In the sand box

You are correct, it was play sand they used, not beach sand. 

Just when I think people in this hobby couldn't get sillier...

"Just when I think people in this hobby couldn't get sillier.."

How naive grasshopper.

Some use Organics such as Rice as the Particulate, others use Metal such as Lead Shot, other use Grain that is a Stone Particulate, in general all must be thoroughly dry before being put to use.

There is also Oil Compacted Sand worthwhile looking into as the Particulate.

There is a thread started on 3-27-2020 by @soma70 entitled "Sandbox Design Advice". In it I recommended silica sand, a non-organic, man-made material used in sandblasting. Silica sand is superior to play sand for this application in that it doesn’t absorb moisture, and is therefore incapable of supporting the growth of bacteria.

Silica comes in different "grades", just like lead shot. When I was using the Bright Star Big Rock isolation boxes, I settled on No. 60 grade sand. It is the optimum size, anything smaller (the particle size, a larger number connoting a smaller size grain) being too powdery.

I found Silica at a construction supply yard, but I think even Home Depot sells it. It’s cheap, like under ten bucks for a fifty pound bag (this was twenty years ago).