Rice, Prudence . Potleey Analysis. A sourcebook , Te University of (hiagr Pees , 1467 ,USA. PP: S4- SB 4 Properties of Clays I: The Clay/Water System ‘A number of characteristics of clays are significant to potters because they determine whether the clays canbe easily and safely formed, dried, and fired into durable containers or other objects. These qualities vary among the clay mineral groups and are in part the bases for some of the industrial or commer- _ cial use classifications of clays. The most fundamental property of actay sits “plasticity, which isin tun founded on the idea ofthe “clay/water system” A ‘lay becomes plastic only when mixed with some amount of water; hence this primary property is based on the relationship between the two most basic in- srediens ofa ceramic, clay and water. This chapter looks at the nature ofthe clay/watr relationship, at plasticity, and at shrinkage—the consequence of removing water from the clay. The dis- cussions of clay and the clay/water system here refer almost exclusively (and except where noted) tothe layered silicate clays. 3.1. The Clay/Water System Clay is defined in most general, functional terms as @ material that becomes «plastic when mixed witha limited amount of water. One must understand the nature ofthis added water to comprehend the property of plasticity as well as, other important characteristics of ceramic materials. The water of interest “here is not the interlayer lattice water between the silica and alumina sheets of the smectite minerals: nor is it the chemically cbmbined water—the hy- sdroxyls—or water of hydration that is part of the chemical structure of clas. Instead, the water involved inthe clay/wvater system is that which is adsorbed ‘by the clay particles, or weakly bound to their surfaces and edges. This con- sists primarily of the water that-makes'dry'clay malleable when'**mechani ‘cally combined” withthe clay (or “physisorbed” rather than being bound as ‘part of the chemical structure (“chemisorbed”) of the clay particles (PhelpsPropERTiES OF CLAYS I: THE CLav/WATER SYSTEM. 55 and Maguire 1956; Dal and Berden 1965; Norton and Johnson 1944; William- sot 1947). This weakly:bound water is easily removed from the clay at low temperatures (see sec. 3.3) ‘The atomic structure of water molecules is similar in some ways to that of silica, As discussed in section 2.3.3, atoms of silicon join with oxygen to form tetrahedrons, and these in turn combine into hexagonal rings of tetrahed- rons, a structural arrangement common in many clays. Water molecules, like silica, tend to join with each other into tetrahedrons, and these tetrahedrons will jin into hexagonal rings. ‘Water motecutes are made up of one oxygen anion (O?-) and two cations of hydrogen (H'*) (fig. 3.14). Because of a noncentrosymmetric charge distri- bution, the water molecule acts as if it had two charges, positive and negative, even though the net charge of the molecule is zero. One end of the molecule, the end with the two hydrogen ions, has a partial positive charge, while the other end associated with the oxygen ion has a partial negative charge. A mole- cule with this structure is called a dipole because it has two electrical charges, or poles, separated by some distance Dipolar molecules may interact with other molecules and ions in two im= portant ways. One results because their dipolar character (dipole moment) leads them to interact electrostatically with other dipolar molecules. Water molecules commonly associate with four other water molecules, the positive “ends” forming four ionic bonds with the negative “ends” of other molecules (ig. 3.1), leading to a hexagonal ring structure (fig. 3.1¢). Unlike the lay- ered clays, however, in water this hexagonal structure develops in three di- ‘mensions, not just in a two-dimensional sheet structure. The bonds holding the molecules together are weak because the dipolar charges are only partial, and hence the electrostatic attraction is reduced. Further, the weak dipole! dipole interaction is a strong function of distance and falls off rapidly with increasing separation. Figure 3.1 The strcture of water: a, a single molecule, showing the dipolar structure resulting from the arrange- ment of two hydrogen nuclei. After Lawrence and West 1982, fig. 3-1, b, the tetrabedral arrangement of mole- cules and, c, the hexagonal ing structure of water, formed by arrangements of tetrahedrons. After Lawrence and Wiest 1982, fig. 3-2, b a b56 ‘Tue Raw MATERIALS OF POTTERY MAKING ‘The second important result ofthis dipolar charge distribution concerns in- teraction with single ions. lons with a positive charge (cations) will be at- tracted to the negative side ot water molecules, while tons with negative charges (anions) willbe attracted to the positive side. In addition to the effect ‘of the charge of the ion, the size of the ion is also important. Small ions. such as Ca or AD, that fit into the center of the hexagonal water structure will enhance its stability. Conversely, ions with radii larger than the size ofthe cen- tral area (greater than approximately one Angstrom [A], oF 1.0 * 10-Hem, oF (0.0000001 mm), such as K'*, will disrupt the hexagonal arrangement (fig 3.2). Table 3.1 gives the ionic radii of some common ions, ‘One more aspect ot the 1on/dipole relationship of water deserves men- tion—the effect of adding ions tothe water structure, An electric field is cre- Figue 3.2. Accommodation of ions of citfereat sizes in the hexagonal water stuctre;arow indicates disruption ‘ofthe water structure caused by the large K” fn. After [Lawrence and Wes 