|Clay Materials - for the Self-Reliant Potter (GTZ, 1990, 98 p.)|
1. Raw materials
Only a few clays can be used as they are found in nature for pottery production. Most often, addition of other clays or materials like sand, limestone or feldspar is needed to produce a mixture suitable for specific forming and firing techniques. A mixture made up from different clays and materials is called a clay body.
Different methods of shaping demand different types of bodies. For the potter's wheel a good plastic clay is desirable, whereas much less plasticity is needed for pressing of tiles.
The ingredients of a clay body can be divided into four groups according to their function: plasticizer, filler, opener and vitrifier. The plasticizer is used for increasing the plasticity, the filler is the cheap bulk material, the opener promotes drying and reduces plasticity, and the vitrifier or flux is makes the body more fusible.
All the non plastic materials like feldspar, whiting etc. also work as openers of the body.
kaolin: This is a primary clay (see p.9) and therefore rather coarse, with little plasticity. Kaolin opens up a plastic clay, so that it dries more easily. It is a refractory clay, increasing the melting point of the body, and it makes the body more white. The clay mineral in kaolin is kaolinite (see p.35), which benefits bodies. Kaolin is a more costly material for clay bodies. Raw kaolin, i.e. unwashed kaolin containing high amounts of silica sand, is cheaper and beneficial if the clay body can tolerate the extra sand.
ball clay: This is a secondary clay with fine clay particles, which make it very plastic. Ball clay normally fires white, but the term is sometimes used for any highly plastic clay used to increase plasticity of a body. The white firing types increase maturing temperature of fusible clays and add whiteness. Many ball clays are grey or black when dug, indicating a high content of vegetable matter (carbon). 10% - 20% addition of ball clay greatly improves the plasticity and green strength of a clay body. A white firing ball clay is normally expensive.
native clay: This simply means a secondary clay, firing to a red or buff colour. It makes up the bulk of the body for buff or dark firing earthenware and stoneware. Preferably, it should be dug close to the site of the workshop to keep its cost down. In the best of cases, it can be used without any additions. Adjusting the properties of a native clay can be done by adding kaolin for opening up the clay to reduce drying cracks, or by adding ball clay to improve its plasticity, as listed in fig.47-1. However, it is better and cheaper to try to find another native clay that can do the same job.
bentonite: This is the commercial name for an extremely plastic clay also known as montmorillonite clay. It exists in smaller quantities in many natural clays, but in pure form it is used as a drilling fluid, in paint and chemical industries and for binding molding sand for iron casting.
In ceramics it is used to increase plasticity and green strength of clay bodies, and an addition of 1-2% to a glaze helps to keep the glaze materials suspended. An addition of 1% bentonite may increase plasticity of a clay body as much as 10% ball clay.
When montmorillonite is present in larger amounts it may cause drying trouble, because of its high shrinkage and reluctance to dry at all.
feldspar: K,Na,CaO . Al2O3 . 6SiO2 , S.G. 2.6 , Hardness 6 , M.P. 1100-1260° C.
Feldspar is a very common mineral found in most primary rocks in amounts up to 60%. It is used as a flux in clay and glazes. Many different types of feldspars exist, the main groups are: potash spar, soda spar and lime spar. Melting point, 1100° to 1260° C. A mixture of 65% potash spar and 35% soda spar has the lowest melting point.
Feldspar is mainly used in porcelain bodies and white firing stoneware but the cost of grinding the feldspar limits its use for other type of ceramics. As a substitute, most rocks (both granitic and volcanic) can be used. A cheap source of ground rock is the dust produced where stones are crushed for road or building construction.
Feldspar can be recognized by its pearly lustre and opaque appearance. The crystal has two main cleavages, that are nearly at right angles to each other. The colour is whitish, grey or different shades of red. Once a feldspar has been shown to you, you will easily recognize feldspars on your own.
nepheline syenite: Nepheline syenite resembles feldspar but contains less quartz; so its melting point is lower, and it can be used as a body flux at temperatures above 1060° C. Rocks containing nepheline occur widely. Nepheline rocks contain about 2% iron oxide, which will color white bodies. Commercial nepheline syenite has had its iron oxide removed by magnetic separation.
glass powder: Ground glass is a cheap source of flux for both clay bodies and glazes. Broken glass can be collected from breweries or window glass shops. The low melting point of glass makes it a good body flux for production of vitrified earthenware, but it has to be ground to a fine powder and mixed very well with the other materials.
limestone: CaCO3, S.G. 1.8 - 2.7 , Hardness 2-3 , Decomposition 900° C.
