Nature of clay

A look into the chemical and physical properties of clay will help in understanding its behavior, and how to correct problems like cracking and warping which occur during drying or firing.

1. Simple Clay Chemistry.

Chemistry is the science which describes what substances are made of and how they combine with each other. This science makes use of special names and symbols. Once learned, they are quite simple to understand.

Elements: An element cannot be broken down into more simple substances, and it consists of only one kind of atom. Oxygen (O) is the most common element on earth .

Compounds: A compound is composed of different elements bound together chemically. Water (H2O) is a compound made up of two parts, or atoms, of hydrogen (H) and one part, or atom, of oxygen (O). Silica (SiO2) is another compound and consists of one silicon atom (Si) and two oxygen atoms (O). This is the most abundant material in the earth's crust.

Ceramic minerals are usually in the form of oxides: this means oxygen (O) is a part of the compound. Minerals (page 7) are compounds.

mixture: A mixture is a physical, not chemical, combination of compounds (and sometimes elements), and each compound remains chemically unchanged in the mixture. For example, a glaze made of feldspar, quartz and limestone is initially a mixture, but during firing a chemical combination takes place and the fired glaze changes into a compound.

Chemical symbols: There are about 100 elements and each of these has a name and a chemical symbol, which is used as a shorthand name. Oxygen is written as capital O and hydrogen as H, whereas other symbols have two letters: silicon = Si and aluminum = Al. Compounds are written in a similar way, with capital letters marking the individual elements:
Water = H2O and salt = NaCl. The small number (2) in H2O indicates that there are two atoms of hydrogen for each atom of oxygen in water.

Formulas of complex ceramic materials are written as combinations of oxides with a high period (.) between them to show they are chemically combined. For example, potash feldspar is written:

K2O.Al2O3.6SiO2

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

Chemical reactions: The formation of clay from feldspar can be written like this in chemical symbols:

K2O. Al2O3. 6SiO2 + H2O ® Al2O3. 2SiO2. 2H2O + K2O + SiO2
(feldspar) (water) (clay) (potash) (silica)

Compare this with the description in fig.

All materials are made of elements which are chemically bonded together. However, under certain conditions, a material may be changed into another by changes in the elements making up the material. Heat often provokes such chemical changes, and the production of quicklime by heating limestone to 900 C is an example of this: CaCO3 ® CaO + CO2 (limestone)(heat)(quicklime) (carbon dioxide)

Carbon dioxide (CO2) mixes with air, and the remaining quicklime (CaO) is slaked with water and can then be mixed with sand to form a mortar for house construction. The trick is, that the mortar sets (becomes hard) when the calcium oxide (CaO) takes back carbon dioxide (CO2) from the air and thereby regains the hardness of the original limestone (CaO3):

CaO + CO2 ® CaCO3 (soft mortar) (from air) (set mortar)

2. Chemical Changes in Clay Crystals

Kaolinite: There are several types of clay minerals, so in individual clays the clay particles or crystals may differ. The clay mineral found in kaolin clay is called kaolinite and its formula is:

Al2O3. 2SiO2. 2H2O

This shows that for each part of alumina there are two parts of silica and two parts of water. The water (H2O) of the clay mineral is not the physical water added to the clay to make it plastic, but chemical water existing within the kaolinite crystal itself.

When kaolin clay is fired, several changes occur within the clay crystal.

100° - 200°C

The physical water evaporates.

450° - 600° C

The chemical water in kaolinite is released and steam can often be seen coming out of the chimney at this temperature.

Al2O3. 2SiO2. 2H2O3 ® Al2O3. 2SiO2 + 2H2O

The release of the chemical water causes a weight loss of 13.95 % and the kaolinite crystals are permanently changed. This is called the ceramic change, in which the clay loses its plasticity forever.

950° C

A new crystal is formed at this temperature and the process is:

2(Al2O3. 4SiO2 ) ® 2Al2O3. 3SiO2 + SiO2

One part of silica is released and adds to the free silica in the clay body. Free silica may already be present in the form of sand.