1982, fig. 6b, “Table 2.1_lonie Rai of Some Common Elements (Coordination Number ~ 6) ‘Element lon ‘Atomic Number Tonic Radius (A) O78 140 102 on 03 040 1st 138 100 os on os 0.96 on. 075, 030 093 06 136 8 Barium Lead RRSSEESBERRRBSSESS Zee ‘Source: Nfer Kingery, Bowen, and Unlmann 1976, table 33,PROPERTIES OF CLays I: THE CLAY/ WATER SYSTEM 7 ated in the water surrounding the ion. The water molecules that are adsorbed immediately onto the ion are in different physical state than ordinary water; they become organized and “immobilized” into a state sometimes referred to as “nonliquid” or *quasi-erystalline” (fig. 3.3). The molecules-are fixed OF with a cation, i their negative regions "are directed inward; with an anion, the positive ends are directed inward. AS a result, the ater inthe immediate region nf the on is income eee ie thin layer of a solid analogous to ice. There may be sot of disordered “buffer” between the immobilized region and the normal liquid water struc- ture of dipole/dipole attraction. The thickness or size of the area of nonliquid water around the ion is in turn a function of the ion’s size and charge. This area is ‘These considerations are important to the elayiwvatereystem because wet, i Clay platelets have active, electrically charged sites on their surfaces and edges that, depending on imperfections and the location ofthe broken bonds, result from the exposure of unsatisfied Al'*, Si**, O?-, and OH ions. Most gener- ally, in kaolinites at least, the break is such that the surfaces of the platelets consist of O° or OH” ions, while the edges are A1* ot Si** ions. In other words, ‘pave chase (nso) Ps ome ewan wall ‘To satisfy the charge deficiency of these ions, the surface sites of the clay Bartle atact the dipole water solecule (and othe ons), te OH and O bonding with H'* in water and the AI’* and Sit* bonding with the O2- in water. Which then acts as a large ion or series of ions surrounded by a water layer. This in the way de- scribed above. ‘The degree of structuring (i.e, the size ofthis immobilized region) is partly on the clay Platelet as well as their positioning. It also varies among the clay minerals Fique 3.3. Mode! forthe mesication ofthe stnvcture of water produced by the presence of small on +), Heavy stipple indicates repion of immobilization; ight stipple indicates re- sion of high disorder between immobilized water and normal water structure outside stip pling. After Lawrence and West 1982, fg. 3-458 ‘Tue Raw MATERIALS OF POTTERY MAKING (Grim and Cuthbert 1945). Kaolinite platelets, for example, have a pattern of charges on the edges that matches the hexagonal structure of water, making an “interlocking and stable structure. ‘The water structuring is also in part a conse {quence of the ions in the water solution. For example, both sodium and cal- cium cations have the same ionic radius, 0.98 A, but sodium is monovalent (Na'*) while calcium is divalent (Ca®"). In clays with adsorbed sodium ca- tions, the nonliquid water zone is smaller than in clays with calcium cations, and there is a considerably greater transition region to liquid water (Grim 1962, 251-52). This is doubtless also related to the fact that sodium-rich clays require less water to develop plasticity than do calcium-rich clays. 3.2 Plasticity "Plasticity allows a clay, upon addition of a limited amount of water, to be © shaped by pressure, and to retain that form when the pressure is relaxed, The property of plasticity is last when this adsorbed water is removed from the clay in drying (see sec. 3.3), butthe:formwill be:retained, Plasticity can be restored by wetting the clay again, but:the:shaped form will: disappear. Upon ‘or burning (see chap. 4), clays become hard and extremely resistant to “weathering, and above certain temperatures the capacity for plasticity is per- ‘manently eliminated. 3.2.1 Factors Influencing Plasticity Plasticity has its origins inthe clay/water system and arises from a number of factors (Bloor 1957; Norton 1948; Marshall 1955; Grimshaw 1971, 496-504). "Amiong the major concerns are the following: (1) clay particle size, (2) elay par ticle shape, (3) surface tension of the water, (4) rigidity of the water, (5) ad- sorbed ions, (6) clay mineral component, (7) clay deposit location, (8) or- {ganic content, (9) nonclay mineral component of the material, and (10) temperature (important primarily in commercial applications of clay working). Of the many factors influencing plasticity, among the most important are the size and shape of the clay particles. As discussed in section 2.3.2, clay particles are extremely small, generally less than 2 um in diameter, and a ‘considerable proportion of the smallest clay particles behave like colloids (see sec. 3.4.2). In addition, they have a flat lamellar or platelet shape, with a ratio of thickness to diameter on the order of 1: 12. These shape and size considera- tions mean the clay particles in a clay mass have a very large total surface area. Most of the properties of clay/water systems (especially plasticity) in large part result from the interaction of clay and water at the surfaces of the clay patticles. ‘Some idea of the surface area involved with clay-sized particles, compared with sand or other particle-size grades, may be given by calculating the sur- face area produced by successively subdividing a cube of some material (Law rence and West 1982, 21). A cube of one cubic centimeter (I cm) has @ total surface area of 0.93 square inches. Dividing that cube into particles on the order of 0.1 mm in diameter would produce 93.0 square inches of surface