Calcium carbonate is the chemical name for whiting, limestone, chalk, marble and coral. All these materials can substitute for one another in clay bodies. They are cheap and easy to grind, and can be used in both clay bodies and glazes as flux. Carbon dioxide is given off before 900° C., and what is left is calcium oxide. The weight loss amounts to 44%. The carbon dioxide gas may produce pinholes in once fired raw glazed ware. Wollastonite (CaO.SiO2) does not give off carbon dioxide during firing and can substitute for limestone in fast firing bodies.
Calcium oxide should be kept under 2% in stoneware bodies. It is mainly used in earthenware. Its fluxing action is strong, and gives the clay a narrow firing range. Above 1100° C. fluxing action increases rapidly, causing high limestone ware to collapse suddenly. Calcium oxide in the body decreases the tendency of glaze crazing, and it decreases the red coloring effect of iron oxide.
Talc: 3MgO . 4SiO2 . H2O , S.G. 2.6-2.8 , Hardness 1-1.5, M.P. 1490° C.
Talc is usually cheap, and it occurs in many locations throughout the World. Solid forms of talc are known as steatite or soapstone. Talc deposits may contain impurities like limestone, quartz, clay and iron. Its colour varies from white, greenish, or grey to brown, but it fires to a cream or white colour or grey if its iron content is high. Talc from different deposits varies, therefore testing is needed before using talc from a new source.
Talc improves resistance to thermal shock and acid. It reduces the tendency of glaze crazing, by preventing expansion of the clay after firing due to moisture absorption . During firing the water in the talc mineral is released, causing a weight loss (loss on ignition) of about 6%. Talc starts to work as a flux around 1030° C., and it produces a stiff glass compared to other fluxes. That gives it a long vitrification range, and the ware will not suddenly collapse when overfired as is the case of limestone. Talc is able to produce a vitrified body and at the same time reduce the tendency of warping and firing shrinkage . Small additions of talc reduce the melting point of the clay body; large additions make it more refractory.
Talc powder has a small amount of plasticity. In plastic forming and slip casting talc causes no problems, but in semi-dry press molding, some types of very fine grained talc may produce lamination problems. This can be overcome by ensuring proper wetting of the talc particles, followed by granulation of the body allowing air to escape during pressing.
Talc is especially valuable for fast firing bodies and for production of wall tiles and electrical insulators.
Dolomite: CaCO3 . MgCO3 , S.G. 2.9 , Hardness 3.5 - 4 , Decomposes at 900° C.
Dolomite behaves in a body as a more or less equal mixture of
and talc would do, except that no silica is introduced. Loss on ignition is around 45% .
Quartz: SiO2 , S.G. 2.65 , Hardness 7 , Melting point 1710° C.
Quartz is a crystal form of silica or silicon dioxide. Silica is found as part of rocks and clays and it is so common that it makes up 60% of all materials in the crust of the Earth. As a free mineral, not combined in clay and other materials, it occurs as quartz, silica sand, sandstone, and flint pebbles. The cheapest source of silica is sand. All sands contain silica in the form of small quartz crystals, but a particular sand may contain small crystals of other minerals e.g. mica. That should not cause problems in a clay body, unless the potter attempts to produce white flawless porcelain.
Additions of silica make the clay more refractory and open up the body, thereby reducing shrinkage and drying problems. As is the rule with all other ceramic materials, the finer the particles, the more actively the silica will combine with other minerals and form a glassy substance with the fluxes in the clay during firing. When the silica in the body remains free (uncombined chemically) , it expands and shrinks suddenly at certain temperatures, as shown on the graph in fig. 51- 1. These sudden changes may cause the fired body to crack, but they also assist in prevention of crazing as explained below.
