Quicksand Fans

Forgive me if this has been brought up somewhere here before, but I’ve always wondered: when building a quicksand pit for filming, why are materials like peat, bentonite clay, and cork so often used? And why not just use the natural soil from digging out the hole?

From my point of view, the answer is because natural soil generally does not work. Natural soil is almost always a mixture of different things; organic matter, sand, silt, clay, and so on. If these are mixed with a lot of water and stirred up the sand and heavier particles sink to the bottom. The result is the six-foot hole you worked so hard to dig is now only three and a half feet deep. The same result would very likely occur if you tried to use ordinary clay soil, because it is again really clay and other things. The term "clay soil" really means soil that is MOSTLY clay. The material we buy from a hardware store or home supply store is much closer to 100 percent what the label says.

Fred588 wrote:From my point of view, the answer is because natural soil generally does not work. Natural soil is almost always a mixture of different things; organic matter, sand, silt, clay, and so on. If these are mixed with a lot of water and stirred up the sand and heavier particles sink to the bottom. The result is the six-foot hole you worked so hard to dig is now only three and a half feet deep. The same result would very likely occur if you tried to use ordinary clay soil, because it is again really clay and other things. The term "clay soil" really means soil that is MOSTLY clay. The material we buy from a hardware store or home supply store is much closer to 100 percent what the label says.

Fred sums it up pretty well.

Peat and cork are separate animals, but suffice to say, they don't 'settle' the way soil would. Clay stays in suspension - some varieties better than others. And this is why I am adding to what what Fred mentioned. Fire Clay, specifically, is the only (pottery) clay that I have seen settle out. So I would avoid fire clays for our application. Ball clays and Kaolin work very well. From what I have seen, Bentonite stays suspended best of all.

Thank you to Fred and Dave for the replies! I never knew settling was that much of an issue. I always thought it’d be simpler and cheaper to just use the natural soil from the hole, but I figured there must be a practical reason to use other materials.

On a side note: it’d be interesting to see difference in appearance (if any) between, Ball, Kaolin, and Bentonite clays. Sometimes I think I can see subtle differences in the clay used in different videos, but I have no idea which one it actually is.