and boron are the building blocks of a glass or glaze. Other materials are only used to modify their behavior in the glaze.

Titanium oxide (TiO2), tin oxide (SnO2) and zirconium oxide (ZrO2) also belong to this group. Sometimes they are called the acidic oxides or network former, or the acid portion of the glaze.

5.2.4. STABILIZERS

Aluminum oxide, Al2O3, is added to make the melted glaze stiffer, so that it will not run off the pots during firing. It is called a stabilizer. Other words for stabilizer are: amphoteric, neutral or intermediate oxide.

Aluminum oxide has a high melting point and will increase the melting point of the glaze. It is usually added to the glaze as kaolin (china clay).

(Boron is termed a stabilizer in the USA but a glass former in Europe.)

5.3. Effect of heat

As heat is increased, the molecules in the glaze move faster, resulting in drying, sintering, melting and gas escape. All of these effects occur when the glaze molecules move so fast that they start to break down, releasing some of their atoms and combining with other molecules to form the glaze.

5.3.1. DRYING

When the powdered glaze on the surface of the ceramic ware is heated, the water evaporates above 100°C (no matter how dry the glaze seems to be, there will always be some water remaining in it). The glaze layer should be as dry as possible before setting in the kiln. If the glaze layer dries too fast when firing starts, it may crack. This can cause crawling of the glaze after it melts.

5.3.2. SINTERING, MELTING, GAS ESCAPE

Sintering

As the temperature rises above 600°C, the sintering of the glaze powder starts. Sintering also takes place in the clay at this temperature. Sintering means that the glaze (or clay) particles start to stick to one another where they touch. The finer the glaze particles are ground, the earlier the sintering will start and the stronger the bond will become.


Figure 5.3.2.A. The glaze particles are enraged many thousand times showing sintering in a glaze heated to 600°C. At the points of contact (arrow) a weak bond is formed.

Fusion

As the temperature rises further, the most fusible (easy melting) materials in the glaze start to melt. This is celled fusion. The refractory (hard melting) particles are surrounded by the liquid materials and are slowly included in the liquid.

The temperature at which melting starts depends on the materials in the glaze. Silica alone melts at 1715°C, but with additions of other materials the melting point will go down. Aluminum oxide (Al2O3) melts at 2050°C and calcium oxide (CaO) at 2570°C, but a mixture of 62% silica, 14.75% aluminum oxide and 23.25% lime melts at only 1170°C. A mixture which has a lower melting point than any of the single materials in the mixture is called an eutectic.

A mixture with many different materials will form eutectics (and will melt) at a lower temperature. Fine grinding of the glaze materials and prolonged firing time above the sintering temperature will also lower the melting point.

When fusion starts, the compounds also start to change. The chemically bonded water in clay has already been released. Around 900°C, limestone (CaCO3) releases carbon dioxide (CO2) and so do other materials containing carbonates, like barium carbonate (BaCO3). Gases of sulfates, oxides etc. are also released both from the glaze and from the body. These gases have to pass through the glaze layer. This action mixes the glaze, helping it to become homogeneous.

In the beginning the melted glaze is very stiff (high viscosity), but as the temperature keeps rising the glaze becomes more fluid and, when watching the melting glaze surface through a spyhole in the kiln, bubbling or even boiling can be seen. When the glaze reaches its maturing temperature, the reactions stop and the glaze becomes smooth.


Figure 5.3.2.B. A cube of glaze is gradually heated up to 1000°C. At 500°C the glaze shrinks slightly (sintering), but at 600°C it swells as gases develop. Melting starts before 700°C and is completed at 1000°C.