Grog: Grog is crushed, already fired clay. The quality and behavior of the grog depends on the original clay. It is extensively used in production of firebricks, saggars and other refractory products, for reducing firing shrinkage and increasing thermal shock resistance. In clay bodies for crockery and tiles it is mainly used to improve the forming and drying characteristics, without changing the final composition of the body. Grog gives "bone" to the body during plastic forming, eases its shaping and prevents it from collapsing during throwing on the wheel. It reduces the problems of warping and cracking during drying (as sand also does), but without adding the problems of cooling cracks that quartz sand may cause.
Grog is normally produced by crushing unglazed waste products in a hammer mill or in a pan grinder. The grog particles should be sharp edged.
colour oxides: Oxides used for coloring clay bodies have to be fairly cheap. That excludes most oxides, leaving us with iron oxide, manganese dioxide and ilmenite. Coloring of engobes for decoration will not be dealt with here.
Iron oxide exists in two main forms. Red iron oxide (Fe2O3) is the same as rust, and has a dark red color. Black iron oxide (Fe3O4) has a coarser particle size than red iron oxide. Black iron oxide can be produced by roasting iron metal to 400° to 700° C in the flue channel or chimney of the kiln. The black crust of oxide is knocked off the metal and ball milled. Ochre is a yellowish material often used for painting houses. It contains iron oxide in a mixture of clay, sand and sometimes limestone. When the ochre contains manganese, it is called umber. Both materials can be used as coloring agents in clay and glazes. Ilmenite (FeO.TiO2) is a black crystal, often occurring as black stripes in beach sand together with zircon sand.
The coloring effect of iron oxide depends very much on the atmosphere and temperature in the kiln:
- Iron oxide in an oxidizing firing below 1020° C will produce a brick red colour.
- Oxidizing firing to 1100° C turns the red colour darker and brownish.
- In a reducing firing the colour will be grey or black.
Above 1000° C iron oxide acts as a strong flux in reducing atmosphere, but when the condition is oxidizing, its fluxing action only starts above 1200° C. Whiting present in the clay has a bleaching effect on the red colour of iron oxide. In red firing surface clays iron oxide content is often 10% .
Manganese dioxide (MnO2)is dark brown to black. As a colorant in clay it produces yellow, brown, purple or black colors. It acts as a strong flux. Only half a percent of manganese will give red clay a brown colour, and with increasing amounts the clay will become black. Black colors are obtained by adding a mixture of iron oxide and manganese dioxide.
2. Classification of Ceramics
The traditional classification of ceramic ware is in three groups - earthenware, stoneware and porcelain - is mainly based on the firing temperature. In common usage, the terms often overlap e.g. a term like "low fired stoneware" is sometimes used to describe ware that resembles real stoneware, but is made from clay bodies vitrifying at earthenware temperatures (additions of fluxes). Here we shall only deal with earthenware and stone- ware.
Earthenware means pottery that is porous when fired . The firing range normally is 9000-1100° C. A wide field of different products fits in this group. Traditional pottery made from local, red firing clay ware (whether glazed or unglazed) is the most common type of earthenware. Unglazed red pottery is often called terracotta. Other types, glazed with white opaque glazes, are known as Faience or Majolica. Glazed wall tiles are normally made from a porous earthen- ware body.
fuel saving: Earthenware can be as durable as stoneware, and the lower firing temperature saves fuel. The cost of additional fluxes needed for maturing the glaze at earthenware temperature compared to that of stoneware is easily paid by the lower firing cost.
dark bodies: Yellow, red, brown or black bodies are made from local plastic clay containing iron oxide. The starting point is a fairly plastic clay from a nearby source. First test the clay (see p.62), and then modify it as necessary :
- if it is too plastic, add sand, grog or a less plastic clay.
- if it has little plasticity, add a plastic clay or remove some of its sand content by washing and screening.
- if it warps during firing, add a more refractory clay or sand.
- if it has little firing strength, add a vitrifier like limestone, talc or a more fusible clay.