1100° - 1400° C

Gradually the clay changes into a crystal called mullite:

2Al2O3.3SiO2 ® 2( Al2O3.SiO2) + SiO2 ® 3Al2O3.2SiO2 + SiO2
(pseudo mullite) (mullite)

More silica is released during mullite formation and the alumina content increases. Mullite crystals are long and needle shaped, and form a lattice structure which reinforces the clay body in much the same way as iron bars reinforce concrete structures. The silica is released in the form of cristobalite crystals which may cause dunting on fast cooling (see page 53).

It is, however, not necessary to remember these chemical reactions in detail. They serve here as illustrations of chemical changes taking place in the fired clay.

montmorillonite: There are several other types of clay minerals, but we shall only discuss montmorillonite, which is often present in native clay along with kaolinite.

The montmorillonite mineral has this formula:

Al2O3.4SiO2.xH2O

This mineral contain 4 silica (SiO2) for each alumina (Al2O3) which is twice as much compared to the amount of silica in kaolinite. Its crystal structure is also different from kaolinite, and it easily breaks into smaller particles. That makes the clay extremely plastic and gives it a soapy feeling. An addition of 1 % pure montmorillonite to a clay body may improve its plasticity as much as an addition of 10% of ball clay. Bentonite is a primary montmorillonite clay mined in the U.S.A., but the term bentonite is often used for other commercial montmorillonite clays as well.

The release of free silica, takes place in montmorillonite above 950 C, but almost double the silica is released, compared to kaolin. Therefore, clay bodies with high amounts of montmorillonite contain a high percentage of free silica after firing, which may cause the ware to crack during cooling.

3. Physical Nature of Clay.

shape and size of clay minerals: The clay crystals of kaolinite are shaped as flat hexagonal flakes (fig.38-2). They are extremely small and can only be seen with the help of an electron microscope. Each crystal contains thousands of layered sheets stacked on top of one another as in a pack of playing cards. The sheets are only loosely bonded together and they easily break into thinner flakes, which retain their hexagonal shape.

water of plasticity: Plasticity can be defined as the property of clay that enable it to be shaped without cracking and keep its new shape. This property is only found in clay.

Clay owes its plasticity to its thin plate like particles. When the clay is in a stiff plastic state, a thin film of water surrounds each clay particle. This film of water acts as a lubricant and enables the particles to slide past one another when the clay is formed, but the particles stick to one another and retain the shape once forming stops.

When more water is added to the plastic clay the clay particles start to move more freely and cannot hold onto one another as before. The clay becomes very soft and cannot retain its shape. After adding more water the clay becomes liquid, and in this state it is called a slip.

particle size: Plasticity, or the ability of the clay particles to hold onto one another, is directly related to the size of the clay particles. The smaller they are, the greater the total surface area and the more there is to hold onto.

A clay with large particles cannot pass our rope test (page 13), whereas a fine plastic clay can be bent without breaking. Each of its fine particles needs only to move a little to accommodate the bending, whereas the particles of the coarser clay have to move so far that they break apart.

Electrical charge: Clay particles behave like small magnets, which attract each other when they have opposite polarity (North-South or plus-minus) but repel each other when they have the same polarity. The polarity of the particles depends on the non-clay materials. When the clay particles repel each other, the plasticity of the clay is low; whereas when they attract each other, the plasticity is high and more water is needed to make the clay soft.

Souring: Many sedimentary clays contain carbon (decayed vegetable matter), which make the clay sour (acid) during storage. The acid polarizes the clay particles so they attract each other, thereby increasing the plasticity of the clay. Adding calcium to a clay has a similar effect.

Additionally, aging clay allows bacteria to produce colloidal gel, a sticky slippery substance that adds to plasticity.