5.3.3. MATERIALS WHICH INCREASE/LOWER MELTING POINT

This chart shows the oxides according to their influence on melting temperature:

OXIDES WHICH RAISE MELTING TEMPERATURE

Al2O3

High

SiO2

­

MgO

|

Cr2O3

|

SnO2

|

ZrO2

|

NiO

|

Fe2O3

|

TiO2

|

CaO

|

ZnO

|

BaO

|

FeO

|

CoO

|

CuO

|

MnO

|

PbO

|

B2O3

|

Na2O

|

K2O

¯

Li2O

Low

Note this scale is not linear and depends on firing temperature and amount of oxide in the glaze

OXIDES WHICH LOWER MELTING TEMPERATURE

5.4. Melted glaze behavior

Fluid state

The fluid state of the glaze should be maintained long enough to allow all bubbles time to escape, so the glaze layer can heal over the holes left by the escaping bubbles. If a glaze tends to produce pinholes and craters, it can be given a soaking period (keeping the kiln at maturing temperature for some time) or the firing temperature can be raised in order to make the glaze more fluid (reduce viscosity).

If the glaze is too fluid, it will run off the pot or the fluid glaze will soak into a porous body leaving matt, dry spots on the surface.

The following chart shows materials which increase or decrease viscosity.

MATERIALS THAT INCREASE VISCOSITY

Al2O3

High

ZrO2

­

SiO2

|

Cr2O3

|

SnO2

|

NiO

|

Fe2O3

|

TiO2

|

CaO

|

MgO

|

ZnO

|

SrO

|

BaO

|

CoO

|

MnO

|

PbO

|

K2O

|

Na2O

|

B2O3

¯

Li2O

Low

Materials at top increase viscosity most. Note these materials are mainly stabilizers and glass formers. (Scale is not linear.) Most materials in this group are fluxes. Materials at bottom decrease viscosity most.

MATERIALS THAT DECREASE VISCOSITY

5.4.1. SURFACE TENSION

To understand surface tension, fill a glass with water to the rim and look at the water surface. The middle of the water surface will be higher than the rim, but the water will not run over. The surface tension of the water holds it as if it were held by a plastic membrane.

A small amount of water forms a spherical drop. Larger amounts of water flatten the spherical form because the force of gravity increases with the weight of water. The fluid glaze behaves in a similar manner, and if the surface tension of the fluid glaze is too high the glaze will pull itself into small islands, leaving the clay body uncovered. This is called crawling.


Figure 5.4.1.A. Surface tension is created by the difference of forces acting on water in the center (B) and at the surface (A). A water particle at B has forces of traction of the water around it evenly distributed. But at A the force is mainly directed away from the surface. This difference causes water to from itself ion spherical drops.

Increasing temperature lowers the surface tension as Fig. 5.3.2.B illustrates. At 800°C the glaze forms a half globe but at 1000°C it

has completely flattened out. Different ceramic oxides influence the surface tension as listed in this chart:

MATERIALS THAT INCREASE SURFACE TENSION

MgO

High

Al2O3

­

ZrO2

|

ZnO

|

CaO

|

SnO2

|

Cr2O3

|

NiO

|

BaO

|

SrO

|

Fe2O3

|

SiO2

|

TiO2

|

Li2O

|

Na2O

|

K2O

|

B2O3

¯

PbO

Low

Note the scale is not linear and the sequence of oxides may change due to other factors like viscosity, flue gas

MATERIALS THAT DECREASE SURFACE TENSION

5.4.2. CRAWLING

Crawling is caused by two factors:

- high surface tension of the glaze;
- difficulty for the glaze to stick to the body.

If the body surface is greasy or dusty the problem is aggravated. Crawling may also happen if the glaze layer cracks before it is sintered. This happens if the glaze contains a high amount of clay or has been ground for too long in the ball mill. The surface tension will then pull the glaze away from the cracks.


Figure 5.4.2.A. Crawling

5.4.3. CRATERS, PINHOLES

The lower the surface tension, the shinier the surface of the glaze becomes and the easier it is for the glaze to heal over craters, bubbles and pinholes.

Interesting effects can be obtained by applying glazes with different surface tensions on top of each other (see page 80).

Surface tension, viscosity and melting temperature are interrelated, so when replacing materials all three will be affected.

5.5. Interface between glaze and body

During firing the glaze interacts with the clay body. Some of the glaze will sink into the body and some of the body material will mix with the glaze so that an intermediate layer is formed between the body and the glaze. This layer bonds the clay and glaze together. It is called the glaze/body interface or "buffer" layer.