When looking for clay materials, keep costs in mind. Instead of adding feldspar as a vitrifier it is cheaper to add a more fusible clay. Instead of adding sand or grog, a less plastic clay could be added.
white earthenware: A body with the appearance of porcelain or stoneware can be produced for earthenware temperatures. Such a body is made with kaolin, a white firing ball clay, and large amounts of talc. Additional flux can be feldspar (preferably nepheline syenite), lime- stone, dolomite, frit or glass powder.
These recipes can be used as a starting point for experiments. For tempera- tures in the 1100° to 1200° C. firing range the flux content should be reduced.
High lime content (up to 20%) reduces the coloring effect of iron oxide.
thermal crazing: Crazing of the glaze is a major problem with earthenware. Another problem is that glazed rims of pots easily chip off. Both problems are caused by different rates of thermal expansion of the glaze and the body. When a pot is taken out of the kiln after firing it is exposed to a sudden temperature drop. The glaze layer and the body will contract, but most often at different rates. Shown below is what happens when 1) glaze contracts more than body, 2) body contracts more than glaze.
glaze contracts more: < here insert 57-1> This figure shows a body (white) with a glaze on top (black). The glaze and the body has contracted at the same rate and there is no tension between the two. <here insert fig. 57-2> In this case, the glaze contracted more than the body (leaving it shorter than the body), which puts the glaze with a tensile stress (it is pulled apart). If the body is very thin it will bend as shown. <here insert fig. 57-3> More likely, the tensile stress will be relieved by cracks in the glaze, as shown in this figure. This is called crazing. The stress, caused by high expansion (and contraction) of the glaze, may be relieved by crazing as soon as the pot is taken out of the kiln or it may take days, months or years. If it takes a long time for crazing to appear, this means that the expansion of clay and glaze is almost equal.
When the body has been exposed to humidity for a long period, water enters the body, which expands slightly (moisture swelling). This expansion may cause a glaze to become too short and it will craze. This kind of crazing is called delayed crazing or moisture crazing.
body contracts more: <here insert 57-4> This example shows a body with higher expansion rate than the glaze. The body contracted more than the glaze when it cooled. The glaze is under compression, and if the clay is thin it may bend as shown to relieve the pressure. If body contraction is only slightly higher than glaze contraction, nothing will happen except the glaze will not craze. <here insert 57-5> If a glaze contracts much less than the body, the compression on the glaze becomes too high and the glaze will start to chip off like this. This will not happen by itself, but only if something bangs against the pot. Typically, the rim of a cup will chip off. <here insert 57-6> In extreme cases, high compression of the glaze may cause the body to crack.
crazing cure: Crazing is cured by adjusting the expansion rate of body and glaze. If the glaze is too big for the body, it is under a constant squeeze and will be less likely to craze. This squeeze is obtained if cristobalite is present in the body.
cristobalite: Cristobalite is a crystal form of silica (SiO2) which is formed above 870° C, when some of the free silica in the body changes its crystal form.
The graph in fig. 58-1 shows the contraction of glaze and body during cooling. Both contract gradually until 573° C., when quartz crystals in the body suddenly contract about 1%. The glaze is still soft enough to accommodate this contraction. The glaze hardens around 500° C, and from then on it contracts at its own rate. In this case, the glaze contracts more than the body and it will craze.
The graph in fig. 58-2 shows contraction of a glazed body containing cristobalite. Initially the graph is similar to the upper one, but at 226° C. when the glaze is hard, the cristobalite in the body contracts 3%, leaving the glaze in compression. In earthenware, the effect is usually only present if the body is close to vitrification.
The conversion of quartz into cristobalite is helped by fine grinding of quartz, by a higher firing temperature, and by presence of talc or limestone. Flint, a form of quartz, converts more easily into cristobalite in earthenware bodies. If the glaze is under too much of a squeeze it may crack the pot or cause shivering and peeling of the glaze.
moisture crazing: After firing, the porous earthenware body will absorb moisture and this causes the body to expand. If the glaze is not under sufficient compression it will craze. Such delayed crazing may occur a long time after firing. The moisture expansion of the body is reduced by making the body more vitreous. Additions of talc or limestone to the body reduce moisture crazing.
crazing cure: For both types of crazing the cure is:
- add quartz (or silica), talc, limestone to the body.
