Human Touch

It started with engineering. All I wanted to do was make an arch out of sand. Packing the sand well enough to support the thin-legged arch I wanted turned out to be conterintuitive: it was done best with sand so wet it ran between my fingers. Heuristic That first arch was carved from a pile made by placing dripping wet sand slurry, one layer after another, handful after handful, until it was tall enough to carve. My fingers felt the density of the sand long before I understood why it worked. Packing slurry worked well enough for small sculptures, but my arches didn't stay small. In growing they acquired internal additions, and I started finding soft spots in the pile. Most of these seemed to come from bad adhesion between additions of sand. I learned to work around the problem, but never quit trying to figure out a solution. Another problem was the soft periphery. The pile might measure a foot across, but only about eight inches could be used because the outer sand wasn't well consolidated. It was unpredictable, too. I had to leave Los Angeles before using the solution that came from memories of watching construction crews building a bridge near where I lived in Kansas. The tall piers were of concrete poured into removable forms; for years the concrete held the plywood's imprint. A year after building my first arch I was on a lake beach in Nebraska with some friends. I'd found an old door and cut it into three parts. Bound with rope scavenged somewhere, the form did its job as I piled in slurry. Here it had an additional benefit: the lake sand contained silt, which retained water and caused slumping of uncontained piles. The pile was so densely packed that my spoon, the only tool I had with me, almost bounced off. I punched holes through the sculpture and it stayed together, standing over three feet tall and looking vaguely Southwestern. This was fun. The process problems were solvable, and improving technique led to new questions. Another year, another move. Back in Los Angeles I wanted to go back to the beach, but circumstances required a form more portable than boards. After some false starts, I wound up with a sheet of upholstery material with laths glued to it, the whole thing wrapped around tall stakes driven into the beach and tied with string. This was ocean sand. It had no silt, and water leached through it too rapidly for slurry layers to stick. If I'd been two people, I could have added water continuously with sand, making slurry on the spot, and retaining familiar technology. My solution was more crude, but worked for a single worker: store water atop the pile, inside the waterproof form. I added sand and water alternately, mixing each addition of sand to make slurry, then tamping with a piece of wood. It was a portable revolution. Sand sculpture became regular and a real delight. My technique has evolved so that I now make much better piles, but the essence of the operation remains the same.

It's astounding. You walk to the beach over sand that slides and pushes out from under your feet. If you pick some up, it runs through your fingers, almost as hard to contain as water. Theoretic It doesn't take long to learn that water mixed with sand changes both completely. Two fluids become a formable solid. It may be counterintuitive, but anyone who has spent time in the playground knows this in the fingers. Water tends to pull in on itself. This makes raindrops, rainbows and flying sparks from every sprinkler. Anything in contact with the water feels this pull, imparting a tiny tensile characteristic to damp sand. It sticks together. This adhesion is the essence of sand sculpture. It's also the bane, because damp sand stickily resists being compacted beyond a certain point no matter how hard it's hit. Granular materials naturally form arch structures, tiny but powerfully resistant to compaction. Smack it on top, and your pile simply spreads sideways. The closer the sand grains, the better they stick together. The sand has to be wet, but how do you get it to pack well if the constituent grains hang up on each other, leaving relatively large air spaces? What you want is close grains, each fitting its space as tightly as possible so the surface tension includes as many as possible. The answer is in the water. It is a lubricant. Not a very good one, but effective if there's enough of it to give the grains a little time, as they sink, to become neighborly. In the final milliseconds of their fall through the water, you give them some help by rapidly striking the pile with a stick. This shakes the whole assembly, helping the grains snuggle in next to each other, filling the space, giving the remanent water the strongest possible handle.

You're on the beach, with the day before you. Your hands itch and your mind is overflowing with ideas. Sand sculpture. It's evanescent, a child's activity. Well, things are temporary, and what adult's life couldn't be improved by some childlikeness? Your hands dig into the damp sand. Practical In sand sculpture you're allowed to accomplish anything you can imagine, as long as you're willing to do the preparatory work. There are many ways to accomplish that work and the technique you choose is based on what you want to carve. Sand that is tightly packed gives each grain a stronger grip on its neighbors. This grip gives the piled sand some tensile strength, which can be exploited to make deep undercuts, leaning towers, long thin structures and counterpoised elements. Not all sculptures call for such elements, and other, less labor-intensive packing methods are good enough. So, for big blocky bases and anything else that just has to sit there, packing damp sand with your hands is good enough. This sort of pile will even accept some carved detail, but don't ask for much. In particular, don't ask for big vertical surfaces. More detailed structures require better compaction, such as slurry-packing. This is an art in itself, the piles lending themselves to flowing, spread-out shapes in a sort of Art Nouveau style. It can be done with no more tools than your hands and some decent sand, if you're willing to sacrifice your fingernails. There are limits on size, but with creative use slurry-packing for details, big sculptures are possible. Just ask Sandy Feet. If you want big, well compacted piles, you pretty much have to use a form. Traditional forms are made of plywood, but more portable ones are possible using fabric supported by stakes, or swimming pool sidewall plastic. Forms allow fast, consistent packing, with many people working simultaneously. The usual method is to throw in a lot of sand, add some water to dampen it, and then pound with something heavy. Some groups use car axles. Not caring to carry axles around, I prefer to keep water standing on top of the pile inside the form. Adding sand in small increments allows me to mix it thoroughly, making sure all the grains are surrounded by lubricating water, then pounding rapidly with a lightweight stick. This technique is slow; at the Santa Cruz contest, one professional told me he'd have had my form filled in a third the time. That's true, but he also didn't make undercuts like mine. You make choices. No matter how you make the pile, the truth comes out as soon as you start to carve. The nature of the pile comes from the sand of which it's made, the way you pack it, the shape of the sand grains and the type of water you used. It takes time to know sand. A piece comes off in a chunk, sliding along a fault: bad adhesion to the next layer down. Part of the sculpture collapses, leaving loose sand: bad compaction. Good compaction results in chunks of sand around the failed sculpture, and shear faces that resemble those in sandstone canyons. Perhaps you've done everything you can to ensure good compaction, and the sculpture still fails: this means you're still learning. You've been too adventurous. Good. My failure rate is around ten per cent.

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References What Holds Sandcastles Together? Nature Magazine #387 1997-June-19 page 765 Central Stress Minimum Nature Magazine #382 1996-July 25 page 336 Digging Into Sand Science News Magazine #136 1989-July-15 page 40 Dry Sand, Wet Sand Science News Magazine #152 1997-September-20 page 186 Sand Pile Pressure Dip Science Magazine #273 1996-August-2 Dynamics of Granular Material American Scientist Magazine #85 1997-November-December page 540

sediment.htm 1999 February 14
Error correction and design update 2000 October 6