Casting slips: A typical clay needs a 100% addition of water to make it into a slip. For slip casting it is desirable to have as little water as possible, in order to reduce problems of shrinkage and wet plaster molds. Addition of washing-soda and water-glass (sodium silicate) changes the electrical polarity of the clay particles so that they repel each other and as little as 50% addition of water will make the clay into a slip.

Strength: A clay containing very fine particles will collapse under its own weight during forming, because the particles slide too easily. Addition of coarser particles will give "bone" to the plastic clay by preventing the fine particles from sliding excessively. The additive can be sand, grog or a coarse clay like kaolin.

Effect of clay preparation: Clay crystals tend to cling together in lumps, that behave like large particles. By blunging the clay, especially in a high-speed blunger, these lumps can be broken down. Prolonged storage (also called aging) of the plastic clay under moist conditions gives the water time to penetrate the lumps of clay crystals and surround the individual crystals with its lubricating film. Water helps to break down the individual crystals, and so furthers the plasticity of the clay. Kneading and pugging brings the clay particles into closer contact, and helps to remove air pockets. This improves the strength and plasticity of the clay, and prevents forming or firing problems due to trapped air.

4. Drying.

After forming, the next step in pottery production is drying. All potters have experienced warping or cracking during drying, so let us look at the causes of these problems.

surrounding air: In the drying process, all the lubricating water (also termed water of plasticity) has to get out of the clay and into the surrounding air. When the water content of the clay is equal to the surrounding air the process of drying stops.

drying shrinkage: Fig. 41-1 shows 4 stages; "A", "B", "C", and" "D" of a clay from forming condition to bone dry. "A" shows the clay in its plastic state; all particles are surrounded by water (shown as small dots) and they easily slide when the clay is formed. When the clay is left to dry, the water moves from within the clay to its surface through the pores between the clay particles.

As the water leaves the spaces between the particles, they move closer together and will finally touch one another as shown in fig. B. The clay shrinks as the water disappears, so clearly the more water a clay requires to become workable, the more it will shrink on drying. That also means that the more plastic a clay is, the more it shrinks. At this stage, termed leather hard, there is still plenty of water left between the clay particles, but since these are already in contact no more shrinkage will take place.

pore water: The water between the clay particles will continue to move out of the clay until the moisture content is the same inside and outside the clay. The remaining water is called pore water, and the finer the clay particles, the higher the amount of pore water. Only when the clay is heated to above 100 C will the last pore water escape.

The pore water may be as much as 5% of the clay weight, and it is therefore important that the initial heating of the kiln is done very slowly, so that this pore water can escape before it turns to steam at 100° C. Steam will crack the pot or cause pieces of it to explode.

rate of drying: All potters know that clay dries faster in dry, warm and windy weather, and that the rate of drying can be slowed down by covering the clay with plastic sheets or wet cloth. Clay ware must dry evenly so that it shrinks evenly. A handle on a cup tends to dry faster than the cup itself, and the different rate of shrinkage will produce a crack in the handle unless care is taken to let the whole cup dry slowly.

Water from the core of the clay travels through the thickness of the clay to the surface, through all the small gaps between the clay particles. Therefore, clay with very fine particles dries slowly, and coarse clay dries faster. Fig. 42-1 shows how the water in a very fine clay cannot pass through the outer layer of the clay, which has already dried and closed the gaps. This can be corrected by "opening up" the clay: i.e. adding grog, sand or another coarse grained clay (fig. 42-2). Ware with thin walls dries quickly and evenly. Thick-walled designs are more likely to warp or crack. They should be dried very slowly, and additions of sand or grog up to 20% are very helpful.