- biscuit fire to a higher temperature.
- glaze fire to a higher temperature.
- add silica to the glaze.
- replace alkaline fluxes (soda and potash) in the glaze with boron oxide.
chipping cure: When the glaze is peeling or chipping off:
- reduce quartz content of the body.
- reduce silica content of the glaze.
- increase alkaline fluxes in the glaze.
- add feldspar to the body (above 1100° C)
If fuel cost is not important and good refractories for kiln furniture are available, the firing range from 1180° to 1300° C offers several advantages over earthenware. At this temperature less fluxes are needed, and it is often possible to find a natural clay that fires to a strong dense body with little addition of other materials. Crazing of the glaze is less of a problem, since at this temperature there is a better body to glaze bond. Even if crazing occurs, the vitrified body will remain waterproof. Another advantage is that the higher firing temperature makes it possible to rely on non-fritted fluxes, like feldspar and limestone, for the glaze.
Examples of stoneware are hotel crockery, floor tiles, sanitary wares, salt-glazed sewage pipes and utensils, electrical insulators and corrosion free vessels for the chemical industry.
vitrification: Stoneware is hard, strong and dense. Its color varies from buff or grey to brown or black. Due to the high firing temperature, the fluxes in the body gradually melt and fill the space between the clay particles with a glassy mass. This strongly binds the clay particles together, but if the body is overfired it becomes brittle like glass and loses its strength. Feldspar is the preferred flux for stoneware, because it has a long vitrification range and it produces a viscous glass, that does not cause the ware to collapse suddenly. To vitrify means to become glass-like.
native clays: Many native clays can be used for stoneware, as dug or in combination with other clays. Stoneware bodies soften during firing, and large items tend to warp or sag unless grog or sand is added to the clay. Earthenware clays low in iron oxide can often be used with additions of fireclay or kaolin.
The optimum recipe for a stoneware body is found by making tests of clay bodies with varying additions of fluxes and/or refractory clay and openers like sand and grog (as listed for earthenware).
crazing: Crazing is seldom a problem for stoneware since the body itself is waterproof. Further- more, the body and glaze materials partly melt together and form a strong bond. At stone- ware temperature, quartz readily changes into cristobalite, which further reduces the tendency to craze. When crazing does occur the corrections listed under earthenware apply.
cracking: A more common problem with stoneware is cracking during firing, or more often during cooling. The main cause is the change in silica crystals at 226°C and 573°C. Free silica in the form of cristobalite expands nearly 3% when heated to 226°C and contracts again when cooled to the same temperature. Free silica in the form of quartz goes through a 1% expansion and contraction at 573°C (see fig. 53-1). These changes take place suddenly, and because large items do not cool evenly, one part of a pot may reach contraction tempera- ture before another. When one area contracts suddenly, the stress within the pot may cause it to crack. If pots only crack occasionally, the cure is to cool the kiln more slowly, par- ticularly from 700° - 150°C. Otherwise changes have to be made in the body.
remedy: Since the problem is caused by free silica, the cure is to reduce the amount in the body. Free silica comes from 1) quartz or flint added to the body or present in the raw clay 2) release of silica from other clay minerals during firing (see p.35). One or more of the following changes can be made:
- grog can replace quartz sand.
- clay containing high amounts of sand could be washed or replaced by a less sandy clay.
- the release of free silica from the clay minerals cannot be avoided, but since montmorillonite clays (bentonite) release twice as much free silica as kaolin, it may help to reduce the amount of montmorillonite clay in the body. A well equipped ceramics laboratory can determine the content of montmorillonite clay minerals in a clay, but potters without access to such services will have to rely on practical testing of the different clays available. A clay which is difficult to dry, has high plasticity and a greasy feeling to it, should be suspected of containing montmorillonite.
- Free silica in the body is also reduced by adding a flux that will combine with the silica and form a glass. When silica is part of a glass it will not cause cracking, because glass is non-crystalline.