Figure 5.5.0.A. Interphase layer created during firing by mixing of materials in the body and the glaze.

Effects of interface

Some of the coloring oxides in the body may enter the glaze and change its colon The higher the firing temperature the stronger the interface layer. The interface layer produces a strong bond between glaze and body that reduces the tendency to craze or peel.

Glazing on greenware (raw glazing or green glazing or single firing) promotes interaction between body and glaze. If too much of the glaze's flux combines with the refractory materials in the body, the glaze may become matt or dry.

5.6. Cooling and crystal formation

Glaze or glass is called a supercooled liquid because, during cooling, crystals have no time to form in the rather sticky mass, and glass by definition does not contain crystals. But some matt glazes and opaque glazes depend on the formation of crystals. For these, cooling should be slow to allow the crystals to grow. ZnO, BaO and TiO2 are used for making matt glazes, but if cooling is rapid the glaze will become glossy instead of matt.

To avoid crystal formation, glossy transparent glazes should be cooled quickly after the maturing temperature has been reached.

5.7. Transparency and opacity

Transparency is the property of allowing light to pass through the glaze to-the clay below. Transparent glazes may be colorless or have color in them - transparent blue, green, brown etc. It is necessary to use transparent glazes in combination with underglaze decoration. Transparent glazes are always shiny.

Opacity is the property of not allowing light to pass through the glaze. Colorless opaque glazes usually look white or gray. When coloring oxides are added, they can be any possible colors. They generally are used with overglaze or on-glaze.

It is possible to make glazes with every degree of transparency or opacity, such as semitransparent or semiopaque.


Figure 5.7.0.A. Section of a window glass. A beam of light passes through it - it is transparent. The lights dissection is slightly bent when passing from one medium (air) to another (glass). This is called refraction.

5.7.1. REFRACTION OF LIGHT

Transparency and opacity are determined by the glaze's ability to transmit light. When light strikes a transparent glaze, most of it passes through the glaze layer to the clay underneath, and the color we see is determined by the color of the clay. Thus, a transparent glaze on a brown clay body will look brown whereas the same glaze on a white clay body will look white. If the transparent glaze is colored, the clay body color will be changed by the fact that the glaze is green or blue, etc.


Figure 5.7.1.A. A transparent glaze reflects the color of the underlying body.

Opaque glazes have a large number of particles in them that reflect light, without allowing it to pass through the glaze. So we are not able to see through the glaze. Thus what we observe is only the surface of the glaze, which is not affected by the color of the clay underneath.

Semitransparent glazes have smaller numbers of light-reflecting particles, so they look cloudy or milky, and their color will be affected by the clay color underneath.

Transparent glazes can be made opaque by the addition of opacifiers, which are finely ground particles that do not enter into the melting of the glaze. These particles stay suspended in the glaze and reflect light. This is similar to mixing clay with water, which makes the water opaque.

Opaque glazes cannot be made transparent without changing their formula (unless they are transparent glazes with opacifier added).

The causes of opacity in glazes can be divided into 4 groups:

1. Presence of very fine particles, which do not dissolve in the glaze melt. The light going through the glaze is scattered by the fine particles. Tin oxide (SnO2) and zircon (ZrSiO4) are used for this.


Figure 5.7.1.B. Fine particles of zircon or tin oxide in the glaze scatter the light and produce opacity.

2. Crystals formed in the glaze during cooling will scatter the light, causing opacity. Titanium dioxide (TiO2) recrystallizes if the cooling is slow and can make glazes opaque.


Figure 5.7.1.C. Two glaze phases, A and B, in the melt cause opacity. Both glaze phases may be transparent but the light gets lost passing from one phase to the other.

3. Opacity is also caused when two melting phases of the glaze do not mix. The light will be scattered when it passes through the border between the two different melts. This takes place in boron glazes and with calcium phosphate (bone ash).

4. Gas bubbles scatter the light and produce opacity. This type of opacity is difficult to control and the method is not recommended.

