Don't worry about "French Sand." There is also some stuff called "Yankee Sand" from the USA that you shouldn't worry about either. You also shouldn't use "tube sand" or dig a hole in your backyard although both are good studies in what you could get by with in desparation. Any larger city greater than a 1000 or so people has something called a ready mix concrete plant or a sandblasting place. You should call them both up and ask for their finest white silica sand less then $10 for a 50 lb bag. You may ask the gradation, or fineness, or % passing the #70 sieve (depending on how they grade it) just out of curiosity. Then you can mix it dry with the bentonite (not kitty littler) that is also found anywhere in the USA if you know who to ask. The fine while sand should be anywhere from a #70 sieve size to smaller down to 120 or less. The best results are with a blend of different sizes for good aggregate interlock.
The French sand is a naturally graded sand of varying particle size that has good aggregate interlock and fine particles but also good porosity. the bad thing is it's hard to find around here and isn't good for larger castings unless only used as a facing sand.
You can get fingerprints to come out in good silica sand thats blended correctly, the tube sand has larger particles without the finer sizes needed to get good detail.
The backyard sand I would think would be hit or miss and would need to be processed by getting all the organics and other stuff out of it before use.
--- In firstname.lastname@example.org, Jim <darkhors5@...> wrote:
> Instead of buying sand, you might try digging a hold in your back yard, for most of Long Island, after about a foot, you will hit sand, occasionally mixed with a touch of clay.