I I 13 Appendix 2 !f?w We want to increase your ur,dersta nm of clq, -&at it is ad :kat hapgeps to it between taking it out of the b~cund and becoming a finis&d stove lbe= Sat is clay? The ter;3. day refers to a certain particle size of a soil miner& Clay cams from weathered rocks; !&en rocks GJZ exposed to sunshine, to hoat szd cold, to rain ad ice, they decompose, or break dohn into smaLL pzrticles. Thic is Use a very Si3W process, xd some rocks t&o .zuch longer to dccoxpose than o-iihcrs * Some rocks break up rzthr easily, atxd get carried downhill by gizciers or streaL7s. The finest pzcticlcs get cried the farthest. they are often drs;~cd oziLy when the river spreads out over a flood pla& and slxs dolm. %q- lqcrs of tbesc tiny pdic-les become ciay deposits, rock decomposes inplace. (Some clqs ze formed differently: the These cliqs zre usually very white.) Clay is defined as a particle size which 'i3ezxres 10 microrz or less in size. That means t5z.t 100 of the largest clay pazticles, side by side, would measure only one millimeter across. Silt is the nelrt laxgest particle size: 40 of the largest silt pa rticles measure one millineter across. Sand is the next largest particle size: fti sand measures up to one third of a millimeter across. I iky does clay stick to&her? e OH on OH OH CJM aH OH (1" I I 1 qKh%yfs cwaw) 5; Gi Si s Si Si Si %4knu\ r I I AI-&+-A\-A;-ii- Aluminum 4 Z & si , 1 OH an & & s 4 ji sKcor\ d, 0: dH qmxyls Gaiikw) shape of clay particles chemical structure of clay (simplified) Clay particles are -ted, that is, they are chemically bound to water. Even after clay ham dried in the sun, there is still water between the the clay particles. This bonded water oticks to other water, and so the particles of we% clay are both attached and lubricated by the water bet-+:aen them. The clay particles slide apart or closer together as the clay is pressed and formed. Smaller particles of clay have more surface axea, and can hold more water in betwzn them relative to 1-r clay pzzticles. Clay that is verystickyhaa small particles of clay. The ability to stick together, and to be formed vithout cracking, is called PUSTICITY, If clay ciul be easily shaped, we say it is hi&ly plastic. If it breaks when coiling around your finger, it is not very plastic. Plastic clay is flexible due to having agoodmixof particle sizes. Plastic clay has some very fine (small) c&y particles. snt very fine clay, all by itself, is too sticlq to work with. / . non-plastic 4 @ ‘s plastic clag ' , clay 4 , 1 Clay znd ce,mmic lecture notes, sage 2 14 One. zf 'the reasons we blend different clays together to make pots and stove liners i;s to get good plasticity. This enables the clay to be formed without cracking, and also dry without cracking.(The clay grips end holds onto itself.) Plasticity also increases with ageing the clay. Ih many parts of the world, potters ti clay weeks or mcmths before they use it. In China, potters work with porcelain, a non-plastic clay. !Pheymixclayfortheirgrandchildrenandusa claymadebytheir grandpmti. After a clay piece has been formed, it is left to dry. While drying, most of the water betweenthe particles of clay evaporates into the air. Just like drying clothes, this happens faster when the air is -and dry and the piece is inthe sun. Howen, with clay pieces, it is very importentthattheydryevenly-from side to side, top to bottom, and inside end out. Cxcefuldryingis important because as the water between the clay parti&l.es evaporates, the cley particles more closer together. This results in the piece shrw in size. If one.paxt of the pot or stove liner dries faster m another park, then it *L&s more and crdcs away from the rest. Th2nnerpartsdryfasterthsn thickerpartcs, so it is importankthatthe clay walls be of all the same thickness. 'Theedges af anypieces shouldbe sli&tly%icker because they dry faster. Traditional folk pattezx usually add materials with large particle sizes to the cley mix to help the pieces dry evenly. The relativelylerge grains of sand, mica, or "grog" (crushed fired cley) do not shrink, They also allow a path for water between particle inside the clay wall to pass toward the ouside of the clay wall and evaporate. The tiny holes in auoh clays make them porous, we say they have high porosiQ. (A sponge is another exunple of porosity.) Very plastic clays, all by themselves, tend to wZp andcrack (unless they are very slowly and very carefully dried). mica (a soft'rock that hastMnleyeredcrystals) !l!heclayisa&zaU.ystroagestduringth~ * leatherhard d+ng stqg, This ia when theclayis stiff endfeelslikeleather- notquitevatandnotquitedry, It is ~fi~ttodefanaas~ap~.Whenclayisvergdry(muchl~~in~), it essi;ly chips when