In practice, a combination of the four methods is used. For example, an opaque glaze can be made with boron and additions of lime, zinc oxide and zircon.

5.7.2. MATERIALS CAUSING OPACITY

The best opacifier is tin oxide, which will make most glazes opaque in additions of up to 7%. However, it is a very expensive material and today is only used for special high-cost products.

Commercially available opacifiers are based on zirconium silicate, prepared with other additions such as magnesia and zinc oxide. They are marketed under names such as "zirconium opacifier", "zirconium silicate", "zinc zirconium silicate" and "magnesium zirconium silicate". Most of these are added to glazes from 5 to 10% and produce different results depending on the type of base glaze. They also vary widely in quality, and it is important to test them before ordering a large quantity. Zirconium opacifiers have the disadvantage of making glazes more refractory and often cause pinholing problems.

The main opacifiers are:

Tin oxide, SnO2
Zircon, zirconium silicate, ZrSiO4
Titanium dioxide, TiO2
Alumina, Al2O3 (high content in boron glazes will reduce opacity)
Calcium oxide, CaO (improves opacity in boron glazes)
Zinc oxide, ZnO
Calcium phosphate, bone ash, Ca3(PO4)2.

Particle Size

The finer the particle size of the opacifier, the better it works. Zircon is often included in the frit batch for greater opacity. In this way opacity is obtained with less zircon, thus reducing some of zircon's bad side effects like high viscosity and the tendency to cause pinholes. Unfortunately the addition of zircon to the frit increases its melting point, making it more difficult to run it off the frit kiln. It also increases the hardness of the frit so much that it may be difficult to grind it with ordinary pebbles and ball mill lining.

It is important to make sure that the opacifier is well dispersed in the glaze. The fine particles tend to lump together. This reduces the opacity effect. By ball milling the opacifier together with the glaze a good dispersion is assured.

5.8. Shiny or matt glaze

Glazes are also defined by the way they reflect light: they may be shiny or matt or in between.

Shiny

Shiny glazes are also known as "glossy" or "bright". They have the property of reflecting light like a mirror. They are best for utilitarian wares, sanitary ware and insulators, as they are easy to wash and do not scratch easily.


Figure 5.8.0.A. A glossy glaze with a smooth surface reflects the light without scattering it.

Matt

Matt glazes are also known as "dull" or "non-reflective". Their surface can vary from smooth to very rough. They are useful for decorative wares and are very popular for floor tiles, which need to be beautiful but not slippery. The matt surface is not functional for dinnerware, because used with cutlery it makes an unpleasant sound and scratches easily.

5.8.1. MATERIALS CAUSING MATTNESS

There are several ways to produce a matt glaze:

Underfiring

As glaze begins to melt, it becomes glassy. If the firing is stopped before the glaze is completely melted, even glossy glazes will appear matt. Often these underfired glazes will have other problems such as blisters and pinholes, but some glossy glazes make very good matt glazes if fired a few cones below their normal temperature. Similarly, adding refractory oxides to a glaze (such as china clay or calcium carbonate) will produce a matt glaze that really is just an underfired glossy glaze.

Crystalline matt

Crystalline matt glazes develop small crystals which break up light (Fig. 5.8.1.A). This type of matt glaze usually produces a more smooth surface than underfired matt glazes. Some matt glazes depend on slow cooling to have time for the crystals to develop.


Figure 5.8.1.A. Surface of crystal matt glaze enlarged several hundred times. Crystals in the glaze scater the light by sending it in many different directions.

Barium carbonate, zinc oxide, titanium dioxide, magnesium oxide and calcium oxide are the agents for crystal matt glazes. For more details see page 113.

5.8.2. OTHER CAUSES

Sometimes glazes that should be glossy will become matt. Some of the reasons are:

- Some of the flux materials may evaporate during firing, leaving a matt surface.
- Sulfates from fuel may settle on the surface of the glaze.
- The glaze is applied too thin.
- The glaze was not mixed sufficiently or not sieved finely enough.

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