For a class that I have taught, I used 18" low plastic bowls and place the tufa on the inside. The plastic bowl is intended to be used as a planter for annuals - so you can purchase at a place like Lowes.
I spray the inside of the bowl with cooking spray - not too much, and that will help with removing the mold the next day. This work great for a 1 day class, cause they can take the tufa trough home the same day (in the container).
Mix should be not to wet and it will not slump too much.
If it does slump, I just correct with a flat masonary tool.
Mike

I agree. There is always a tendency to slump. Verticle walls are difficult because of it. If you want a piece with verticle sides, I suggest making the piece between inside and outside boxes or containers. If you use cardboard boxes, be aware the boxes will get wet and try to cave in or bulge out. I have made some large ones with verticle sides using wooden boxes that are rigid and can be easily removed--just unscrew the sides. Others have been made using inside and outside forms of black nursery pots.
Several of my classes have made troughs with verticle sides using wire mesh basket shape forms made of one inch hardware cloth. The mix is built up on the mesh, both inside and outside, and the form stays in the trough. The mix must be very dry and will it try to slump off. It just takes a little patience and constsant pushing the mix through the holes in the form. In a recent class troughs were made on pieces of plywood, all turned out great and they were taken home on the board the same day.
If you want to try that, I suggest making a wire "basket' about 12 x 10 x 4.5 inches high. (could be round instead of rectangular). Lay down a 3/4 in thich layer of wet mix on a board. Tamp it down then place the basket on the mix. Push the basket into the mix slightly. Then put a layer of mix in the bottom of the basket and tamp it flat and into the bottom of the basket. then start working up the sides to form the walls. Add mix inside and outside of the sides of the basket while pushing it through the wire mesh holes. Work all around and gradually build up all four sides layer by layer. Don't try to build up one side at a time to its full height-- that will not work. Just be patient. This does work and the finished piece should be about 13.5 x 11.5 x 6in high. Once youve mastered this you can go larger, but there is a limit to how high the verticle sides can be made without too much trouble. I suppose you could build up sides over a larger wire form--two inches at a time-- wait until each layer sets up some then add another layer--and so on, and your sides might end up a foot tall. Just some thoughts. Remember you will always learn something from each attempt even if it is a "failure".

Perhaps I am singing to the choir on this one, but a ONE TIME ONLY mold can be a woven basket headed for the dump! Mold tufa on the inside - tendency to slump is less with this very rough surface. You get a delightful pattern on outside of your trough!!

I am new here and haven't had a chance to read lots of the archives and many photo's don't pull up anymore. I just had never read that there was a certain height to vertical walls.
Sounds like a good use for old easter baskets..

I've always found "inside the mold" easier than outside the mold just because it's a smaller project. The project will always be the size of the mold. This photo is two "inside bowl" molds that I "tufa glued" for a giant pot. For the top bowl I just leave out the center. For the bottom bowl, I just make as usual. I always cover my bowls with plastic bags, sometimes using two or three, and they pop right out. The bags leave texture and make the release super easy. Once released, I carve off the jagged center edge, wire brush the outside and join together with soupy hypertufa. Spread a little outside the seam, let sit for a few hours, and wire brush the to blend in. Afterwards, I put on a wood stain for a textural color. Don't like the concrete colorants. If you're making a trough type object, it's always easier to use an inside mold than an outside mold. It's too hard to keep the mud in place on vertical walls that are more than about 4" tall. Hope this helps.

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"inside outside", depends on the size or look I want, with the mix on the outside I can get a nice smooth look if that's what I want, example: a plain 1/2" sided small concrete bird-bath, sometimes the edges need a bit of chiseling for ease of removal-but easy enough while green.

Wow! valolson1-great looking pot! never would have know that this was 2 put together-what color stain did you use? - such a nice sandy tone! Yes I like the look that plastic wrap gives also, recently I've been casting small s'hroom using plastic in the molds it gives a nice natural look to these also a nice aged look to bowls etc. (not that I've done so many of these) not to mention the ease of removal for so many other items. Great work! Wendy

The pot, valoson1, is just about the best I have seen in a long time .........wowowowow

1) For beginners, working inside a mold is easier.
2) As long as you have the proper mold or sub-structor there's no limit on the vertical.
3) You can use cardboard boxes, place the whole box in a plastic bag and wrap sides with duct tape. Depending on the size, you may need cinder blocks or something similar to brace the sides.
valolson1, Nice pot, really goes well with that palm!
You can also use this to make round candle holders. You either cut out while still green or use styrofoam pieces to make openings in each bowl. Make sure you place a piece of styrofoam in the bottom of one bowl mold for the top opening. After 24 hours unmold, gently wire brush the two bowls, cement the two halves together with the same recipe, spritz with water and let sit for 20 minutes in the shade then gently wire brush to blend where the join is. Spritz with water, wrap in plastic and leave to cure for 24 hours. Make sure the top opening is big enough for a pillar candle and to have room to put your hand or a pair of tongs in there to remove the candle. My friend put a small, shallow glass dish in her's for the candle drips.

For the above pot, I used a light oak type stain. The secret is to paint it on, but just hit the high spots so you still get that textural look. I hate concrete colorants because you virtually have the same color and it adds unneeded expense to your project. I always use Behr Wet Look Sealer as a finish seal to the project. I've tried most of them and it's the best.

valolson1, thank you so much for posting the color of the stain you used, your pot here look so grand and a great display I might add also! Didn't plan to color any of my 'tufa' pots but may have to try it on at least one! I do a lot of acrylic paint washes on my leaves-takes LO-Ts of time with so many different leaves and colors to experiment. On my sphere fountain after painting I used a craft gloss ext/int turned out looking the same as the wet sealer, this comes in a small bottle and the instructions say it's a 'super tough polyurethane' as I don't like the glossy look on my leaves this is a small bottle and a lot cheaper to buy for the odd piece.

I really like the Behr Wet Look Sealer because it gives a "natural" sheen instead of a poly look that is totally opposite of primitive stone. And, while it costs $21 a gallon, it also goes a looooooong way. It's especially useful for those projects I place in the house.

I was thinking that the 'Behr Wet Look Sealer' would give everything the wet/very shiny surface but the pic. above of your pot don't have this? you used this type of sealer on this??? I would be very interested in this BWLS if it only gives a slight sheen :~)
I wanted the 'wet look' on my fountain sphere but rarely like this effect on other items I have made-Wendy

If you want a shinier sheen with the Behr Wet Look Sealer, you can just put on another coat or two. When I do an orb as a topper for a fountain, I really like to shine it up.