If that was 24"x24"x24", you need some support for the wet mix to cling to, at least on the sides if not the bottom. Try wrapping some chicken wire around the form. It shouldn't be really tight, there should be some space between the form and the wire. The mix should be thoroughly worked through the wire so it will "hold hands" around the wire.

Also when using wire mesh of some kind, it's kind of hard to do it all in one coat. I find that working it through the wire and smoothing it on top of the wire, letting some of the wire show through, letting it set up a bit, then putting on another layer works best.

Also make sure that your mix isn't too wet. Squeeze a handful and let it set in your hand. It shouldn't be slumping, or much, and no water should drip out when it's squeezed.

Sue

Jean try tinfoil.
Skim coat it with a rough slurry coat, let it dry and then apply your tufa. Use a trowel and keep troweling up. Be patient, it will eventually stay put.:)
I use a recipe of my own that is almost no slump even on vertical surfaces.
It contains lime and a few other goodies including a bit of powdered fired clay so it may not be as strong as others.
I'm still testing this recipe for strength.

I should add that the tinfoil should be draped over the container so the wet of the tufa can't pull it down.
Plastic cling wrap actaully works for this too, if you drape it over the top. Tufa sticks nicely to cling wrap. But for cling wrap wet your container and then throw on the cling wrap. the wet makes it stick to the contaner with vacuum and helps keep it in place. this works well for plastic but it might not with white styrofoam.

If your tufa is too slumpy for tinfoil then you might need the wire.:)
I've never had to use wire even going perpendicular.

Thank you, thank you.

I have some larger styrofoam boxes I was wanting to use to make planters and troughs. Some are pretty big...like 3 feet. I guess I'd really need to use chicken wire or something similar. The boxes don't fit inside each other, so I can't use them that way.

I think I'll try the plastic wrap with the smaller box first. One of the problems it seemed using a plastic bag, was air was between the bag and the box.

I was gonna give up on this notion, but I'll give it another try...

Jean

If you have a bigger styrofoam box, you can make the inside box from anything, tape together styrofoam blocks for the needed size, cardboard box filled with sand and wrapped in plastic bag, even bricks in plastic bags, anything with solid sides.

Don't give up! Have fun!

Andrea