Monday, March 19, 2007

Clay is Strange Stuff

Clay is strange stuff. It is not mud, dirt, or silt. As gardeners know, clay takes a long time to saturate, and when it finally does, it is not something fun to step in. Clay also takes a long time to dry out, and when it finally does, it is not something fun to shovel.

Clay will hold shapes, while dirt or sand will not. Detectives in novels race the clock to analyze footprints in sand, but they can take their time if their quarry walked on clayey soil. Children know that clay is much better for making mud pies than is typical dirt. At the beach, damp sand is great for sand castle, but when it dries out (if the tide lets it survive that long), the castle collapses. However, a castle of clay will remain standing long after it has dried out. Even the wind won’t blow it down. Why? What makes clay different from dirt or sand?

The potter’s answer lies in the fact that the shape of the clay particle is flat—as mica is flat—whereas the shape of even the finest particle of the finest silt is round. This elongated flatness creates a slight electrical attraction between clay particles, causing them to tend to stick together. When water molecules do manage to squeeze in between the clay molecules (which takes them a long time), they get caught in the attraction and tend to stay put. Hence, the long time in the garden for clay to absorb water, and the long time for clay to give it up again. And hence the characteristic “stickiness” of clay: if you ever wet-mounted glass slides in biology class, you know those long, flat plates of glass stick together using nothing but water as “glue.” Or, if you ever tried using chopsticks to pry half an almond (flat side down and covered with sauce) off your plate, you know firsthand that wet flat things stick together!

This is the same property present in clay, that makes it more sticky than dirt. This stickiness or adhesion is what allows walls of clay to stand up tall, too—–enabling a potter to create a tall vase (or a small castle).

When I was learning the potter’s craft, I had to learn how to properly “wedge” clay. Wedging means taking a softball-to-cantaloupe-sized lump of the stuff and pushing on it with the hands in a rotating motion—VERY much like kneading dough for bread. Wedging continues until a “done” state is reached (and, as with kneading, it’s tough to know the feel of “doneness” except by experience). I understood years ago that wedging was supposed to eliminate air bubbles, which cause annoying problems in throwing clay and disastrous explosions in firing clay. Wedging tables have a taut wire strung next to the table where potters can periodically slice their lumps of clay during the wedging process to inspect a cross-section of the lump for internal air bubbles. (Actually the wire is used primarily for injuring the less-than-alert potter’s fingers, rendering further pottery work impossible until the wound heals. Eventually, I learned to avoid the wedging table altogether, using other surfaces for wedging. I don’t even own a wedging table today).

Twenty years later I learned about the flatness of clay particles, and the electrical charge thing. I found out that the purpose of wedging is really to align clay particles. The motion of forcing the clay around and around in a spiral imparts a spiral alignment to the lump. Thus—air bubbles lacking in each case—a wedged lump of clay throws better on the wheel than an unaligned, unwedged lump of clay; the circular motion of the throwing teams up with the alignment of wedged clay to build taller, stronger, thinner clay walls on the wheel than is possible with unwedged clay. More alignment. More adhesion. Nicer, thinner pot walls. Happier potter.

“Ageing” clay also makes sense to me now. Aged clay throws better than new clay. Oriental potters of long ago knew this fact, mysterious though it seemed. Now I know that the longer clay gets to age in its wet state, the more time is available for those flat little plates to align themselves in the lump. And they do align themselves! Thus, aged wedged clay throws better than newly processed wedged clay. Learning about the properties of clay particles has helped me--an artistically-gifted anomaly in an extended family of engineers and doctors--better appreciate what I can and can’t make clay do. I can daydream all day long about making large and sturdy--but lightweight--water pitchers (much easier to heft than heavy pitchers), but when it comes to throwing time, I’d better make sure my clay is aged and well-wedged, so I can make those walls go up tall and thin without collapsing. All those clay platelets are holding hands, singing together. I’ve helped them “do their thing.”

1 comment:

Anonymous said...

Thank you. The "why" of things is so helpful, but so often lost as we pass on how to's. I'd tried to crush some dried clay to dust, and couldn't figure out why the clumps that I'd mushed flat, when wet, were so much harder than the rest of the clay. That explains it.