Allen A. Denio Department of Chemistry University of Wisconsin-Eau Claire Eau Claire, WI 54701
Clay was formed by the action of wind and water upon the earth's crust long before man evolved on this planet. This natural clay-forming process continues today. Clay was apparently one of the first shelter building materials, and it was also fabricated by hand into utilitarian objects such as howls, oil lamps, and storage vessels. More recent technological breakthroughs have heen the potter's wheel and glazes. The potter's craft gradually evolved almost into its present state without much help from chemists. Many of the current generation of potters still function quite well with little or no knowledge of chemistrv. There&, however, a growing awareness among potters that chemistry is indeed relevant to their functioning in the studio. Considerable progress has been made on this campus in "bridging the gap" between the chemistry laboratories and the pitt&ystudio. In fact, the transition state actually involves a bridge over the Chippewa River, a necessity for those not aualified to walk uoon the water. The author functions as a guest lecturer in one of the potterv courses taueht bv Professor Richard Joslin of the Art ~epartmentA . text with audio tapes and slides has been prepared to help introduce basic chemical concepts that are of value to pottery students. This paper will focus upon some of the chemistry that is important to them. The Origin and Composition of Clay
Most ,,f rile earth's crust is compllsedof a nulllhcro i e l e m e n ~Oxveen. . silicon, and aluminum reoresent 82.7% by weight of the &2;'s crust. The ten most a h k d a n t elements, which comprise 99.2%, are listed in Tahle 1. Silicate minerals contain a basic unit in which each silicon atom is bonded to four oxygen atoms in a tetrahedral arrangement. One such isolated unit would represent the orthosilicate ion, Si044-. An example of such a mineral is zircon, ZrSi04. More complex silicate structures generally are found in which the tetrahedral silicate units share oxygen atoms. Thus, two tetrahedral units with a shared oxygen would have the formula Siz07" (Fig. 1). I t is possible to have single strand silicates in which the neighboring tetrahedra each share two oxygen atoms to form a long chain. In a similar manner i t is possible for each tetrahedral unit to share three or even all four oxygen atoms and form two dimensional sheet type structures or three dimensional arrays. Luckily for potters, some silicate minerals also contain A13+ ions in place of some Si4+ ions in the silicate tetrahedra. The difference in charge is compensated for by the inclusion of
Chemistry for Potters other cations such as Na+, K+, Ca2+,Mg2+,and Fe3+ in the structure. These minerals are called alumiuosilicates, a common example being a type of mica with the name muscovite, KA12(AISi3010)(OH)2.The feldspar minerals are also aluminosilicates and make up about 54% of the earth's crust. One common type of feldspar is anorthite, which has the formula CaAI&gOa. - - - The basic reneat unit in this mineral is Al2Si2Os2-; the Ca2+ions maintain electrical neutrality. I t is believed that clavs are formed bv the slow weathering of feldspar minerals. consider the conversion of anorthite into clay by the very slow reaction with water and carbon dioxide. CaAl&i~Os+ 3H10 + 2C02 Anorthite
Ca0. A1203.2Si02 Anorthite
272 1 Journal of Chemical Education
+ 3H20 + 2C02
-
AI2O3.2Si02.2H20 + Ca(HC03)s Kaolinite
This "ideal" formula for clay is given the mineral name kaolinite. The clay that is found in deposits in the earth's crust normally contains many impurities which can alter the color and other properties of the material. In Tahle 1,the ten most abundant elements in the earth's crust were oresented. I t is noted that oxveen . is bv far the predominant element. If the other nine elements are assumed to he Dresent as oxides. a simnle calculation leads to the weight percentages of these oxides in the earth's crust. This ass u m ~ t i o nienores the Presence of NaCl (halite). CaF? (fluorite); and F& (pyritejdeposits. Note, hokever,' that chlorine accounts for only shout 0.2%by weight of the earth's crust, and fluorine and sulfur are even less common. The nine most ahundant oxides are given in Tahle 2, alonx with the weight percentages of the oxides in kaolinite and two typical clay samples, North Carolina kaolin and common red clay. I t should be noted that the natural kaolin sample has a
-
Elements in the Earth's Crust: The Ton Ten
Eiemem
Weight %
Element
Weight %
oxygen silicon aluminum
49.5 25.7 7.5 4.7 3.4
sodium potassium magnesium hydrogen titanium
2.6 2.4
iron
ion.
AlnSizOs(OH)&+ Ca(HCOd2 Kaalinite
The first product is the clay mineral kaolinite. Thus, the feldspar mineral anorthite is slowly converted into this clay mineral and calcium bicarbonate; as the latter product is washed away small, hexagonal shaped plate-like crystals of the kaolinite remain. The ratio of the length to thickness of these plates is about 10:1, and the average particle diameter is in the 0.1-100 @ m range. The previous paragraph gives the chemical formulas of anorthite and kaolinite as written hv chemists. However. potters have traditionally used a different approach in which the minerals are nm;idered to be nmposrd of oxide;. The pre\.ious chrmiual equation is prefcrc.ntii~Ily written IIV porters as:
Table 1.