5. Particle Orientation.

unstable particles: Playing cards standing on their edge are very unstable arrangements and clay minerals, having a rather similar shape, behave in the same way. When pressure is applied, clay particles position themselves with their flat sides facing the pressure.

particle orientation: An example of this is shown in fig. 43-1. A clay with its particles randomly positioned is left to mature for a long time. Gradually the force of gravity causes the particles to orientate themselves with their flat side facing the pull of gravity.

differential shrinkage: Such alignment of particles produces higher drying shrinkage in one direction, since more particles (and more water) are stacked in that direction. The additional water causes greater shrinkage during drying, and the shrinkage in one direction may be several times greater than in the other direction (fig. 43-2). This difference may cause problems in drying and firing.

throwing: When throwing a pot on the wheel, pressure is applied to the wall of the pot from both sides and the clay particles will position themselves parallel to the wall (fig. 43-3). In forming the bottom of the pot, pressure should be applied towards the wheelhead, while moving the fingers from outside to the centre. Otherwise, the particles will not be aligned, and the bottom will crack (fig. 43-4).

extrusion: When forming clay by extrusion, particle orientation takes place when the clay is forced through the die (fig. 43-5). A screw extruder produces another problem, called lamination, by the pressure from its screw blades. In the extruded column of clay a spiral lamination is formed. This may cause products to warp or crack during drying or firing.

slip casting: In a slip casting mould, the suspended clay particles are sucked towards the inner wall of the plaster mould, and they align themselves with their flat sides towards the mould face (fig. 44-1). If the design of the cast has sharp corners, the particle orientation (and thereby the direction of shrinkage) will be at right angles to each other and the pot may crack here during drying.

kneading: During prolonged storage the clay particles position themselves according to the pull of gravity and one purpose of kneading (wedging) the clay before forming is to break up this particle orientation.

firing: During firing, shrinkage also takes place, and particle orientation may create problems of warping similar to those mentioned for drying.

6. Firing.

The clay body goes through a number of stages during firing.

Up to 120° C Water smoking:

The water of plasticity evaporates first and then the pore water. Rapid increase of temperature will build up steam pressure, and may crack the clay.
220° C Cristobalite expansion: Cristobalite is created from silica at temperatures above 900° C. When the clay is fired a second time it will expand nearly 3% at 220° C. On cooling, cristobalite shrinks again. Rapid cooling at this temperature may crack ware.

350 - 600° C Ceramic change:

As described above, the chemically bound water of the clay crystal is released. The clay is very fragile and porous at this point. The clay particles are held together by a sort of "spot welding" at the points of contact. This process is called sintering (fig. 45-1).

573° C Quartz expansion:

The quartz crystal (SiO2) expands suddenly and will shrink again at this point during cooling (about 1%). The clay structure during heating is still open enough to accommodate this change, but if cooling is too rapid, the ware may crack.

500° - 900° C Oxidation:

Organic matter in the clay is burned out. If the clay has a black core after firing, then this stage of firing was done too fast. When the rise of temperature is very rapid, the surface may vitrify before the carbon dioxide inside the clay has escaped, and the entrapped gas will bloat the clay at a later stage of firing. "Bloating" is seen as bubbles or voids, which occur inside the clay and on the surface.

Limestone (CaCO3) gives off its carbon dioxide (CO2) at 825° C.

900° C - upwards Vitrification:

At this temperature the soda and potash in the clay will start to form a glass by combining with the free silica. As the temperature rises, more and more glass will be formed, involving materials like limestone, talc and iron oxide. The melted glass will gradually fill the pores between the clay particles as shown in fig. 46- 1. This vitrification process also causes the clay to shrink.

firing shrinkage: Firing shrinkage is a simple indication of how much a clay is vitrified. Another indication is how much water the fired clay can absorb. Vitrified clay has more glass filling its pores, so it absorbs less water.

glass melt: The melted glass formed in clay bodies is normally stiff, and will not cause the clay to collapse suddenly. Feldspar produces a stiff glass that allows for a long firing range. Limestone, on the other hand, only becomes a strong liquid flux when fired to above 1100° C. This will cause the ware to collapse suddenly

Glass of vitrification produces a strong finished body. But if the body is fired too high it will lose strength and become brittle.