The Si,O;'
-
calcium
1.9 0.9 0.6
composition very close to that of the "theoretical" composition of kaolinite, while the red clay sample approximates the earth's crust rather closelv. The term "clay" has many definitions, depending upon the perspective taken. Thus, a potter, a chemist, and a geologist might have trouble in arriving a t a common statement. The American Ceramic Society has provided the following: Clay is a fine-grained rock which, when suitably crushed and pulverized, becomes plastic when wet, leather-hard when dried and on firing is converted to a permanent rock-like mass.
This "ceramic" clay is composed of clay minerals and impurities. The clay minerals include kaolinite, illite, chlorite, smectite, and several others. The impurities include quartz, a large variety of iron compounds, several alkali and alkaline earth metal oxides, calcium and magnesium carbonates and sulfates.. oreanic ., materials. and manv more. Clav cornnositions vary widely, even when taken from a single location. I t is little wonder then that these incredihlv comolex mixtures are not easily defined. Properties of Clay The plasticity of clay, or its ability to be manipulated when wet, is a complex phenomenon. There are several relevant factors such as the nature of the verv small hexaeonal olate crystals having a tremendous amount of surface area per gram, the type of cations adsorbed onto the clay surface, the unique high surface tension of water, and the temperature of the system. It is often necessary to combine different twes .. of clays tb obtain suitable working properties. The aging of clay to improve its plasticity and workability adds to the mystique of the process. Since clay has been formed slowly over countless centuries, it seems strange that it should need an aging period of a couple of weeks. It appears that a bacterial process is responsible for this clay "ripening" stage. This is favored by keeping the wet clay relatively warm and by adding a small amount of starch to feed the bacteria. When a new hatch of clay is mixed, it is common to add a portion of previously aged clay, which in turn promotes the bacterial process in the new material. The Firing of Clay After a clay body has been formed, it is set aside to dry before the firing process, which is used to form a hard and strong mass. Several changes occur during "firing," and the temoeratures for different Drocesses depend to some extent upon the origin of the clay s&nple. Residual water in the structure is removed in the 100-125'C range. The chemically combined or OH lattice water is removed from about 350-525% Recall from Table 2 that kaolinite contains about 14% of this lattice water. This dehydration process occurs without shrinkage. Quartz is a normal component of clays, and a phase change or quartz inversion occurs a t 573% At this temperature the more random a form changes to the more orderly @-quartz with a slight volume increase. Most clay samples will contain some organic matter which Table 2. Clay Compositions: Weight Percentages. North Carolina
Oxides Si01 A1203
H20
Fez03 MgO
Kaolinite 46.6 39.5 13.9
kaolin 46.2 38.4 13.2 0.6 0.4
CaO
Na20 KzO Ti02 Others
1.2
Common red clay
57.0 19.2 3.5 6.7 3.1 4.3 2.4 2.0 0.9 0Q
Earlh's crust
55.0 14.2 8.1 6.7 3.2 4.8 3.5
2.9 1.0 O
R
is burned off in the firing cycle, assuming an adequate supply of air is present. This oxidation process proceeds a t a variety of temperatures from 200-l,OOO°C, depending upon the types of organic residues present. Magnesium carhonate is decomposed to the magnesium oxide and carbon dioxide a t 790°C, while calcium carhonate undergoes a similar change a t 880°C. The calcium and magnesium sulfates dissociate in the 1,100 to 1,300°C range. When a kaolin sample is fired to almost the complete dehydration point, a slight change in the structure results to give the new product called metakaolin. Its exact structure is not well understood. Further heating eventually leads to a defect spinel-type structure a t about 950°C. In the 1,000 to 1,250°C O~) ranee. " . lone" needlelike crvstals of mullite (3 . A 1- ~..0 ~ 2 S-.i are formed, these enhance the strength of the structure. The a to B-auartz transition a t 573°C is eventuallv followed by the iormnti;m ot'tridymite at 8;u0(. and finally c;iLtohalite ar 1.17U'C. 'l'heze solid phase tmnsitinni wcur sl~wl\,, ; m l thr. quartz usually remain; as cristohalite after coolingto room temperature. The firing process must he done slowly to permit these several changes to occur without damaging the clay body. The initial "green ware" having very little strength is transformed into a fired piece that is very hard, strong, resistant to abrasion and having good chemical inertness. About 5-10% shrinkage occurs during this process. Kiln temperatures are very important to the potter, hut of greater value is the "heat work," a comhination of temperatures and time. The kiln is often e a. u i.o. ~ e dwith a thermocouple. 11utmolt pottersfill rewrt t u u ~ i n gp y r n ~ e t r i ccones to eitirnate I he "heat w r k " quantity. These cones are sl~mdrr. three-ridrd pvrnrnirli 01 k n o w refractnrinesa \.aluec. R'lwn a cone o t n ch(
